Chemical Sciences for the New Decade: Volume 2 Biochemical and Environmental Applications 9783110783629, 9783110783582

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Ponnadurai Ramasami (Ed.) Chemical Sciences for the New Decade

Also of interest Chemical Sciences for the New Decade Volume : Organic and Natural Product Synthesis Ponnadurai Ramasami (Ed.),  ISBN ----, e-ISBN ----

Chemical Sciences for the New Decade Volume : Computational, Education, and Materials Science Aspects Ponnadurai Ramasami (Ed.),  ISBN ----, e-ISBN ----

Chemical Sciences in the Focus Volume : Pharmaceutical Applications Ponnadurai Ramasami (Ed.),  ISBN ----, e-ISBN ----

Chemical Sciences in the Focus Volume : Green and Sustainable Processing Ponnadurai Ramasami (Ed.),  ISBN ----, e-ISBN ----

Chemical Sciences in the Focus Volume : Theoretical and Computational Chemistry Aspects Ponnadurai Ramasami (Ed.),  ISBN ----, e-ISBN ----

Physical Sciences Reviews e-ISSN -X

Chemical Sciences for the New Decade Volume 2: Biochemical and Environmental Applications Edited by Ponnadurai Ramasami

Editor Prof. Dr. Ponnadurai Ramasami Computational Chemistry Group, Department of Chemistry, Faculty of Science, University of Mauritius, Réduit 80837, Mauritius E-mail: [email protected]

ISBN 978-3-11-078358-2 e-ISBN (PDF) 978-3-11-078362-9 e-ISBN (EPUB) 978-3-11-078371-1 Library of Congress Control Number: 2022941766 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the internet at http://dnb.dnb.de. © 2022 Walter de Gruyter GmbH, Berlin/Boston Cover image: sorbetto/DigitalVision Vectors/Getty Images Typesetting: TNQ Technologies Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Preface of the Book of Proceedings of the Virtual Conference on Chemistry and its Applications (VCCA-2021). A virtual conference on chemistry and its applications (VCCA-2021) was organized online from 9th to 13th August 2021. The theme of the virtual conference was “Chemical Sciences for the New Decade”. There were 197 presentations for the virtual conference with 400 participants from 53 countries. A secured platform was used for virtual interactions of the participants. After the virtual conference, there was a call for full papers to be considered for publication in the conference proceedings. Manuscripts were received and they were processed and reviewed as per the policy of De Gruyter. This book, volume 2, is a collection of the thirteen accepted manuscripts within the fields of biochemical and environmental applications. Yandri et al. reported on the increase in the stability of the α-amylase enzyme from Aspergillus fumigatus using dimethyladipimidate. Yakubu and co-workers synthesised squarylium dyes and they were characterized using spectrometric techniques namely FT-IR, UV-visible and GC–MS. Mkhondwane and Pullabhotla reviewed on the oxidation of cyclohexane using advanced oxidation processes with metals and metal oxides as catalysts. Pantcheva et al. isolated and elucidated the structural features of novel bis-salinomycinate complexes of Mg(II), Ca(II), Sr(II) and Ba(II) ions. Sowemimo and Adeniyi synthesised and characterized the alkaloid derived hydrazones and their metal(II) complexes namely Mn, Ni and Co. Oguegbulu et al. optimised the molecular prototype (small molecule analogue), in a bid to minimise cellular toxicity, off target activities and improve activity. Halilu and Muhammad investigated on the phytochemical, physicochemical and antioxidant activity of the seed oil with the view of providing data for the effective utilization of the oil in food, pharmaceutical and other industrial applications. Aswathanarayan and Vittal discussed the biofilm formation in medically relevant bacteria and the use of small molecules for preventing and controlling biofilm formation in infectious bacteria. Miles and co-workers synthesised a pharmaceutical cocrystal of ascorbic acid + para-aminobenzoic acid and ascorbic acid + paracetamol and they were characterized by PXRD, DSC, and FTIR. Nwokonkwo and Nwafor extracted, isolated and characterised the secondary metabolites from the leaves of morinda lucida from Oshiegbe in Ebonyi state. Alfred et al. reported on the ethnopharmacology, phytochemistry and a new chemotaxonomic marker in Oldenlandia affinis (Roem. & Schult.) DC. Rubiaceae. Oni et al. investiaged on the oil obtained from Cola lepidota seed and concluded that the oil contains some phytochemicals and nutraceutical properties. Kayode and co-workers reviewed on cancer

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Preface

therapeutics as cancer is a remarkable enemy to health and over the past years, advances have been made in the study and understanding of this disease. I hope that these chapters of this volume 2 will add to literature and they will be useful references for researchers. To conclude, VCCA-2021 was a successful event and I would like to thank all those who have contributed. I would also like to thank the Organising and International Advisory committee members, the participants and the reviewers. Prof. Ponnadurai Ramasami Computational Chemistry Group, Department of Chemistry, Faculty of Science, University of Mauritius, Réduit 80837, Mauritius E-mail: [email protected]

Contents Preface V List of contributing authors

XV

Yandri Yandri, Nurmala Nurmala, Tati Suhartati, Heri Satria, and Sutopo Hadi 1 The stability increase of α-amylase enzyme from Aspergillus fumigatus using 1 dimethyladipimidate 1 1.1 Introduction 2 1.2 Materials and method 2 1.2.1 Materials and instrumentations 3 1.2.2 Procedure 3 1.3 Results and discussion 3 1.3.1 Production and isolation of α-amylase 4 1.3.2 α-Amylase purification 5 1.3.3 Characterization of native and modified enzymes 6 1.3.4 Native and modified enzymes 1.3.5 Determination of the native and modified enzymes’ thermal 7 stability 1.3.6 Constants of thermal inactivation ate (ki), half-life (t1/2), and energy Gibbs changes due to denaturation (ΔGi) of the native and modified 7 enzymes 8 1.4 Conclusions 9 References Yakubu Ali, Joseph O. Odey, Giwa A and K. A Bello 2 A novel application of synthesised based squarylium dyes on nylon 6, and 11 silk woven fabrics 11 2.1 Introduction 13 2.1.1 Results and discussion 13 2.1.2 Synthesis of squaric acid 14 2.1.3 General procedure for synthesis of squarylium dyes 14 2.1.4 Synthesis of dye A, B, C, D and E 14 2.1.5 Physical properties of the synthesized dyes 14 2.2 Discussion 14 2.2.1 Physical characteristics of the synthesised dye 16 2.2.2 Infra-red analysis of the synthesized dyes 16 2.2.3 Mass spectroscopy of the synthesised dyes 18 2.3 Solvatochromism 18 2.3.1 Molar extinction coefficient (ε) 19 2.3.2 Dye exhaustion for nylon 6 and silk

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2.3.3 2.3.4 2.3.5 2.3.6 2.3.7 2.3.8 2.3.9 2.4

Contents

Effect of pH 20 20 Effect of time Effect of carrier concentration 21 Effect of temperature 21 Wash fastness 22 Light fastness 23 Perspiration fastness 23 Fastness to hot pressing 26 References

20

Siphumelele Thandokwazi Mkhondwane and Viswanadha Srirama Rajasekhar Pullabhotla 3 Cyclohexane oxidation using advanced oxidation processes with metals and 29 metal oxides as catalysts: a review 29 3.1 Introduction 31 3.2 AOPs for cyclohexane oxidation 32 3.2.1 Photocatalysis 34 3.2.2 Photocatalysis in oxidation of cyclohexane 39 3.2.3 Fenton oxidation process 41 3.3 Heterogeneous Fe Fenton catalysts 41 3.3.1 Ferriyhydrite 41 3.3.2 Ferrites 42 3.3.3 Hematite 42 3.3.4 Pyrite 43 3.3.5 Magnetite 43 3.3.6 Fenton oxidation of cyclohexane 49 3.3.7 Ozonation 52 3.3.8 Ozonation of cyclohexane 55 3.4 Conclusion and outlook 56 References Ivayla Pantcheva, Nikolay Petkov, Svetlana Simova, Rumyana Zhorova, and Petar Dorkov 4 Alkaline-earth metal(II) complexes of salinomycin – spectral properties 65 and antibacterial activity 65 4.1 Introduction 67 4.2 Experimental 67 4.2.1 Materials 67 4.2.2 Preparation of complexes 1–4 67 4.2.3 Methods 68 4.2.4 Antimicrobial activity assay

Contents

4.3 4.3.1 4.3.2 4.3.3 4.4

Results and discussion 68 Spectral properties of complexes 1–4 72 Structure of complexes 1–4 Antimicrobial activity of complexes 1–2 75 Conclusions 75 References

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69 73

Mutiu Sowemimo and Adeleke Adeniyi 5 Synthesis and characterization of alkaloid derived hydrazones and their 79 metal (II) complexes 80 5.1 Introduction 81 5.2 Materials and methods 81 5.2.1 Chemicals and reagents 81 5.2.2 Synthesis of ligands 82 5.2.3 Synthesis of the metal complexes 82 5.2.4 Physicochemical measurements 82 5.2.5 Antimicrobial screening 82 5.3 Results and discussion 90 5.4 Conclusions 90 References Joseph C. Oguegbulu, Abedawn I. Khalaf, Colin J. Suckling, Margaret M. Harnett, and William Harnett 6 Lead optimisation efforts on a molecular prototype of the immunomodulatory 93 parasitic protein ES-62 94 6.1 Introduction 97 6.2 Methods 97 6.2.1 In silico screening 97 6.2.2 Synthetic methods 100 6.2.3 HPLC method for LogP determination 100 6.3 Results 100 6.3.1 In silico Screening (selection of amines) 100 6.3.2 Synthesis of compounds 102 6.3.3 HPLC method for LogP determination 102 6.4 Discussion 107 6.4.1 Conclusions 107 6.5 Experimental section 107 6.5.1 General 108 6.5.2 Synthesis of compounds 108 Abbreviations 109 References

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Contents

Emmanuel M. Halilu and Baraka Muhammad 7 Phytochemical and antioxidant studies of Hibiscus Cannabinus seed 111 oil 112 7.1 Introduction 113 7.2 Methodology 113 7.2.1 Collection, identification and preparation of seeds 113 7.2.2 Physicochemical studies 113 7.2.3 Extraction of oil 113 7.2.4 Organoleptic examination of fixed oil 113 7.2.5 Solubility testing 113 7.2.6 Determination of specific gravity 114 7.2.7 Oil analysis 114 7.2.8 Qualitative phytochemical test 114 7.2.9 Thin layer chromatography 114 7.2.10 GC-MS analysis 114 7.2.11 Toxicity studies 114 7.2.12 Antioxidant assay using DPPH free radical 115 7.3 Results and discussion 115 7.3.1 Physicochemical evaluation 7.3.2 Extraction, organoleptic evaluation, solubility studies on the oil 7.3.3 Specific gravity, oil analysis and qualitative phytochemical 116 screening 117 7.3.4 Thin layer chromatography (TLC) 118 7.3.5 GC-MS analysis of Hibiscus cannabinus oil 118 7.3.6 Acute toxicity studies 119 7.3.7 Free radical scavenging activity 120 7.4 Conclusions 120 References

116

Jamuna Bai Aswathanarayan and Ravishankar Rai Vittal 8 Small molecules as next generation biofilm inhibitors and anti-infective 123 agents 123 8.1 Introduction 124 8.2 Biofilm formation–composition and mechanism 124 8.2.1 Composition of biofilms and antibiotic tolerance 125 8.2.2 Quorum sensing in biofilms 125 8.3 Biofilm inhibition strategies 127 8.3.1 Inhibition of bacterial surface adhesion 129 8.3.2 Signal inhibitors 131 8.3.3 Disruption and dispersal of mature biofilms 132 8.4 Conclusion and future prospective 132 References

Contents

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Fatima Miles, Fayrouz Djellouli, Nourelhouda Bensiradj and Abdallah Dahmani 9 Pharmaceutical cocrystal consisting of ascorbic acid with p-aminobenzoic 137 acid and paracetamol 138 9.1 Introduction 139 9.2 Experimental 139 9.2.1 Preparation of compounds 139 9.2.2 Characterization of Cocrystal 140 9.3 Results and discussion 141 9.3.1 Cocrystal AB 142 9.3.2 Cocrystal AP 143 9.3.3 Results of DFT calculations 146 9.3.4 FT-IR spectral study 146 9.3.5 Dipole moment and frontier orbitals (HOMO and LUMO) 147 9.3.6 Thermodynamic properties 149 9.4 Conclusion 149 References Dorathy Chinasa Nwokonkwo and Emmanuel Ikechukwu Nwafor 10 Extraction, isolation and characterization of secondary metabolites in the 151 leaves of Morinda lucida from Oshiegbe in Ebonyi State 151 10.1 Introduction 153 10.2 Materials and methods 153 10.2.1 Sample collection and authentication 154 10.2.2 Column chromatography 154 10.2.3 Structural determination of the bioactive compounds 154 10.3 Results and discussion 154 10.3.1 Extract yield and phytochemical analysis 156 10.3.2 Structural elucidation of the isolated compounds 161 10.4 Conclusions 161 References Francis Alfred Attah, Augustine E. Mbanu, Uche M. Chukwudulue, Ugochukwu J. Jonah, and Ngaitad S. Njinga 11 Ethnopharmacology, phytochemistry and a new chemotaxonomic marker in 165 Oldenlandia affinis (Roem. & Schult.) DC. Rubiaceae 166 11.1 Introduction 168 11.2 Materials and methods 168 11.3 Botanical description and geographical distribution 169 11.4 Traditional uses and ethnopharmacology of Oldenlandia affinis 169 11.4.1 Traditional uses 171 11.4.2 Ethnopharmacology

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11.5 11.5.1 11.5.2 11.5.3 11.6 11.7

Contents

Documented metabolite profile in Oldenlandia affinis 175 175 Nonpeptidic metabolites of Oldenlandia affinis 176 Peptidic metabolites of Oldenlandia affinis 178 Phytochemistry of cyclotides expressed in Oldenlandia affinis Taxonomic conflict and a new chemotaxonomic marker in Oldenlandia 180 affinis 181 Conclusion and future prospect 182 References

Sarah Oni, Akinola Akinlabi, Abayomi Bamisaye and Josephine Ojo 12 Physico-chemical and nutraceutical properties of Cola lepidota seed 187 oil 187 12.1 Introduction 188 12.2 Methodology 188 12.2.1 Proximate analysis 188 12.2.2 Boiling point 188 12.2.3 pH determination 189 12.2.4 Saponification value 189 12.2.5 Peroxide value 189 12.2.6 Iodine value 189 12.2.7 Acid value 190 12.2.8 Determination of specific gravity 190 12.2.9 Percentage yield determination 190 12.3 Results and discussion 193 12.4 Conclusions 194 References Abolanle A. A. Kayode, Izuchukwu Emmanuel Eya and Omowumi Titilola Kayode 197 13 A short review on cancer therapeutics 197 13.1 Introduction 198 13.2 Some hallmarks of cancer 198 13.2.1 Sustained proliferative signalling 199 13.2.2 Evading growth suppressors 200 13.2.3 Activating invasion and metastasis 200 13.2.4 Enabling replicative immortality 200 13.2.5 Resisting cell death 201 13.2.6 Reprogramming energy metabolism 201 13.2.7 Evading immune response 202 13.3 Cancer therapeutics 204 13.4 Cancer therapeutics on some of the hallmarks of cancer 204 13.4.1 Therapeutics on sustained proliferation

Contents

13.4.2 13.4.3 13.4.4 13.4.5 13.5

Index

Therapeutics on cell death resistance 205 206 Therapeutics on replicative immortality 206 Therapeutics on evading immune response 206 Therapeutics on growth suppressor evasion 206 Conclusions 207 References 211

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List of contributing authors Kayode A. A. Abolanle Department of Biochemistry School of Basic Medical Sciences Babcock University Ilishan-Remo Ogun State Nigeria E-mail: [email protected] Adeleke Adeniyi Department of Chemistry Lagos State University Ojo Lagos Nigeria E-mail: [email protected] Akinola Akinlabi Department of Chemistry Federal University of Agriculture Abeokuta Nigeria Yakubu Ali Ahmadu Bello University Polymer and Textile Engineering Zaria Kaduna Nigeria Jamuna Bai Aswathanarayan Department of Microbiology School of Life Sciences JSS AHER Mysore Karnataka 570015 India E-mail: [email protected] Francis Alfred Attah Department of Pharmacognosy and Drug Development Faculty of Pharmaceutical Sciences University of Ilorin Ilorin, Kwara State Nigeria E-mail: [email protected]

https://doi.org/10.1515/9783110783629-202

Abayomi Bamisaye Department of Chemical Sciences Lead City University Ibadan Nigeria; and Department of Chemistry Federal University of Agriculture Abeokuta Nigeria Giwa A Bello Ahmadu Bello University Polymer and Textile Engineering Zaria Kaduna Nigeria K. A Bello Ahmadu Bello University Polymer and Textile Engineering Zaria Kaduna Nigeria Nourelhouda Bensiradj Laboratoire de Chimie Théorique Computationnelle et Photonique Faculté de Chimie USTHB BP32, 16111 El Alia Algiers Algeria Uche M. Chukwudulue Chukwuemeka Odumegwu Ojukwu University Igbariam Anambra State Nigeria E-mail: [email protected] Abdallah Dahmani Laboratoire de Thermodynamique et de Modélisation Moléculaire Faculté de Chimie USTHB, BP32, El-Alia 16111 Bab-Ezzouar Alger Algerie

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List of contributing authors

Fayrouz Djellouli Laboratoire de Thermodynamique et de Modélisation Moléculaire Faculté de Chimie USTHB, BP32, El-Alia 16111 Bab-Ezzouar Alger Algerie E-mail: [email protected] Petar Dorkov Research and Development Department Biovet Ltd., 39, P. Rakov Str. Peshtera 4550 Bulgaria Izuchukwu Emmanuel Eya Department of Medicine & Surgery School of Clinical Sciences Babcock University Ilishan-Remo Ogun State Nigeria Sutopo Hadi Department of Chemistry Faculty of Mathematics and Natural Sciences The University of Lampung Bandar Lampung Lampung 35145 Indonesia E-mail: [email protected] Emmanuel M. Halilu Faculty of Pharmacy Cyprus International University Haspolat/Nicosia North-Cyprus Via Mersin 10 Turkey; and Department of Pharmacognosy and Ethnomedicine Faculty of Pharmaceutical Sciences Usmanu Danfodiyo University Sokoto Nigeria E-mail: [email protected]

Margaret M. Harnett Institute of Infection Immunity and Inflammation University of Glasgow Glasgow G12 8TA UK William Harnett Strathclyde Institute of Pharmacy and Biomedical Sciences University of Strathclyde Glasgow G4 0RE UK Ugochukwu J. Jonah Riyadh Al Khabra General Hospital Al Qassim Saudi Arabia E-mail: [email protected] Omowumi Titilola Kayode Department of Biological Sciences College of Basic and Applied Sciences Mountain Top University Kilometer 12, Lagos-Ibadan Expressway Prayer City Ogun State Nigeria Abedawn I. Khalaf Department of Pure and Applied Chemistry University of Strathclyde Glasgow G1 1XL UK Augustine E. Mbanu Department of Pharmacognosy Faculty of Pharmacy University of Ibadan Ibadan Nigeria E-mail: [email protected] Fatima Miles Laboratoire de Thermodynamique et de Modélisation Moléculaire Faculté de Chimie USTHB, BP32, El-Alia 16111 Bab-Ezzouar Alger Algerie

List of contributing authors

Siphumelele Thandokwazi Mkhondwane Department of Chemistry University of Zululand Private Bag X1001 Kwa-Dlangezwa, 3886 South Africa Baraka Muhammad Department of Pharmacognosy and Ethnomedicine Faculty of Pharmaceutical Sciences Usmanu Danfodiyo University Sokoto Nigeria Ngaitad S. Njinga Department of Pharmaceutical and Medicinal Chemistry University of Ilorin Ilorin Nigeria E-mail: [email protected] Nurmala Nurmala Department of Chemistry Faculty of Mathematics and Natural Sciences The University of Lampung Bandar Lampung Lampung 35145 Indonesia Emmanuel Ikechukwu Nwafor Department of Industrial Chemistry Ebonyi State University Abakaliki Nigeria E-mail: [email protected] Dorathy Chinasa Nwokonkwo Department of Industrial Chemistry Ebonyi State University Abakaliki Nigeria Joseph O. Odey University of Calabar Pure and Applied Chemistry University of Calabar Calabar Nigeria E-mail: [email protected]

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Joseph C. Oguegbulu Department of Chemical Sciences Bingham University PMB 005, Karu Nasarawa State Nigeria E-mail: [email protected] Josephine Ojo Department of Chemical Sciences Lead City University Ibadan Nigeria Sarah Oni Department of Chemical Sciences Lead City University Ibadan Nigeria; and Department of Chemistry Federal University of Agriculture Abeokuta Nigeria E-mail: [email protected] Ivayla Pantcheva Faculty of Chemistry and Pharmacy Sofia University “St. Kl. Ohridski, 1, J. Bourchier Blvd. Sofia 1164 Bulgaria E-mail: [email protected]fia.bg Nikolay Petkov Faculty of Chemistry and Pharmacy Sofia University “St. Kl. Ohridski 1, J. Bourchier Blvd. Sofia 1164 Bulgaria Viswanadha Srirama Rajasekhar Pullabhotla Department of Chemistry University of Zululand Private Bag X1001 Kwa-Dlangezwa, 3886 South Africa E-mail: [email protected]

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List of contributing authors

Heri Satria Department of Chemistry Faculty of Mathematics and Natural Sciences The University of Lampung Bandar Lampung Lampung 35145 Indonesia

Tati Suhartati Department of Chemistry Faculty of Mathematics and Natural Sciences The University of Lampung Bandar Lampung Lampung 35145 Indonesia

Svetlana Simova Institute of Organic Chemistry with Centre of Phytochemistry Bulgarian Academy of Sciences Acad. G. Bontchev Str., bl. 9 Sofia 1113 Bulgaria

Ravishankar Rai Vittal Department of Studies in Microbiology University of Mysore Mysore, Karnataka 570006 India E-mail: [email protected]

Mutiu Sowemimo Department of Chemistry Lagos State University Ojo Lagos Nigeria E-mail: [email protected]

Yandri Yandri Department of Chemistry Faculty of Mathematics and Natural Sciences The University of Lampung Bandar Lampung Lampung 35145 Indonesia E-mail: [email protected]

Colin J. Suckling Department of Pure and Applied Chemistry University of Strathclyde Glasgow G1 1XL UK

Rumyana Zhorova Integrated Micro-Electronics BG EOOD Industrial Zone Microelectronica Botevgrad 2140 Bulgaria

Yandri Yandri*, Nurmala Nurmala, Tati Suhartati, Heri Satria and Sutopo Hadi

1 The stability increase of α-amylase enzyme from Aspergillus fumigatus using dimethyladipimidate Abstract: This study’s purpose is to improve the α-amylase enzyme’s stability from Aspergillus fumigatus applying dimethyladipimidate (DMA). It was conducted in different stages, including production, isolation, purification, modification, and the characterization of native and modified enzymes by the DMA addition. The enzyme activity was specified using the Fuwa and Mandels methods, while the protein level was conducted by the Lowry method. The results indicated that the native enzyme contains a specific activity of 7010.42 U/mg, with an increase of 7.8 times than the crude extract, which contains 904.38 U/mg. Meanwhile, the native enzyme contains an optimum pH of 5 at 55 °C, with residual activity of 17.17% after 60 min of incubation at 55 °C and a half-life of 25.86 min. After the DMA addition with 0.5, 1, and 1.5% concentration, the enzymes had 5.5 optimum pH and 65 °C temperature. Meanwhile, after 60 min of incubation at 65 °C, the modified enzymes had 54.17, 46.18, and 34.44% of residual activity, and 85.55 58.25 and 37.46 min of half-lives, respectively. This showed that the addition of DMA to the native α-amylase from A. fumigatus increased the stability of the modified enzymes by 1.5–3.3 times than the native enzyme. Keywords: α-amylase; A. fumigatus; dimethyladipimidate.

1.1 Introduction Enzymes are biocatalysts widely used in industries to convert one compound to another without harmful effects and environmental pollution. They work under physiological conditions selectively and specifically without requiring high energy [1] and can be used repeatedly through the immobilization process [2–7]. Furthermore, enzymes have unique functions and activities that are easily deactivated when the need is no longer required.

*Corresponding author: Yandri Yandri, Department of Chemistry, Faculty of Mathematics and Natural Sciences, The University of Lampung, Bandar Lampung, Lampung 35145, Indonesia, E-mail: [email protected] Nurmala Nurmala, Tati Suhartati, Heri Satria and Sutopo Hadi, Department of Chemistry, Faculty of Mathematics and Natural Sciences, The University of Lampung, Bandar Lampung, Lampung 35145, Indonesia, E-mail: [email protected] (S. Hadi) As per De Gruyter’s policy this article has previously been published in the journal Physical Sciences Reviews. Please cite as: Y. Yandri, N. Nurmala, T. Suhartati, H. Satria and S. Hadi “The stability increase of α-amylase enzyme from Aspergillus fumigatus using dimethyladipimidate” Physical Sciences Reviews [Online] 2022. DOI: 10.1515/psr-2021-0138 | https://doi.org/ 10.1515/9783110783629-001

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1 The stability increase of α-amylase enzyme

According to Sarrouh et al., enzymes are naturally used as biocatalysts in industries because they are environmentally friendly [8]. Meanwhile, one of the enzymes that play an essential role in the industry is amylase due to its large amount of starch [1, 9]. Industries need thermostable enzymes that work optimally within the temperature range of 60–125 °C and a wide pH range [10]. Therefore, it is necessary to find out stable enzymes under extreme conditions. Mozhaev and Martinek [11] stated that stabilization from mesophilic microbes is the preferred way to obtain stable enzymes due to the weakness associated with the direct isolation from thermophilic bacteria, which requires the design of bioreactors and new processing methods [12]. Mozhaev and Martinek [11] reported three stabilization methods, namely immobilization, chemical modification, and directed mutagenesis. However, some weaknesses are associated with the immobilization method, such as a decrease in binding capacity and enzyme reactivity because of mass transfer inhibition by the immobilizing matrix. Directed mutagenesis also requires full information on the primary and three-dimensional format description. Therefore, chemical modification is the preferred method for obtaining stable enzymes. According to Tananchai, chemical modification using bifunctional reagents increases enzyme stability because it forms cross-links with protein molecules [13]. Meanwhile, Elangovan stated that intermolecular and intramolecular cross-links in protein molecules maintain the stability of the enzyme’s tertiary structure [14]. One of the compounds that can be used for its formation is a homobifunctional compound such as dimethyladipimidate (DMA), which contains two reactive functional groups with equal reactivity to the side chains of the same amino acid or DNA base [15]. The chemical modification using DMA was carried out by Yandri et al. [16] on the cellulase enzyme from Bacillus subtilis ITBCCB148. The results revealed that the thermal stability was 1.4–1.6 times greater than the purified enzyme. Similarly, the α-amylase from B. subtilis ITBCCB148 was also chemically modified using dimethyladipimidate [17]. This revealed that the modified enzyme raised its stability 1.5–3.5 times than the purified, as revealed by a rise in half-life and Gi as well as a reduction in the ki value. Meanwhile, the chemical modification of the α-amylase enzyme from Aspergillus fumigatus in this study was conducted using dimethyladipimidate (DMA). The results showed an increase in the stability of the modified enzyme 1.5–3.3 times than the purified, as revealed by a rise in the half-life.

1.2 Materials and method 1.2.1 Materials and instrumentations UV–Vis spectrophotometric analysis was carried out at the Biochemistry Laboratory, Department of Chemistry, Faculty of Mathematics and Natural Sciences, University of Lampung. Moreover, the

1.3 Results and discussion

3

microorganism utilized was a local isolate of A. fumigatus from the Microbiology Laboratory of the University of Lampung, while the materials were those with a pro-analysis degree.

1.2.2 Procedure This study was conducted in several stages, namely production, isolation, purification, modification, as well as purified and modified enzymes characterization. Assay of α-amylase enzyme activity was conducted by Mandels [18], while the protein level was analyzed by the Lowry method [19]. The α-amylase enzyme was isolated from A. fumigatus and purified using the fractionation method with ammonium sulfate. The enzyme obtained was further purified using the dialysis method and modified with DMA [20]. During the modification process, the DMA solids that had been dissolved in phosphate buffer pH 8.0 were added to the 10 ml enzyme until the concentration reached 0.5, 1, and 1.5% (v/v). The solution was blended using a magnetic stirrer at room temperature for 1 h. The native and modified enzymes’ characterization contained the finding of optimum pH and temperature, enzyme kinetic data (KM and Vmax), and thermal stability tests. In addition, the determination of half-life (t1/2), inactivation rate constant (ki), and energy changes because of denaturation (ΔGi) of the modified -amylase enzyme were carried out utilizing the first-order inactivation kinetics equation [20] as follows: ln(Ei /E 0 ) = −k i t Energy changes due to denaturation (ΔGi) of the native and modified enzymes were conducted using the equation [20]: ΔGi = −RT ln (ki h/kB T) All measurements were analyzed by the Student’s t-test between the native and modified enzymes to identify the significant differences was set at p < 0.05. The null hypothesis had been accepted and there is significant difference between the native and modified enzymes measurement results.

1.3 Results and discussion 1.3.1 Production and isolation of α-amylase This study produced the α-amylase enzyme by inoculating A. fumigatus into the inoculum medium and shaken in a shaker for 24 h. The inoculum was transferred to the fermentation medium and shaken for 96 h. Inoculum and fermentation media consisted of 1.4% (NH4)2SO4, 2% KH2PO4, 0.3% urea, 0.3% MgSO4, 0.3% CaCl2, 0.005% FeSO4 7H2O, 0.0014% ZnSO4 7H2O, 0.002% CoCl2, 0.75% peptone, and 7.5% cassava starch. The enzyme was separated from other components using centrifugation at 5000 rpm for 20 min. Meanwhile, the centrifuged filtrate obtained was 2500 mL crude extract of α-amylase enzymes with a unit activity of 44.95 U/mL, the protein content of 0.025 mg/mL, and 904.38 U/mg specific activity.

4

1 The stability increase of α-amylase enzyme

1.3.2 α-Amylase purification The enzyme’s crude extract was further purified in two stages, namely precipitation using ammonium sulfate and dialysis. 1.3.2.1 Precipitation using Ammonium Sulfate Salt Enzyme precipitation was conducted by adding ammonium sulfate [(NH4)2SO4] salt in six levels of saturation, namely 0–20, 20–40, 40–60, 60–80, and 80–100%. The relationship between the ammonium sulfate percentage and the α-amylase enzyme’s specific activity from A. fumigatus is shown in Figure 1.1. Figure 1.1 shows that the α-amylase enzyme has the greatest specific activity in the 40–60% fraction, which is 1523.64 U/mg. Meanwhile, in 0–20, 20–40, 60–80, and 80–100% fractions, it has 368.71, 708.75, 667.50, and 401.27 U/mg, respectively, which indicates that the enzyme is still deposited in these fractions. Furthermore, the fractionation process was conducted in two levels of ammonium sulfate saturation, namely 0–30 and 3–95%, to increase the protein yield and activity, and prevent substantial loss. The result revealed that the α-amylase enzyme’s activity in the 30–95% fraction had a 2460.12 U/mg specific activity (Figure 1.2).

Figure 1.1: The enzyme fractions association at different ammonium sulfate saturation levels with α-amylase enzyme’s specific activity from A. fumigatus. The error bars represent standard deviations with a significant difference = p < 0.05.

5

1.3 Results and discussion

Figure 1.2: The enzyme fractions association at two levels ammonium sulfate saturation with α-amylase enzyme’s specific activity from A. Fumigatus. The error bars represent standard deviations with a significant difference = p < 0.05.

1.3.2.2 Dialysis The dialyzed α-amylase enzyme had a unit and specific activity of 33.65 U/mL and 7010.42 U/mg, respectively, and higher purity of 7.8 times greater than the crude extract. The results of the purification stage to the dialysis stage are shown in Table 1.1.

1.3.3 Characterization of native and modified enzymes 1.3.3.1 Determination of optimum pH of native and modified enzymes The optimum pH of the native α-amylase enzyme was at pH 5, while the modified enzyme with variations in DMA concentration of 0.5, 1, and 1.5% had pH 5.5. This showed that the modified enzyme had increased pH stability and a wider range than the native enzyme, which is 5.5–7, as illustrated in Figure 1.3. Table .: Schematic of α-amylase purification from A. fumigatus Stage

Crude extract Fractionation ammonium sulfate Dialysis result

Enzyme Unit Protein Specific volume (mL) activity level (mg/mL) activity (U/mg) (U/mL)  

. .

. .



.

.

Total Purity activity (U) level (times)

. ,. .. ,.

 .

..

.

.

6

1 The stability increase of α-amylase enzyme

Figure 1.3: The native and modified enzymes optimum pH. The error bars represent standard deviations with a significant difference = p < 0.05.

1.3.4 Native and modified enzymes 1.3.4.1 Determination optimum temperature Figure 1.4 reveals that the native α-amylase enzyme’s optimum temperature is 55 °C, while the modified enzyme has variations in DMA concentration of 0.5, 1, and 1.5% is 65 °C. This indicates that the modified enzyme has better resistance to higher temperatures. After modification, all modified enzymes showed a significant (p < 0.05)

Figure 1.4: The native and modified enzymes optimum temperature. The error bars represent standard deviations with a significant difference = p < 0.05.

1.3 Results and discussion

7

increase in thermal stability compared to the native enzyme which indicated by the higher of their optimum temperatures.

1.3.5 Determination of the native and modified enzymes’ thermal stability The native and modified enzymes’ thermal stability was determined through incubation at their optimal temperatures with time intervals of 0, 10, 20, 30, 40, 50, and 60 min. Meanwhile, the test curves of both enzymes are presented in Figure 1.5. This graph showed a significant (p < 0.05) increase in thermal stability for all modified enzymes compared to the native enzyme. The native enzyme’s remaining activity after incubation at 55 °C for 60 min was 17.17%, and the modified enzyme (0.5, 1, and 1.5%) at 65 °C for 60 min was 54.17, 46.18, and 34.44%.

1.3.6 Constants of thermal inactivation ate (ki), half-life (t1/2), and energy Gibbs changes due to denaturation (ΔGi) of the native and modified enzymes Changes in ki, t1/2, and ΔGi of the native and modified enzymes using DMA with a concentration of 0.5, 1, and 1.5% are displayed in Table 1.2. The stability improvement of the modified enzymes were shown in a significant (p < 0.05) decrease of ki, the increase of their half-life (t½) and the increase of ΔGi compared to the native enzyme. The stability increase of the modified enzymes after

Figure 1.5: The native and modified enzymes’ thermal stability. The error bars represent standard deviations with a significant difference = p < 0.05.

8

1 The stability increase of α-amylase enzyme

Table .: The ki, t/, and ΔGi values of native and modified enzymes. Enzyme Native DMA .% DMA % DMA .%

ki (/min)

t/ (min)

ΔGi (kJ/mol)

Stability increase

. . . .

. . . .

. . . .

 . . .

modification by DMA 0.5, 1.0, and 1.5% were respectively 3.3, 2.3, and 1.4-fold higher than the native enzyme based on the increase of each half-life (t½). Hence, the best concentration of DMA to increase the enzyme stability is 0.5%. Based on Table 1.2, the modified enzyme’s ki value is lower than the native, therefore, it is more stable. Meanwhile, the low ki value means that it is less adaptable in water because of the construction of the cross-links between the enzyme and DMA, which drives the structure to become stiffer, thereby increasing its stability. The ΔGi value reveals the total energy demanded to denature an enzyme. Table 1.2 reveals a rise in the modified enzyme ΔGi value compared to the native. It indicates that the increase in stability leads to rigid and inflexible structures. Its stiffer nature makes the enzyme structure more solid, therefore, enough energy is needed to denature the enzyme. The data in Table 1.2 showed that the modified enzyme’s half-life was raised than the purified, therefore, the longer it is, the better its stability. For the modified enzyme’s DMA, 0.5, 1, and 1.5% concentration increased its half-life from 25.86 to 85.55 min, 25.86–58.23 min, and 25.86–37.46 min, respectively. These results are almost the same as those reported by Yandri et al. [16, 17] using the same modifier, which gave a 1.4–3.5 times rise in the modified enzyme’s stability than the purified. According to Stahl [21], an increase in the half-life raises stability.

1.4 Conclusions The chemical modification of the native α-amylase from A. fumigatus was carried out using DMA. The stability of the modified enzyme has increased by 1.4–3.3 times compared to the native enzyme. The stability of the modified enzyme is shown by the increase of half-life and ΔGi and the decrease of ki value of the modified enzyme. Based on these data, the modified enzyme is more stable than the native enzyme.

References

9

References 1. Li S, Yang X, Yang S, Zhu M, Wang X. Technology prospecting on enzymes: application, marketing and engineering. Comput Struct Biotechnol J 2012;2:1–11. 2. Sirisha VL, Jain A, Jain A. Enzyme immobilization: an overview on methods, support material, and applications of immobilized enzymes. Adv Food Nutr Res 2016;79:179–211. 3. Yandri, Suhartati T, Hadi S. Immobilization of α-amylase from locale bacteria isolate Bacillus subtilis ITBCCB148 with diethylaminoethyl cellulose (DEAE-Cellulose). Mat Sci Res India 2010;7:123–8. 4. Yandri, Susanti D, Suhartati T, Hadi S. Immobilization of α-amylase from locale bacteria isolate Bacillus subtilis ITBCCB148 with carboxymethyl cellulose (CM-Cellulose). Mod Appl Sci 2012;6: 81–6. 5. Yandri, Amalia P, Suhartati T, Hadi S. Effect of immobilization towards thermal stability of αamylase isolates from locale bacteria isolate Bacillus subtilis ITBCCB148 with calcium alginate. Asian J Chem 2013;25:6897–9. 6. Yandri, Suhartati T, Yuwono SD, Qudus HI, Tiarsa ER, Hadi S. Immobilization of α-amylase from Bacillus subtilis ITBCCB148 using bentonit. Asian J Microbiol Biotechnol Environ Sci 2018;20: 487–92. 7. Yandri, Suhartati T, Satria H, Widyasmara A, Hadi S. Increasing stability of α-amylase obtained from Bacillus subtilis ITBCCB148 by immobilization with chitosan. Mediterr J Chem 2020;10:155–61. 8. Sarrouh B, Santos TM, Miyoshi A, Dias R, Azevedo V. Up-to-date insight on industrial enzymes applications and global market. J Bioprocess Biotech 2012;S4:002. 9. Yandri Y, Tiarsa ER, Suhartati, Satria H, Irawan B, Hadi S. The stability improvement of α-amylase enzyme from Aspergillus fumigatus by immobilization on a bentonite matrix. Biochem Res Int 2022;2022:3797629. 10. Vieille C, Zeikus JG. Thermozymes: identifying molecular determinant of protein structural and functional stability. Trends Biotech 1996;14:183–9. 11. Mozhaev VV, Martinek K. Structure-stability relationship in proteins: new approaches to stabilizing enzymes. Enzym Microb Technol 1984;6:50–9. 12. Janecek S. Strategies for obtaining stable enzymes. Process Biochem 1993;28:435–45. 13. Tananchai P. Stabilization of enzymes by chemical modification [Thesis]. New Zealand: Massey University; 2011. 14. Elangovan H. Chemical cross-linking protein: a review. J Pharmaceut Sci Innovat 2012;1:22–6. 15. Wong SS, Jameson DM. Chemistry of protein and nucleic acid cross-linking and conjugation. Boca Raton: Taylor & Francis. United States of America; 2012. 16. Yandri, Amalia P, Suhartati T, Hadi S. The chemical modification of cellulase obtained from Bacillus subtilis ITBCCB148 with dimethyladimipidate. Biosci Biotechnol Res Asia 2015;12:2089–93. 17. Yandri A, Apriyanti, Suhartati T, Hadi S. The increase of thermal stability of α- amylase from locale bacteria Isolate Bacillus subtilis ITBCCB148 by chemical modification with dimethyladipimidate. Biosci Biotechnol Res Asia 2010;7:713–8. 18. Mandels M, Raymond R. Measurement of saccharifying cellulose. Biotechnol Bioeng Symp 1976;6: 21–33. 19. Lowry OH, Rosebrough NJ, Farr AL, Randall RJ. Protein measurement with the folin phenol reagent. J Biol Chem 1951;193:265–75. 20. Kazan D, Ertan H, Erarslan A. Stabilization of Escherichia coli penicillin g acylase against thermal inactivation by cross-linking with dextran dialdehyde polymers. Appl Microbiol Biotechnol 1997; 48:191–7. 21. Stahl S. In: Gupta MN, editor. Thermostability of Enzymes. New Delhi: Springer-Verlag; 1999: 59–60 pp.

Yakubu Ali, Joseph O. Odey*, Giwa A and K. A Bello

2 A novel application of synthesised based squarylium dyes on nylon 6, and silk woven fabrics Abstract: Squarylium dyes were synthesized and characterized by different spectrometric techniques using FT-IR, UV-visible and GC–MS, the dyes gave molar extinction coefficient values greater than 5.2812 × 105 L mol−1 cm−1. Their fastness properties in respect to wash, light, perspiration and hot pressing on nylon 6, and silk fabrics were analyzed, effects of time, temperature, carrier concentration and pH was also investigated and reported. The dyed fabrics showed good to very good wash, light fastness, and perspiration good to very good hot pressing on nylon 6 and good to very good on silk fabric, respectively. The dye-bath exhaustion was found to be between 76 and 92% on nylon 6 and 57 and 85% on silk, respectively. The percentage exhaustion on nylon 6 was found to be very good to excellent but on silk it was found to be good to very good. These studies showed that squarylium dyes can be applied to nylon 6 and silk fabrics, but better performance was found on nylon 6 than silk fabric. Keywords: squarylium dyes; synthesis; characterization; silk; nylon.

2.1 Introduction Squarylium dyes possess a high absorption in the visible to the near-infrared region [1]. As a result of their notable absorption, planar, and conjugated structures properties, they have extensive applications in dye-sensitized applications [2], hetero-junction solar cells [3], photodynamic therapy [4], xerographic photoreceptors [5], organic transistors [6], and nonlinear optics [7]. More so, squarylium dyes show discreetly efficient fluorescence in the visible to near infrared region [8] and thus have also been used much as fluorescent probes [9]. Conversely, in many cases, the fluorescence quantum yield of squarylium dyes is unsatisfactory, even in solution, and squarylium dyes lose their fluorescence properties in the solid state [10] (Figures 2.1–2.3). Dyes generally are chemical substances which are colored and have the ability to impart its color onto a substrate from an aqueous dispersion or solution. The substrate may be a textile fiber/fabric, leather, hair, food or cosmetics. In some cases, materials to be dyed possess affinity for the dye and easily absorb the dye from their solution or

*Corresponding author: Joseph O. Odey, University of Calabar, Pure and Applied Chemistry University of Calabar, Calabar, Nigeria, E-mail: [email protected] Yakubu Ali, Giwa A and K. A Bello, Ahmadu Bello University, Polymer and Textile Engineering, Zaria, Kaduna, Nigeria As per De Gruyter’s policy this article has previously been published in the journal Physical Sciences Reviews. Please cite as: Y. Ali, J. O. Odey, G. A and K. A Bello “A novel application of synthesised based squarylium dyes on nylon 6, and silk woven fabrics” Physical Sciences Reviews [Online] 2022. DOI: 10.1515/psr-2021-0233 | https://doi.org/10.1515/9783110783629-002

12

2 A novel application of synthesised based squarylium dyes

Figure 2.1: Keto-enol tautomeric forms of squarylium dye.

Figure 2.2: Reaction scheme for the synthesis of squaric acid.

aqueous dispersion [11]. X-ray crystallographic analysis of these dyes demonstrated the special delocalized nature of the π-electron system, tautomerism occurs when the amino group is secondary [12]. Textile fibers, natural or synthetic are made up of polymer molecules as their basic units. The functional groups within the polymers retain their chemical properties and this determines the type of dyestuff for which the fiber will have affinity for. Generally, fibers with many polar functional groups (hydrophilic fiber) will have better affinity for ionic dyestuffs and those with few polar groups or no polar groups (hydrophobic fiber) will have greater affinity for nonionic dyestuff. Crystallinity tends to impart hydrophobicity to fiber even those having polar groups. This is due to the screening – off of polar functional groups with prevalent intermolecular hydrogen bonds. Also, close packing of polymer molecules leaves little or no space for dye molecule to penetrate [13].

2.1 Introduction

13

Figure 2.3: Schematic routes for the sqaurylium dye synthesis.

Considering the applications of squarylium dyes as mentioned above, to the best of our knowledge, there has been no report on squarylium dyes application on fabric. In this paper, we report the synthesis, characterization and application of squarylium dye on silk and nylon 6, fabric.

2.1.1 Results and discussion Squaric acid 97% purity, 2-amino-6-nitrobenzothiazole 97% purity, and 4-aminoacetophenone 98% purity, were purchased from Senya chemicals China. Other solvents and reagents were purchased from Nigeria.

2.1.2 Synthesis of squaric acid Squaric acid was synthesised by reacting hexachlorobutadiene (0.096 mol, 25 g) with excess morpholine (50 ml) and toluene (25 ml). The mixture was heated in an oil bath for 7 h under reflux. It is then cooled, at 60 °C followed by addition of sodium acetate/ acetic acid buffer (200 ml), and heated for 16 h. After cooling, 60 ml of sulfuric acid was added. The reaction was further refluxed for 24 h followed by filtration and washing with 50 ml water and 50 ml acetone and the final product was air dried.

14

2 A novel application of synthesised based squarylium dyes

2.1.3 General procedure for synthesis of squarylium dyes Squarylium dyes are prepared by condensation reaction of squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione) with arylamine. About 0.290 g (0.0025 mol) of squaric acid was heated under reflux in a mixture of 1-butanol (40 mL) and toluene (20 mL). Then water was distilled off azeotropically using a Dean–Stark trap. After 1 h, the appropriate (different substituted) primary aromatic amines 5 mmol (0.005 mol) was added and the reaction mixture refluxed for additional 4 h. The suspension was cooled at room temperature and the solvent removed on a rotary evaporator. The residue was crystallized from 1-butanol and the solid obtained was dried in an oven at 50 °C The same procedure used for dye A was used for all the other dyes, but different substituted primary aromatic amines were used. Where Dye B R1 = OH R2 = H, Dye C R1 = H R2 = Br, Dye D R1 = H R2 = Cl, Dye E R1 = H R2 = NO2

2.1.4 Synthesis of dye A, B, C, D and E Dye A was synthesized condensing 2-amino-6-nitrobenzothiazole using the procedure above, while dye B, C, D, and E was also synthesized using the same procedure as in dye A but using 3-aminophenol, 4-bromoaniline, 4-chloroaniline, 4-nitroaniline as different condensing component respectfully (Table 2.1).

2.1.5 Physical properties of the synthesized dyes The molar extinction coefficient, molar mass, melting point, molar mass, and the percentage yield of the synthesized dyes are shown in Table 2.2.

2.2 Discussion 2.2.1 Physical characteristics of the synthesised dye The physical characteristics of the synthesized dyes were shown in Table 2.2. Each of the different dyes synthesized possessed distinctive physical characteristics. The dyes exhibited well defined melting points. This generally shows that the values were quite high and ranged between 270–336 °C which correspond with other literature values i.e. squarylium dyes possess high melting points [13]. The difference in their melting point could be due to the difference in molecular structure of the dyes. The dye condensed with p-anisidine gave the highest melting point of 332–336 °C. The color of the dye

2.2 Discussion

15

Table .: Structures of the synthesised dyes. Dye samples

Dye structures

Dye A, prepared from -amino-nitrobenzothiazole

Dye B, prepared from -aminophenol

Dye C, prepared from -bromoaniline

Dye D, prepared from -chloroaniline

Dye E, prepared from -nitroanilne

Table .: Physical properties of the synthesised dyes. Dye

A B C D E

Molar mass (g/mol)

Colour

Melting point (°C)

Percentage yield (%)

Molar extinction coefficient (L mol− cm− × )

    

Red Yellow Yellow Yellow Yellow

– – – – –

    

. . . . .

crystals varied from yellow to red for all the dyes. However, dye A condensed with 2-amino-6-nitrobenzothiazole gave a red color while others condensed with 3aminophenol, 4-bromoaniline, 4-chloroaniline, and 4-nitroaniline, the yields ranged from 53–97%.

16

2 A novel application of synthesised based squarylium dyes

Table .: Visible absorption spectra of synthesised dyes. Dye A B C D E

λmax DMF(nm)

λmax ethanol (nm)

λmax acetone (nm)

    

    

    

2.2.2 Infra-red analysis of the synthesized dyes The basis of infra-red analysis was to ensure that the functional groups characterizing the dyes were present. As seen from the infrared spectra result Table 2.4, the dyes gave absorption peaks due to primary aromatic amine (N–H) stretching vibration at 3400–3500 cm−1, dye A gave N–O stretching vibration at 1350 ± 30, C=C stretching at 1600–1500 cm−1, aromatic C–C stretching vibration band at 1457– 1414 cm−1, C–H aromatic bending vibration at 690–900 cm−1. Dye B gave OH stretching vibration at 3580–3650 cm−1 and bending vibration at 1330–1430 cm−1, C– C stretching vibration band at 1457–1414 cm−1, C–H aromatic bending vibration at 690–900 cm−1. C=C stretching vibration at 1600–1500 cm−1. The same for dyes C. dye D give C–Cl stretching vibration band at 679 cm−1, dye E gave N–O stretching vibration at 1350 ± 30.

2.2.3 Mass spectroscopy of the synthesised dyes The elucidated structural information of the synthesized dyes was shown in Table 2.5. The identified mass spectra of dye A shows fragment with mass-to-charge (m/z) ratio of 42, 55, 69, 74, 87, 98, 126, 137, 151, and parent 194 representing M+ and the corresponding positive charge fragment of CNO+, C2HNO+, C3H3NO+, C3H8NO+, C3H7N2O+, C4H6N2O+, C5H4SNO+, C7H5SO+, C7H5NSO+, and C8H8N3SO+ was found. There was no difference between the calculated and the experimental, i.e. 194 g/mol. The mass spectral of dye B also gave mass-to-charge ratio (m/z) of 27, 41, 43, 57, 74, 94, 115, 129, 144, 186, 198, and parent 298 representing M+ and the corresponding positive charge fragments of C2H3+, C3H5+, C3H7+,C3H5O+, C5N+, C6H8N+, C9H7+, C10H9+, C9H6NO+, C10H6N2O2+, C11H7N2O2+, and C16H12N2O4+. The difference between the experimental (298 g/mol) and the calculated value (296 g/mol), may be due to isotropy. Dye C give a mass spectra of mass – to – charge ratio (m/z) of 27, 41, 55, 69, 74, 87, 98, 123, 138, 180, 222, and parent 264 representing M+ and the corresponding position charge fragments C2H3+, C3H5+,C4H7+, C5H9+, C6H2+, C6HN+, C6H12N+, C9HN+, C9N0+, C10N2O2+, C13H6N2O2+, and C16H12N2O2+. There was no difference between the experimental

A B C D E

Dye

. . . . .

Str

N–H

. . . . .

C–C

.

Str

OH

.

Ben

Table .: Analysis of the infrared spectra of the synthesised dyes.

. . . . .

Ben

C–H

. . . . .

Str . . . . .

Str

C=C

.

.

Str

N–O

.

Str

C–Cl Str

OCH

Str

C=O

2.2 Discussion

17

18

2 A novel application of synthesised based squarylium dyes

Table .: Mass spectra of the synthesised dyes (m/z). Dye Fragment A B C D

+

Experimental Calculated +

+

+

+

CNO , CHNO , CHNO , CHNO , CHNO CHNO+, CHSNO+, CHSO+, CHNSO+, CHNSO+ CH+, CH+, CH+, CHO+, CN+, CHN+, CH+, CH+ CHNO+, CHNO+, CHNO+, CHNO+ CH+, CH+, CH+, CH+, CH+, CHN+, CHN+ CHN+, CN+, CNO+, CHNO+, CHNO+ CH+, CH+, CH+, CHO+, CH+, CH+, CH+ CH+, CHN+, CHN+, CNO+, CNO+, CNO+, CNO+, CNO+, CHNO+

















(264 g/mol) and the calculated. Dye H gave a mass spectral of mass-to-charge ratio (m/z) of 27, 41, 43, 57, 74, 87, 101, 114, 128, 143, 186, 198, 221, 240, 284, 293, and parent 326 representing M+ and the positive charge fragment of C2H3+, C3H5+, C3H7+, C3H5O+, C6H2+, C7H3+, C8H5+, C9H5+, C9H6N+, C10H3N+, C13NO+, C14NO+, C14N2O2+, C15N2O2+, C20N2O+, and C20H10N2O3+. The difference between the experimental (326 g/mol) and the calculated (324 g/mol) may be due to isotropy.

2.3 Solvatochromism Solvatochromism is the effect of solvent polarity on the absorption spectra of colored organic compound. This phenomenon is as a result of solute–solvent interfaces in both the ground and excited states. Solvents used in this research as summarized in Table 2.3, are in the order of decreasing polarity of DMF, ethanol and acetone. The solvents were chosen because they have a wide difference in polarity. The results obtained reveal that, there is a general bathochromic shift with increasing solvent polarity. The wave-length of maximum absorption was highest in DMF and the least is acetone. Generally, the synthesised dyes exhibit positive solvatochromism. The bathochromic effect observed on changing to a more polar solvent such as DMF shows that the compound has high polar excited state to compare to the ground state, a polar solvent was able to stabilized this effect more at the ground state, thus lowering the transition energy [13] the higher the polarization of solvent, the extended wavelength [14].

2.3.1 Molar extinction coefficient (ε) The larger the molar absorptivity, the more probable the electronic transition. The molar extinction coefficient of the synthesized dyes is determined using the concentration of the dyes in dimethylformamide as solvent. The result obtained is shown in

19

2.3 Solvatochromism

Table 2.2. From the Result, it can be seen that the dyes possess high molar extinction coefficient and this may be attributed to the fact that it transmits more light. From the result shown, it can be deduced that majority of the synthesized dyes gave molar extinction coefficient values greater than 5.2812 × 105 L mol−1 cm−1. The higher molar extinction coefficient values of the synthesized dyes implies the probability for more electronic transition.

2.3.2 Dye exhaustion for nylon 6 and silk From the results summarized in Table 2.6 dyes A, B, C, D, E applied to nylon 6 has percentage exhaustion of 92, 80, 84, 86, 76, and 82%, respectively. Also from the same Tables 2.4–2.6 with respect to dye exhaustion on silk it shows that dyes A, B, C, D, and E have percentage exhaustion of 68, 78, 87, 85, 55, and 80%, respectively. From the results obtained it is clear that the percentage exhaustion of both nylon 6 and silk are quite different. However, nylon 6 has relatively the highest percentage exhaustion. Comparing dyes A with percentage exhaustion of 92% dye B 80%. On nylon 6 with the result obtained from silk with the same dyes A and B which gave the percentage dye exhaustion of 60, 78, and 80%. It can be seen that nylon 6 gave better percentage exhaustion of 92% with A and 68% exhaustion on silk, with the same dye. It can be concluded from the result obtained that the synthesised dyes are better on nylon 6 than silk fabric. This may be attributed to the amorphous nature of nylon 6 when compared with silk. Also silk been highly crystalline than nylon 6 which could reduce affinity. Crystallinity also tends to impart hydrophobicity to the fiber even those having polar groups. Nylon 6 is semi-crystalline fiber with amorphous regions for dye molecule to penetrate. However effective dye sites are screened off from the dye molecules in crystalline region [15]. Also, the close packing of polymer molecules leaves little or no space for dye molecules to penetrate [16]. Very low percentage dye exhaustion was achieved on nylon 6 and silk with dyes C, D, and H, this may be attributed to the presence in the structure of the dyes electron withdrawing groups such as Cl, Br, and OCH3. In dyes C, D, and E, respectively, the electron withdrawing groups tend to make the dye structure to be unstable by electron withdrawing. Nylon 6 and silk been hydrophobic requires a low molecular weight dyes. Comparing dye A which is a Table .: Dye exhaustion on nylon  and polyester fabrics. Dye A B C D E

% Exhaustion on nylon 

% Exhaustion on polyester

    

    

20

2 A novel application of synthesised based squarylium dyes

heterocyclic group from 2-amino-6-nitrobenzothiazole is more stable than dyes C, D, and E from benzene ring. Although dye E contains oxygen in the structure, but OCH3 been more effective withdraws electrons from the dye structure thereby making the dye structure to be unstable. Hence the presence of Cl, Br, and OCH3 groups makes the heterocyclic ring electron deficient by electron withdrawing [16].

2.3.3 Effect of pH The effect of pH on nylon 6 was carried out, for each 0.5 g fabric sample, the same dye concentration was prepared, 2% shade, time of dyeing 1 h at 100 °C. But pH of dye bath was varied as follows pH 2, pH 3, pH 4, pH 5, pH 6, pH 8, pH 9, pH 10, pH 11, and pH 12, respectively. It was generally observed that as pH of dye-bath was increased, exhaustion also increase in the acidic medium from pH 3, pH 4, pH 5, and pH 6. Dye uptake was observed to be best for nylon 6 at pH 6. For dyes A, B, C, D, and E giving exhaustion of 92, 80, 84, 86, and 82%, respectively. In alkaline medium it was observed that the exhaustion was decrease as the pH was increased to pH 12, giving percentage exhaustion of 57, 62, 55, 68, and 73%, respectively. The low dye exhaustion observed in the alkaline medium may be due to hydrolysis, because further increase in pH lowered the color strength due to hydrolysis [17]. It can be deduced that pH 6 is the best pH for dyeing nylon 6 fabric.

2.3.4 Effect of time From the result obtained in Table 2.15, it was observed that as the time of dyeing increased the percentage exhaustion also increase for both nylon 6 and silk fabrics, exhaustion was low at shorter time of dyeing this may be attributed to the assumption that at shorter time of dyeing, exhaustion is fairly achieved but as the time increase more dyes diffuse into the fiber (Table 2.14). With time the molecules of the dye are transferred from solution to the surface of the fabric and adsorbed dye diffused mono molecularly into the fabric to form solid solution. This observation may be attributed to the molecular size of the dyes as well as rate of diffusion of the dye [18].

2.3.5 Effect of carrier concentration The squarylium dyes were applied to silk, the result obtained are for the effect of varying carrier concentrations keeping other condition constant and are given in Table 2.17. Carrier concentrations were varied from 0.25–1.25 g/L. It was generally observed that as carrier concentration increase, dye exhaustion also increases. The highest exhaustion were obtained at carrier concentration of 1.25 g/L with dyes A, B, C, D, and E, giving exhausting of 61, 67, 75, 91, 88, 64, and 82%, respectively, at 1 g/L the

2.3 Solvatochromism

21

exhaustion obtained using the same dyes were 59, 65, 72, 82, 81, 57, and 80%, respectively. It can be deduced that from the result obtained carrier concentration of 1.25 g/L gave the best dye exhaustion. The least exhaustions were obtained at the lowest carrier concentration 0.25 g/L, i.e. 41, 47, 25, 52, 64, 70, 41, 23, and 57%. Silks are highly hydrophobic with a very compact structure and are highly crystalline [19] and possess high glass transition temperature (70–80 °C) therefore, phenol as carrier, to enhance the dye penetration of the synthesized dyes with silk, carriers function by facilitating dye diffusion, opening the fiber pores and increasing segments mobility of silk fiber. Hence, at higher carrier concentration, dye exhaustion is also higher.

2.3.6 Effect of temperature On raising the temperature of the dye-liquor bath, the more agitation of the dye molecules will be experienced, hence, rapid adsorption on the textile fiber. Diffusion is directly proportional to thermodynamic temperature [20]. Result obtained from studying effects of varying temperature on percentage exhaustion is shown in Tables 2.16 and 2.18. The temperature was varied from 40 to 100 °C for both nylon 6 and silk, it was generally observed that as the temperature of the dye bath was increased, migration improved, adequate leveling, and penetration also occurred, thereby increasing the percentage exhaustion. Dye uptake was observed to be best on nylon 6 at 100 °C for dye A, B, and E, with other factors kept constant, giving exhaustion of 85, 84, 88, 94, 60, and 87%, respectively (Table 2.19). On silk using the same dyes the exhaustion obtained were 68, 78, 87, 85, 76, and 80% on dyes A, B, and E, respectively. It was also observed that dye uptake was also best at 100 °C for silk. At 40 °C the exhaustion was found to be 42, 45, 57, and 48% for dyes A and E, respectively. It can be seen that exhaustion was low at 40 °C for nylon 6, when compared to silk at 40 °C. The exhaustion were 40, 42, 52, 46, and 45% for dyes A, B, E, on silk fabric, high percentage exhaustion was obtained on nylon 6, at 40 °C low percentage exhaustion was observed. Temperature in general increase the rate of any chemical reaction, the same thing occurs with dye but also with the water that the dye is dissolved in. It was observed that the highest absorbance was obtained at 100 °C for both the nylon 6 and silk fibers. At low temperature, the absorption is lower because the dye was not in the fiber, but only in the water when it gets hot.

2.3.7 Wash fastness The results were summarized in Tables 2.7 and 2.8. It was clearly shown that almost all the synthesized dyes gave a very good washing fastness on both nylon 6 and silk. All of the dyes on nylon 6 gave fastness rating of four for change in color and four for staining of adjacent fabric, while silk gave a fastness rating of 5 and 5 for staining of adjacent

22

2 A novel application of synthesised based squarylium dyes

fabric. The dyes tend to have better wash fastness on silk when compared to nylon 6. The wash fastness on nylon 6 was good while that of silk was very good. The wash fastness property on nylon 6 and silk dyed fabrics were generally good to very good. The very good fastness property on silk dyed fabrics may be attributed to the crystalline nature of silk. Which makes it difficult for the dye to diffuse into the fiber, and once in, it is difficult for the dye molecules to diffuse out thereby giving dyeing of high fastness to washing. Also, the good to very good wash fastness observed on both nylon 6 and silk on dyes A, B, and E, may be attributed to the nature of bond (hydrogen bond) that bind the dye and the substrates. Also presence of substituent enhances the results seen.

2.3.8 Light fastness The results of light fastness are summarized in Table 2.9. It was observed that the light fastness of the nylon 6 dyed fabrics were higher compared with the silk dyed fabrics. The dyed nylon 6 fabric gave a fastness rating of 6 i.e. very good light fastness, while that of silk gave rating of 5 i.e. good, this could be as a result of the molecular architecture of the compound and its substituents which provides protection from the sunlight to its nitro chromophores group and amino auxochrome [21].

Table .: Wash fastness of dyes A–E on nylon  using ISO  standard. Samples Dyed nylon  fabrics A B C D E

Wash fastness Change in color

Staining on cotton

    

    

Table .: Wash fastness of dyes A–E on polyester using ISO  standard. Samples Dyed polyester fabrics A B C D E

Wash fastness Change in color

Staining on cotton

    

    

23

2.4 Fastness to hot pressing

Table .: Light fastness of dyes A–E on nylon  and polyester. Dyed samples

Rating on nylon 

Rating on polyester

    

    

A B C D E

2.3.9 Perspiration fastness Both alkaline and acid perspiration test were evaluated and the results of the perspiration fastness test (Tables 2.12 and 2.13) shows that both acidic and alkaline perspiration were good to very good. Almost all the dyes gave a rating of 4–5 for change in color and staining of adjacent fabric. The good perspiration fastness of the dyes may be due to the state of the dyes in the fabrics being in form of insoluble particle which resist attack of the chemicals in aqueous solution. From the results obtained it can be found that the rating in the acidic condition was slightly higher than in the alkaline condition, for both nylon 6 and silk fabrics.

2.4 Fastness to hot pressing The hot press fastness nylon 6, and silk was also tested according to ISO and the results are shown in Tables 2.10 and 2.11. From the results obtained, it was observed that both nylon 6 and silk gave a very good to excellent fastness to pressing. The results it shows that the synthesized dyes had a very good pressing fastness, rating of 4–5. The heat fastness of dyeing of equal dye concentration on the same substrate will, at a specific Table .: Fastness to hot pressing of dyes A–E on polyester. Samples Polyester A B C D E

Wet

Dry

Damp

Color change

Staining on cotton

Color change

Staining on cotton

Color change

Staining on cotton

    

    

    

    

    

    

24

2 A novel application of synthesised based squarylium dyes

Table .: Fastness to hot pressing of dyes A–E on nylon . Sample

Wet

Nylon 

Damp

Dry

Color change

Staining on cotton

Color change

Staining on cotton

Color change

Staining on cotton

    

    

    

    

    

    

A B C D E

Table .: Fastness to perspiration of dyes A–E on polyester. Polyester

Acidic condition

Dye

Alkaline ccondition

Color change

Staining on cotton

Color change

Staining on cotton

    

    

    

    

A B C D E

Table .: Fastness to perspiration of dyes A–E on nylon  fabric. Nylon 

Acidic condition

Dye

Alkaline condition

Color change

Staining on cotton

Color change

Staining on cotton

    

    

    

    

A B C D E

Table .: Effect of dye bath pH on % exhaustion on nylon  using dyes A–E. Dyes A B C D E

pH 

pH 

pH 

pH 

pH 

pH 

pH 

pH 

pH 

pH 

    

    

    

    

    

    

    

    

    

    

2.4 Fastness to hot pressing

25

Table .: Effect of time on dyeing on nylon % exhaustion using dyes A–E. Dyes

 min

 min

 min

 min

 min

 min

    

    

    

    

    

    

A B C D E

Table .: Effect of temperature on % exhaustion of nylon  using dyes A–E. Dyes A B C D E

 °C

 °CT

 °C

 °C

 °C

 °C

 °C

    

    

    

    

    

    

    

Table .: Effect of carrier concentration on % exhaustion of silk using dyes A–E. Dyes

. g/L

. g/L

. g/L

 g/L

. g/L

    

    

    

    

    

A B C D E

Table .: Effect of temperature on % exhaustion of silk using dyes A–E. Dyes A B C D E

 °C

 °C

 °C

 °C

 °C

 °C

 °C

    

    

    

    

    

    

    

temperature and time, be dependent on the size and polarity of the molecules of the dye within the substrate and the volatility of the molecules of the dyes involved which in tend determines the rate of diffusion of the dye within the substance and the volatility

26

2 A novel application of synthesised based squarylium dyes

Table .: Effect of time on % exhaustion of silk using dyes A–E. Dyes A B C D E

 min

 min

 min

 min

 min

 min

    

    

    

    

    

    

of the dye [22]. Generally the hot press fastness of the synthesized dyes on nylon 6 and silk gave a very good to excellent result. This may be due to the chemical structure of the fabric as well as the chemical structure of the dyes [23].

References 1. Yagi S, Nakazumi H. Squarylium dyes and related compounds. In: Heterocyclic Polymethine Dyes. Berlin, Heidelberg: Springer; 2008. 133–81 pp. 2. Nesheli FG, Tajbakhsh M, Hosseinzadeh B, Hosseinzadeh R. Design, synthesis and photophysical analysis of new unsymmetrical carbazole-based dyes for dye-sensitized solar cells. J Photochem Photobiol Chem 2020;397:112521. 3. Kubota Y, Nakazawa M, Lee J, Naoi R, Tachikawa M, Inuzuka T, et al. Synthesis of near-infrared absorbing and fluorescent bis (pyrrol-2-yl) squaraines and their halochromic properties. Org Chem Front 2021. https://doi.org/10.1039/d1qo01169c. 4. Pontremoli C, Moran Plata MJ, Dereje DM, Sansone E, Chinigò G, Fiorio Pla A, et al. The effect of substitutions on cyanine dyes on their photodynamic activity. In: XXXII annual congress of the Italian society of photobiology; 2021. p. 1. 5. Hyodo Y, Nakazumi H, Yagi S, Nakai K. Synthesis of bisquaraine dyes. Novel homologues of 1, 2-squaraines bearing symmetrical and unsymmetrical structures. J Chem Soc, Perkin Trans 1 2001; 1:2823–30. 6. Gsänger M, Kirchner E, Stolte M, Burschka C, Stepanenko V, Pflaum J, et al. High-performance organic thin-film transistors of J-stacked squaraine dyes. J Am Chem Soc 2014;136:2351–62. 7. Prabhakar C, Bhanuprakash K, Rao VJ, Balamuralikrishna M, Rao DN. Third order nonlinear optical properties of squaraine dyes having absorption below 500 nm: a combined experimental and theoretical investigation of closed shell oxyallyl derivatives. J Phys Chem C 2010;114:6077–89. 8. Mayerhöffer U, Fimmel B, Würthner F. Bright near-infrared fluorophores based on squaraines by unexpected halogen effects. Angew Chem Int Ed 2012;51:164–7. 9. Prabhakar C, Bhanuprakash K, Rao VJ, Balamuralikrishna M, Rao DN. Third order nonlinear optical properties of squaraine dyes having absorption below 500 nm: a combined experimental and theoretical investigation of closed shell oxyallyl derivatives. J Phys Chem C 2010;114:6077–89. 10. Matsui M, Fukushima M, Kubota Y, Funabiki K, Shiro M. Solid-state fluorescence of squarylium dyes. Tetrahedron 2012;68:1931–5. 11. Soo-Youl P, Kun J, Oh S-W. The novel functional chromophores based on sequarylium dyes. Bull Korean Chem Soc 2005;26:430–8. 12. Peter RH, Ingamells W. Methods for fibre identification. J Soc Dye Colour 1960;3:76.

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13. Yuanwei Z. Squaraine dyes design and synthesis of various functional materials applications. Orland Florida: Elsevier science Limited; 2008. p. pp143–167. 14. Marini A, Monoz-Losa A, Biancard A, Mennueci B. Salvatochromism. J Phys Chem 2010;114. https://doi.org/10.1021/jp1097487. 15. Trotman ER. Dyeing and chemical technology of textile fibres, 5th ed London: Griffin; 1975: 17–23 pp. 16. Obu QS, Louis H, Odey JO, Eko IJ, Abdullahi S, Ntui TN, et al. Synthesis, spectra (FT-IR, NMR) investigations, DFT study, in silico ADMET and Molecular docking analysis of 2-amino-4(4-aminophenyl) thiophene-3-carbonitrile as a potential anti-tubercular agent. J Mol Struct 2021; 1244:130880. 17. Agnew-Heard KA. Spectroscopic measurements of noncovalently labeled human serum albumin with near-infrared dyes. Georgia State University; 2002. 18. Ahmad AI. Reactive dyes development: a review of ― textile dyer and printer. J Soc Dye Colour 2012; 28:19–24. 19. Bello KA, Ibrahim S, Yakubu MK. Kinetics and thermodynamic studies of disperse dyes derived from 4-amino-3-nitrobenzaldehyde on silk fabrics. Int J Res Sci 2016;2:11–5. 20. Radei S, Carrión-Fité FJ, Ardanuy M, Canal JM. Thermodynamic and kinetic parameters of silk dyeing with Disperse Blue 56 using bio-based auxiliaries and co-solvent microemulsion. Textil Res J 2020;90:523–36. 21. Louis H, Onyebuenyi IB, Odey JO, Igbalagh AT, Mbonu MT, Eno EA, et al. Synthesis, characterization, and theoretical studies of the photovoltaic properties of novel reactive azonitrobenzaldehyde derivatives. RSC Adv 2021;11:28433–46. 22. Odey JO, Louis H, Agwupuye JA, Moshood YL, Bisong EA, Brown OI. Experimental and theoretical studies of the electrochemical properties of mono azo dyes derived from 2-nitroso-1-naphthol, 1nitroso-2-naphthol, and CI disperse yellow 56 commercial dye in dye-sensitized solar cell. J Mol Struct 2021;1241:130615. 23. Sharma KK, Chowdhury AR, Srivastava S. Chemistry and applications of lac and its by-product. Nat Mater Prod Insects: Chem Appl 2020:21. https://doi.org/10.1007/978-3-030-36610-0_2.

Supplementary Material: The online version of this article offers supplementary material (https://doi. org/10.1515/psr-2021-0233).

Siphumelele Thandokwazi Mkhondwane and Viswanadha Srirama Rajasekhar Pullabhotla*

3 Cyclohexane oxidation using advanced oxidation processes with metals and metal oxides as catalysts: a review Abstract: Selective oxidation of cyclohexane has gained substantial interest in the field of research due to the prominence of its products in industrial processes. Particularly, advanced oxidation processes (AOPs) constitute a positive technology for the oxidation of cyclohexane owing to their high oxidation potentials and environmental benign properties. This review entails to address the progress made in advanced oxidation of cyclohexane over nanostructured metals and metal oxides catalysts. The main focus is directed toward the photocatalysis, Fenton oxidation and ozonation as advanced oxidation processes. Mainly, the fundamental principles, prime factors of the AOPs in conjunction with metal and metal oxide catalysts and the mechanistic insight toward the oxidation of cyclohexane are highlighted. The affirmative effects of the metals and metal oxide catalysts mainly focusing on particle size, structure and elemental composition is stressed. Lastly, the advantages and disadvantages of the AOPs and the strategic approaches to counter the disadvantages are also clearly elucidated. Keywords: AOPs; catalyst; cyclohexane oxidation; metal oxides and ozonation.

3.1 Introduction The development of the efficient oxidation process for the hydrocarbon’s remains an enormous challenge in modern chemistry [1–4]. This can be attributed to high stability of these compounds at ambient conditions constitutes due to absence of an empty orbital in their lower energy level which can be stimulated to readily react with other molecules [5]. One such process is the oxidation of cyclohexane to cyclohexanol and cyclohexanone famously known as KA oil. Cyclohexanone and cyclohexanol are prominent feedstock for the production of adipic acid and caprolactum respectively. Adipic acid and caprolactum are imperative chemicals that are associated on the income generating processes. Although the traditional production of adipic acid and

*Corresponding author: Viswanadha Srirama Rajasekhar Pullabhotla, Department of Chemistry, University of Zululand, Private Bag X1001, Kwa-Dlangezwa, 3886, South Africa, E-mail: [email protected]. https://orcid.org/0000-0002-0093-460X Siphumelele Thandokwazi Mkhondwane, Department of Chemistry, University of Zululand, Private Bag X1001, Kwa-Dlangezwa, 3886, South Africa As per De Gruyter’s policy this article has previously been published in the journal Physical Sciences Reviews. Please cite as: S. T. Mkhondwane and V. S. R. Pullabhotla “Cyclohexane oxidation using advanced oxidation processes with metals and metal oxides as catalysts: a review” Physical Sciences Reviews [Online] 2022. DOI: 10.1515/psr-2021-0146 | https://doi.org/10.1515/ 9783110783629-003

30

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

caprolactum were solely for the manufacturing of polyamide 6,6 and polyamide 6 respectively [6–8]. However, the recent growth in their production capacity can be ascribed to the steady increase in the demand for polyurethane and polyester resins. For example, the commercial production of adipic acid at global scale was estimated to be about one million tons per annum in 1997, whereas about 1.3 million tons were reported in 2010 [9]. Therefore, it is more likely that the current figure is doubled. Scheme 3.1 presents the oxidation of cyclohexane to produce KA oil. In general, the mechanism of the cyclohexane oxidation phenomenon has been outlined to transpire via hydrogen abstraction in which hydrogen atom is abstracted from cyclohexane resulting in the formation of cyclohexyl radical in a reaction that is slightly exothermic (ΔH) but overall endergonic (ΔG) in reverence to the starting material [10]. This transpires by numerous number of events such as (i) H abstraction by an oxidant species, (ii) C–H bond cleavage by unsaturated metal centre, (iii) H abstraction by radical species present in reaction solution and (iv) H abstraction by O2 species bound to a metal centre. Each of these events liberate a carbon centred parent radical (cyclohexyl radical) [11] (Eq. (3.1)). The carbon centred parent subsequently reacts with oxygen from an oxidant (in this case oxygen is used as an example) to produce cyclohexayl peroxy radical (Eq. (3.2)). The reaction outlined in Eq. (3.2) does not require any practical energy and is accompanied by great rate of consistence from one cyclohexyl radical to another [12]. The cyclohexyl peroxy radical further reacts with another cyclohexane molecule to liberate cyclohexyl hydrogen peroxide (CHHP) and cyclohexyl radical through Eq. (3.3). However, this reaction requires the activation of the C–H bond of the cyclohexane. Therefore, few kcal/mole of activation energy are required [12, 13]. The formation of the cyclohexanol encompasses the subsequent cleavage of the cyclohexyl hydrogen peroxide to cyclohexyl monoxide and hydroxide radicals (Eq. (3.4)). Cyclohexyl monoxide radical subsequently abstract hydrogen from cyclohexane to produce cyclohexanol and cyclohexyl radical through Eq. (3.5). In contrast, the formation of the cyclohexanone encompasses the reaction that liberate cyclohexyl hydrogen peroxide and cyclohexyl hydrogen peroxide radical from cyclohexyl hydrogen peroxide and cyclohexyl peroxy radical (Eq. (3.6)). Alternatively, cyclohexyl hydrogen peroxide radical decomposes to form cyclohexanone and hydroxide radical (Eq. (3.7)). In principle, cyclohexanol and

Scheme 3.1: Oxidation of cyclohexane to KA oil.

3.2 AOPs for cyclohexane oxidation

31

cyclohexanone is also formed from decomposition of the two cyclohexyl peroxy radicals [10–14] (Eq. (3.8)). C6 H12 → C6H•11

(3.1)

C6 H•11 + O2 → C6 H11 − OO•

(3.2)

C6 H11 − OO• + C6 H12 → C6 H•11 + C6 H11 − OOH

(3.3)

C6 H11 − OOH → C6 H11 − O• + •OH

(3.4)

C6 H11 − O• +C6 H12 → C6 H11 − OH + C6 H•11

(3.5)

C6 H11 − OOH + C6 H11 − OO• → C6 H10 (•) − OOH + C6 H11 − OOH

(3.6)

C6 H10 (•) − OOH → C6 H10 = O + •OH

(3.7)

2C6 H11 − OO• → C6 H10 = O + C6 H11 − OH + O2

(3.8)

Commercially, the oxidation of cyclohexane is exploited at 150–160 °C and 1–2 MPa of temperature and pressure respectively using cobalt based homogenous catalysts. However, the production of the enormous amount of the unwanted reaction products (succinic and glutaric acids) in conjunction with the use homogenous catalysts are major crisis of the current commercial process. As a result, there is trademark compromise between the percentage conversion and selectivity. Typically, the percentage conversion is kept at 3–4% to obtain 70–83% selectivity toward KA oil [7, 10, 14]. For sustainable development of human society, the establishment of the highly efficient and environmentally viable cyclohexane oxidation process which can be executed at ambient reaction constitutes is a topic of the modern research. Among various approaches underway, advanced oxidation processes (AOPs) have unveiled their judicious potential as the promising podium toward the solution [15–17]. For this reason, after a brief introduction on the economic prominence of the cyclohexane oxidation products and the stumbling blocks of the current commercial production process of these products, we focus on summarizing the in-depth exploration of the AOPs as the leading approach for optimizing the cyclohexane oxidation efficiency. The fundamental principles of the AOPs and their effects on cyclohexane oxidation are clearly articulated by inference from the literature reports.

3.2 AOPs for cyclohexane oxidation AOPs encompasses the rapid regeneration of hydroxyl (•OH) radical species. Owing to the comparatively higher oxidation potential than its contenders, •OH provide effective oxidative productivity when compared to conventional oxidants such as oxygen (O2), ozone (O3) and hydrogen peroxide (H2O2). As a result, the past decades have

32

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

witnessed a prodigious deal of research in to AOPs for degradation of pollutants, preferential and total oxidation of hydrocarbons. Of particular interest ozonation, Fenton and photocatalytic advanced oxidation processes have been studied to a great extent [18–23].

3.2.1 Photocatalysis Generally, the photocatalysis process entails the combination of both catalysis and photochemistry, implying that catalyst and light irradiation are capable to accelerate the redox reaction [17, 20, 24–27]. Currently, a wide variety of semiconductive transition metals and metal oxides have been extensively studied in photocatalytic oxidation processes, focusing in shape such as crystallinity and anisotropy [20], size [28] and composition dependent catalytic performance [29, 30]. The semiconductor photocatalysis phenomenon encompasses seven fundamental steps [31] (shown in Figure 3.1): (i) semiconductor absorption of the photon which results to the formation of the charge carries (electrons and holes), (ii) Recombination of the photo generated charge carriers, (iii) trapping of electrons in the conduction band at surface cation sites, (iv) trapping of the valence band holes at surficial M−OH sites, (v) Instigation of the oxidative corridors by valence band hole at surficial M−OH sites, (vi) Instigation of the reductive corridors by a conduction band electrons and (vii) further photocatalytic and thermal reactions to produce mineralization products. Step (v) is the rate limiting step and can be further alienated in to five independent steps which are: (1) Mass transfer of the reactants, (2) Surface adsorption of the reactants on to the photon activated semiconductor, (3)

Figure 3.1: Seven fundamental steps of photocatalysis [31].

3.2 AOPs for cyclohexane oxidation

33

Photocatalytic oxidation of the adsorbate by the valence band hole, (4) Desorption of the reaction intermediate from the surface of the semiconductor material, (5) intermediates mass transfer from the interface region to the bulk fluid [26, 31]. In general, the rate of the photocatalytic reaction is equivalent to the slowest essential step of the chemical reaction. As the mass transfer steps (step 1 and 5) are generally faster than the reaction steps (step 2, 3 and 4), the particles in the active site vicinity are non-divergent from those in the bulk fluid phase. In this respect, the mass transfer phenomena do not disturb the photocatalytic rate of the reaction. In this manner, the rate of the photocatalytic reaction is determined by steps 2, 3 and 4 [26, 31, 32]. In this essence the adsorption-reaction-desorption interfaces between the surface of the catalyst and the molecules during the steps 2, 3 and 4 is highly important [33]. Generally, the oxidation aptitude of the photocatalytic processes is relatively far smaller than most of the photogenerated charge carriers due to the recombination of the electrons and holes which renders the process efficiency. However, a lot of tremendous ground work have been explored in the past to increase the photonic efficiency by inhibiting the recombination of the photogenerated charge carrier [18–49]. The recombination of the photogenerated e−/h+ pairs transpires within nanoseconds after generation. However, some photogenerated electrons and holes become trapped at surface state of the semiconductor to form eshallow−/hshallow+/hdeep+ within femtoseconds after generation for later use [26]. Accordingly, the decrease in particle size of the semiconductor material have been reported to effectively enhance the photocatalytic process by subsequent increase in surface area which results to an enhanced separation of the photogenerated charge carriers by surface state trapping

Figure 3.2: Schematic presentation of the important roles of the facets in the surface design of photocatalytic materials (A) adsorption and activation of reactant molecules on different facets, (B) redox abilities of photogenerated charge carriers tuned by the surface electronic band structures of different facets, (C) accumulations of photogenerated electrons or holes on different facets [35].

34

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

phenomenon. In this manner, lifetime of the photogenerated excitons in the reaction mixture is prolonged [26, 31]. The size dependent catalytic efficiency can be envisaged from the general aspect of surface-to-volume ratio for surface catalyzed reactions. In this manner, the decrease in particle size of the catalyst results to an increase in surface area. Subsequently, the increase in surface area increases the number of active surface sites which enhances the catalytic reaction [34, 35]. Moreover, once the size of a semiconductor falls underneath the critical radius, then the photogenerated charge carriers start to act quantum mechanically confined and the charge quarantine results to a sequence of discrete electronic states. In this manner, it is likely to improve the oxidation and reduction pathways of the valence band holes and the conduction band electrons respectively [42]. Furthermore, cation/anion doping has also been adopted as another podium to minimize electron-hole recombination owing to the prominent competence of the dopant molecules to trap the photogenerated charge carriers [44, 45]. Recently, numerous studies have appointed the nature of the exposed facets (surface planes) to also provide a fresh venture for fabrication of highly effective photocatalysts through several mechanisms. These mechanisms are presented in Figure 3.2 [35]. Typically, (1) the arrangement of the surface atoms regulates the surface adsorption and activation phenomena of the reacting molecules, and therefore affecting the activity and percentage selectivity toward the desired products (Figure 3.2A). (2) The charge carriers are generated from the surface electronic states, the redox abilities of the photogenerated charge carriers vary with the type surface atomic configuration (facets) (Figure 3.2B). (3) The exposure of multiple facets leads to the formation of hybrid electronic structures that result to the construction of the exceptional surface heterojunction which separates the charge carries (Figure 3.2C) [45]. These considerations are prolific when tailoring the surface atomic arrangement of the single crystal toward specific facet exposure configurations. Accordingly, the semiconductor particle size reduction, confining the nature of the exposed facets and the cation/anion doping have been persuade as leading strategies to counter the recombination of the photogenerated charge carriers [26–49].

3.2.2 Photocatalysis in oxidation of cyclohexane The photocatalytic oxidation of cyclohexane has been well explored to a great extent using various metals and metal oxides catalysts [50–57]. Most of the effort has been directed toward the influence of the particle size [53], shape [54, 55] anion/cation doping [53, 54] and surface atomic arrangement of the catalysts [56] on solar energy harvesting ability and the separation of the photogenerated charge carriers. For example, Kang and co-workers investigated the catalytic efficiency of the Au, Ag and Cu supported carbon quantum dot composites (Au/CQDs, Ag/CQDs and Cu/CQDs respectively) for photocatalytic oxidation of cyclohexane using H2O2 under visible light irradiation [57]. The Au/CQDs catalyst exhibited higher catalytic efficiency than blank

3.2 AOPs for cyclohexane oxidation

35

Figure 3.3: (a) Cyclohexane adsorption-desorption isotherm of the Au/CQDs catalyst, (b) cyclic voltammograms of bare glassy carbon electrode and Au/CQDs-modified glassy carbon electrode, Scan rate = 25 mV/s, A 450-W of xenon lamp utilized as the light source with a visible-CUT filter to cut off light with a wavelength of λ < 420 nm, (c) ESR signals of the DMPO–•OH adducts for Au/CQDs–H2O2 system under visible light irradiation, (d) PL spectra (485 nm excitation) of CQDs, AuNPs and Au/CQDs [57].

oxidation and bare CQDs with percentage conversion of 63.8% and selectivity of 99.9%. This has persuaded the authors to probe the origin of catalytic efficiency by exploring the physicochemical properties of the catalysts. Accordingly, the cyclohexane adsorption efficiency, electrocatalytic H2O2 decomposition capability to form • OH radicals and photogenerated charge carriers’ separation efficiency of the Au/CQDs catalyst were investigated. Figure 3.3A presents the adsorption-desorption isotherm of the Au/CQDs catalyst. Typically, Au/CQDs exhibited type-IV curve characteristics, implying virtuous cyclohexane adsorption ability. The Au/CQDs electrocatalytic aptitude to decompose H2O2 was investigated using a three-electrode electrochemical cell (Figure 3.3B). The pure glassy carbon electrode exhibited no electrochemical retort, Whereas, Au/CQDsimproved glass carbon electrode displayed apparent H2O2 decomposition response (onset potential at −0.6 V). On the other hand, visible light irradiation further enhanced the H2O2 decomposition efficiency (onset potential at −0.2 V). To further gain an insight on the photocatalytic efficiency of the catalysts the spin-trapping electron spin resonance was adopted to elucidate the effect of the Au/CQDs catalyst on generation of the

36

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

highly reactive oxygen species (Figure 3.3C). Typically, DMPO–•OH adducts were observed upon visible light irradiation of the Au/CQDs–H2O2 mixture, implying that the •OH radical is the dominant specie responsible for high photocatalytic efficiency of the Au/CQDs–H2O2 reaction system under light irradiation. However, EPR can only give insight on the concentration of the radical species present in the reaction solution but cannot impart information about the lifetime of the photogenerated charge carriers. For this reason, photoluminescence spectroscopy (PL) was carried out to probe the bulk defects sites of the Au/CQDs catalyst (Figure 3.3D). Generally, defect sites can trap the photons from the light source with energy equivalent or great than their band gap energy. Subsequently, the photons are then captured by the metal or metal oxide semiconductor material and promote the excitation of the electrons from the valence band to conduction band generating the holes in the valence band. The photogenerated electron-hole pair is then subjected to recombination, specifically on defect sites centers to produce photoluminescence emission spectra [58]. Therefore, the higher photoluminescence emission spectrum implies higher recombination rate of the photo-excitons (electrons and holes). Typically, the PL spectra of the CQDs, AuNPs and Au/CQDs catalyst indicate that the AuNPs and CQDs in Au/CQDs catalysts have synergic effect in separating the photogenerated charge carriers, since the PL spectrum intensity of Au/CQDs catalyst is significantly weaker than that of bare CQDs and AuNPs. In another study, Henriquez and co-workers studied the catalytic efficiency of the (1 1 0)/(0 0 1) bismuth oxyiodide (BiOI) facet ratio (BiOI-4, BiOI-6, BiOI-10, and BiOI-12) in photocatalytic oxidation of cyclohexane emulsions of water under visible light irradiation using O2 [59]. The BiOI catalysts were highly reactive for photocatalytic oxidation of cyclohexane. Nonetheless, product selectivity toward cyclohexanol and cyclohexanone was highly dependent on the (1 1 0)/(0 0 1) facets ratio. Typically, higher cyclohexanol concentration were obtained from high (1 1 0)/(0 0 1) facets ratio, whereas higher cyclohexanone concentrations were obtained from low (1 1 0)/(0 0 1) facets ratio. Based on these results, the mechanistic insight toward the formation of the products was investigated. Classically, the band gap of a semiconductor material is one of the enormous properties which plays profound role in enhancing the photocatalytic efficiency. For example, a semiconductor material with low band gap exhibit tremendous photocatalytic properties in comparison to a semiconductor characterized with high band gap [60–62]. For this reason, UV–vis reflectance spectroscopy was utilized to counter the effect of the band gap in catalytic activity differences of the synthesized BiOI-4, BiOI-6, BiOI-10, and BiOI-12 catalysts with varying (1 1 0)/(0 0 1) facets ratio. The calculated band gaps using Kubelka–Munk function were 1.85, 1.90, 1.91, and 1.90 for BiOI-4, BiOI-6, BiOI-10, and BiOI-12 respectively. Considering the marginal differences in the calculated band gap values, therefore the major differences in the catalytic efficiencies of the synthesized catalysts could be index to other factors. For this reason, in situ electron paramagnetic resonance (EPR) was adopted as the major characterization

3.2 AOPs for cyclohexane oxidation

37

technique to probe the effect of the (1 1 0)/(0 0 1) facets ratio in generation of the radical species using N-tertbutyl-α-phenylnitrone (PBN) as a spin trap. The radicals produced were detected through a typical triplet signal with 8.1 G hyperfine coupling constant attributed to PBNox (PhCON (O•)). But allegedly produced through further oxidation of the PBN-OH spin adduct (Figure 3.4A). Therefore, the production of PBNox suggests the generation of the •OH radical in the early stages of the oxidation of cyclohexane. Furthermore, the linear relationship between the generation of the •OH radical and the (1 1 0)/(0 0 1) facets ratio was observed (Figure 3.4B), suggesting that only (110) facet is capable of generating the •OH radical (Figure 3.4C). Based on the EPR results the mechanistic pathway toward the formation of the products was elucidated. According to the properties of the individual facet acquired using UV–vis reflectance spectroscopy and EPR in conjunction with the cyclohexane oxidation pathway reported in the literature: the cyclohexane C–H bond activation on the surface of a semiconductor material transpires through hydrogen abstraction by the photogenerated hole to produce cyclohexyl radical. Subsequently, cyclohexanone is liberated from the degenerate branching of the cyclohexyl peroxyl radical produced from the reaction between cyclohexyl radical and molecular oxygen (O2). Nonetheless, the

Figure 3.4: (a) EPR signal observed for PBNox, (b) Dependence of the PBNox area and peak intensity ratio (1 1 0)/(0 0 1) for the BiOI photocatalysts, (c) (1 1 0) and (0 0 1) facet dependent cyclohexane oxidation mechanism [59].

38

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

cyclohexane oxidative pathway is different over (110) facet owing to its ability to generate •OH radical through photocatalytic oxidation of water. Typically, •OH radical directly reacts with cyclohexyl radical to produce cyclohexanol. This rationalizes the high cyclohexanol selectivity over high (1 1 0)/(0 0 1) facet ratio. In this manner, the nature of the exposed facet have profound effect on the photocatalytic properties of a semiconductor material. These studies have provided an insight into major factors responsible for photocatalytic activity of a semiconductor materials and the expected cyclohexane oxidation products. For typical example, features such as surface-to-volume ratio, nature of the exposed facets and anion/cation dopant interaction are major factors responsible for high photocatalytic performance of a semiconductor materials. However, all these elucidations have appointed the surface of the catalyst as the most prolific feature responsible for high catalytic efficiency of the semiconductive photocatalysts. For this reason, more information on the surface state of the catalysts would be more expedient. Such information can be obtained from the correlation between numerous number of characterization techniques such as (a) Fourier transform infrared spectroscopy which can be used to probe the functional groups present at the surface of the catalysts, (b) UV–vis spectroscopy to probe the band gap of the semiconductor material, (c) Photoluminescence emission spectroscopy to study the surface defects sites, (d) X-ray photoelectron spectroscopy (XPS) to evaluate the oxidation state of the elements at the surface of the semiconductor material, (e) Raman spectroscopy to provide information on the structural fingerprint of the atoms and the surface and (f) Auger electron spectroscopy (AES) to probe the surface state of the elements. The correlation of these techniques can provide a critical information about the surface of the semiconductor catalysts and neither can be sufficiently enough alone to elucidate the effect of the surface on the catalytic activity of the semiconductor material. For typical example, Fu and co-workers studied the effect of the surface HF removal post treatment of TiO2 on the photocatalytic properties by evaluating the superficial uncoordinated post HF removal. Typically, the XPS was conducted as the principal surface analyzing technique [63]. In this case, spectroscopic peak corresponding to TiO2 F1S increased with HF concentration. However, there was no observable change in peak intensity and chemical shift for T2P and O1S. This could be attributed to the long electron escaping depth with more than the utmost atomic layer from the TiO2 sample, which outcast the XPS spectroscopy as the trustworthy surface analysis technique. Accordingly, Auger electron spectroscopy (AES) analysis was undertaken. The AES results also indicated no chemical shift during the collection of Auger electrons from electron escaping depth. Nonetheless, in regard to other studies the fluorine surface removal can also be evaluated by Raman spectroscopy [64, 65]. Fu and co-workers further used Raman spectroscopy to evaluate the surface of the TiO2 post to HF removal. The Raman spectroscopy showed the weakening of the B1g and the

3.2 AOPs for cyclohexane oxidation

39

shift of low frequency Eg, which can be indexed to alteration of the symmetrical Ti-O-Ti and surface Ti coordination.

3.2.3 Fenton oxidation process Fenton oxidation process is an advanced oxidation process that encompasses the exploitation of the ferrous ion (Fe2+) and hydrogen peroxide (H2O2) in acidic media. Ferrous ion (Fe2+) is oxidized to ferric ion (Fe3+) by hydrogen peroxide resulting in a subsequent generation of hydroxyl anion and hydroxyl radical (•OH) from hydrogen peroxide [66–68]. The process is shown by Eq. (3.9). Fe2+ + H2 O2 → Fe3+ + OH• + OH−

(3.9)

The Fe2+ ions consumed during the Fenton process are regenerated by subsequent reaction of Fe2+ ions with H2O2 through Eqs. (3.10) and (3.11) [67]. Fe3+ + H2 O2 → [Fe … . OOH] + H+ → Fe2+ + HO•2

(3.10)

Fe3+ + HO•2 → Fe2+ + O2 + H+

(3.11)

The Fenton process can be further alienated into photo-, sono- and electro-Fenton processes. These processes are more efficient because they produce more •OH specie than conventional Fenton process. The photo-Fenton process is otherwise similar to Fenton process, except that it is further induced by UV radiation as the main source of energy responsible for photolysis of Fe3+ to Fe2+ ions (Eq.(3.12)). Subsequently, the liberated Fe2+ reacts with H2O2 to produce the •OH radical as shown in Eq. (3.13) [69–72]. On the other hand, the sono-Fenton process utilizes the ultrasound as the source of energy responsible for homolytic cleavage of H2O2 to produce two •OH species. Whereas, electro-Fenton process is an electrochemical advanced oxidation process based on in situ generation of •OH species. [Fe(OH)]2+ + hv → Fe2+ + •OH

(3.12)

Fe2+ + H2 O2 → Fe3+ + HO− + •OH

(3.13)

In contrast to utilization of the external energy as one of the prominent approaches toward enhancing the efficiency of the process, the concentration of the hydronium ion present in solution is also an important element that plays a profound role in optimizing the process efficiency due to H2O2 and iron ions (Fe2+ and Fe3+) speciation factors [73]. Typically, the Fenton’s process is highly effective at pH value of 3.0 due to the presence of higher concentrations of ferric and ferrous ions compared to other operational pH values. As a result, a marginal increase or decrease in the pH value sharply renders the efficiency of the process [74]. When the pH of the solution reduces below 3.0, the H2O2 solvates with hydronium ion to produce oxonium ions (H3O2+), which subsequently enhances the chemical stability of the H2O2 and decreases its

40

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

reactivity with Fe2+ ions. On the other hand, when the pH rises above the 3.0, the Fe2+ decreases as colloidal Fe + complex (Eq. (3.14)) formation takes place [75]. The attraction between Fe+ ions and ligands in these complexes is stronger than that between ligands and ferrous ions [72]. This is one of the drawbacks of the Fenton process. +

[Fe(H2 O)6]2+ + H2 O → [Fe(H2 O)5(OH)] + H3 O+

(3.14)

Another massive pitfall of this process is related with the production of higher quantity of the Fe slurry with time on stream [76–78]. The accumulation of the slurry is highly associated with the presence of Fe3+ ions and the increase in pH value of the solution. The concentration of the Fe3+ ions increases with time on stream, which subsequently results to an assembly of the stable insoluble slurry complexes [79]. Furthermore, based on Eq. (3.9) it can be postulated that the concentration of the OH− species also increases with time on stream. This phenomenon results to an increase in the pH of the reaction solution, which subsequently induces the production of the Fe slurry [67, 80]. In this manner, the downstream treatment of the generated slurry is essential in Fenton process [77, 78, 80]. Nonetheless, production of the slurry can be minimized by maintaining the pH of the reaction solution at pH = 3 and optimizing regeneration of the Fe2+ ions. The rapid regeneration of the Fe2+ ions can be achieved by utilization of the external basis of energy in the form of UV light. On the other hand, the pH of the solution can only be stabilized at pH = 3 in the electro-Fenton process by subsequent anodic water oxidation. This is one of the major advantages of the electro-Fenton process [77, 80]. Even though Fenton based processes have been studied to a great extent over the past decades, the homogeneous based Fenton processes necessitates complimentary steps, such as precipitation to recover the catalyst, enabling catalyst re-use and preventing contamination. In this manner, the cost effectiveness of the homogeneous processes largely relies on supply of chemicals, labor and power requirements [80–83]. These shortcomings have redirected the focus toward heterogeneous Fe based Fenton catalysts [84–90]. The utilization of the heterogeneous catalysts also possesses some enormous advantages such as the ability to regulate the production of the slurry and control over extent metal leaching [91]. In addition, an increase in pH of the solution and accumulation of the Fe3+ ions, the production of the slurry is also associated with the metal leaching. However, the metal leaching in heterogeneous catalysts is trivial and seldom causes the production of the slurry [92]. The concentration of the utilized Fe2+ ions increases with time on stream in the case of the homogeneous based Fenton process. Whereas, the Fe2+ ions consumed in the heterogeneous Fenton process is insignificant, owing to the rapid regeneration of these species from Fe3+ ions due to the insignificant production of Fe3+ slurry [77, 89, 90]. In homogeneous Fenton process, the production of the •OH species is solely allied with the Fe2+ ions, whereas in the case of heterogeneous Fenton process the •OH and • OH2 radical species are produced from Fe2+ and Fe3+ oxidation and reduction pathways respectively [77]. Some of the enormously used Fe based heterogeneous catalysts

3.3 Heterogeneous Fe Fenton catalysts

41

encompasses ferrryhydrites [92], hematite [93], pyrite [88], ferrites [77] and magnetite [94]. Owing to their high abundance, enhanced stability, high adsorption capacity, environmentally benign properties and cost effectiveness, Fe based heterogeneous catalysts have received a lot of attention in Fenton processes. These materials also possess added prolific advantages [77] over homogeneous Fe Fenton catalysts such as (a) insignificant effect of the inorganic carbonates on the reaction, (b) large effluent pH range of about 3.0–9.0 and (c) long catalyst lifetime. Nonetheless, they are also associated with the slow hydrocarbons oxidation rate, which is regarded as the major drawback of these materials. This shortcoming can be regulated by either doping with the no-metal or metal dopants usually transition metals, introducing a particle size mediating surfactant to increase the surface-to-volume ratio of the catalysts or addition of the chelating agents such as malonate, citrate, oxalate and ethyldiaminetetraacetic acid [77, 88].

3.3 Heterogeneous Fe Fenton catalysts 3.3.1 Ferriyhydrite Ferrihydrite is a highly abundant naturally occurring hydrous Fe3+ oxyhydroxide mineral which exists in great quantities at the surface of the Earth. It is sometimes a constituent of the extra-terrestrial materials [92]. Ferrihydrite is usually used as the precursor for the preparation of the hematite and goethite [93]. Due to its high surfaceto-volume ratio, ferrihydrite is often used as a dominant sink for various metals and nutrients. In the Fenton process, the high surface-to-volume ratio of the ferrihydrite plays a profound role in enhancing the contact between ferrihydrite surface and H2O2 [94–96]. The decomposition of the H2O2 at the surface of the ferrihydrite is largely dependent on the pH of the solution. Typically, the H2O2 decomposition increases with increasing pH of the solution. Nonetheless, at pH values above 8.0, non-reactive oxygen species are produced from H2O2 decomposition. As a result, the process is highly effective at pH 7.0. The ferrihydrite surface modification using citrate mediating agent alters the surface properties and the ferrihydrite which subsequently results in high photo-Fenton catalytic activity [77, 97].

3.3.2 Ferrites Ferrites are ceramic materials formed by mixing and firing of the Fe3+ oxide blended with other transition metal(s) such as nickel, barium, manganese and zinc [98]. In general, the ferrites can be further categorized as hexagonal, garnet and spinel ferrites, based on the crystal structure. Hexagonal ferrites also called hexaferrites are high magnetic Fe oxides with hexagonal crystal structure. They are formed by Fe, O2 and at

42

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

least one metallic element, most often strontium or barium, occasionally contains other metals such as Co and Mn. Garnet ferrites possesses the silicate mineral garnet structure with general formula of M3(Fe5O12), where M is a rare Earth metal such as cerium, yttrium, scandium and dodecahedral coordinated sites. On the other hand, spinel ferrites are Fe oxides with a general formula of AB2O4, where A and B are metallic cations occupying tetrahedral and octahedral sites respectively containing Fe3+ ions as one of the central elements in their structure. The ferrites catalysts can be fabricated using solvothermal method, precipitation and co-precipitation method, emulsion method, hybrid hydrothermal-calcination method and reverse micelle method. Typically, Jauhar and Singhal fabricated MnFe2O4 and CoFe2O4 ferrite catalysts using precipitation method followed by hydrothermal treatment at 90 °C. The Fenton oxidation efficiency of the catalysts was highly dependent on the oxygen vacancies. Furthermore, the Fenton oxidation efficiency of the CoFe2O4 exhibited a steady decrease with annealing temperature [99].

3.3.3 Hematite Hematite is a black to steel, brown to reddish brown or silver grey primary ore of iron that occur in numerous types such as specularite, martite, kidney ore and iron rose [93]. Due to its large surface area, well defined topology, stability, and environmental benign ever after losing reactivity properties, hematite has received a lot of spotlight in Fenton oxidation catalysis [93, 100]. In contrast to its high stability, hematite is often subjected to a blast furnace without losing its reactivity and having any environmental impact when used as a feed material for pig iron production [101]. In the photo-Fenton oxidation process, UV irradiation of the sulphur doped hematite along (110) facet enhances its catalytic activity. Furthermore, the N and S doping also enhance the photo-Fenton process efficiency by creating photo exciton trap states which regulates the electron-hole recombination phenomenon and creating surface heterojunction that plays a massive role in promoting the electron transfer between the surface of the catalysts and H2O2 [100].

3.3.4 Pyrite Iron pyrite is one of the most highly abundant metal sulfides on Earth with molecular formula of FeS2. It is often called fool’s gold and is famously known for its spontaneous reaction with O2 or H2O to produce H2O2 either by reaction of leached pyrite ferrous ions with dissolved oxygen or reaction between ferrous ions at the surface of pyrite with dissolved oxygen species. The surface bound pyrite Fe2+ ions reacts with oxygen through Haber –Weiss reaction pathway to produce H2O2 in which superoxide species is an intermediate. The mechanism is shown in Eqs. (3.15) and (3.16) [77, 101, 102].

3.3 Heterogeneous Fe Fenton catalysts

43

Fe2+ + O2 → Fe3+ + O•− 2

(3.15)

+ 3+ Fe2+ + O•− 2 + 2H → H2 O2 + Fe

(3.16)

The aptitude of iron pyrite to form hydrogen peroxide when reacting with O2 or H2O2 is the leading fundamental character that makes it to be one of the tremendous heterogeneous Fenton catalysts. However, the necessity to regulate the pH of the solution with acidic medium to facilitate the formation of the H2O2 is the main shortcoming of the iron pyrite catalyzed Fenton oxidation process [103, 104].

3.3.5 Magnetite Magnetite is an opaque, black and submetallic ferrimagnetic mixed valent iron oxide of the spinel family with the molecular formula of Fe3O4. It is the most magnetic naturally occurring mineral on Earth. Owing to its unique Fe2+ ion structural properties which plays a significant role in instigating Haber-Weiss mechanism, magnetite has gained a significant attention in Fenton and photo-Fenton oxidation processes [105]. In contrast to Haber-Weiss mechanism instigating aptitude of the magnetite iron oxide, its facile separation from the reaction solution by means of magnetism is another feature that makes it one of the tremendous heterogeneous catalysts [106]. The doping of the magnetite with heavy metals increases its catalytic activity. Typically, vanadiumdoped magnetite exhibit high catalytic activity in comparison to chromium doped magnetite. Typically, vanadium particles increases the concentration of the Fe2+ ions over Fe3+ ions and the hydrocarbons’ adsorption aptitude which subsequently bring about the direct effect on the decomposition of H2O2 [77, 107]. In addition, the vanadium doping of magnetite also results to an increase in surface HO− groups and the Fenton catalytic activity due to enhanced adsorption properties, intensified H2O2 decomposition rate to form HO• species and rapid regeneration of Fe2+ ions due to induced electron transfer by vanadium particles. Similarly, magnetite doping with Nb [108], Ti [109] and Pd [110] also increases its Fenton and photo-Fenton activity. However, other non Fe heterogeneous catalysts, particularly metal supported catalysts and mixed metal supported catalysts have also been reported for Fenton oxidation processes and most of these catalysts possess high catalytic activity than Fe based heterogeneous catalysts.

3.3.6 Fenton oxidation of cyclohexane Fenton oxidation of cyclohexane has been well explored mostly exploiting metal complex homogeneous catalysts [111–120]. Even though homogeneous catalysts are associated with numerous number of drawbacks, nonetheless they have displayed a prudent potential as future catalysts for Fenton oxidation of cyclohexane in terms of

44

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

H2O2 decomposition capacity, cyclohexane percentage conversion and regulating product selectivity [113]. Some of the homogeneous cyclohexane oxidation processes extracted from pertinent literature reports are presented in Table 3.1. Although little has been done in Fenton oxidation of cyclohexane over heterogeneous catalysts, however, some few tremendous studies have been reported. For typical example, Bordoloi and coworkers [119] studied the Fenton oxidation of cyclohexane over AgW-SBA-15 in acetonitrile. When SBA-15 was used as a catalyst, the minimal cyclohexane percentage conversion of 4.4% was obtained with 100% selectivity toward KA oil. However, when W-SBA-15 was used the cyclohexane percentage conversion tripped up to 50.6% with percentage selectivity of 80.4% toward KA oil and Cyclohexanediol, cyclonexanedione and oxepane-2,7-dione as side products. Surprisingly, when W was coupled with Ag in SBA-15 an upsurge of 89.9% cyclohexane conversion was obtained with 99% selectivity toward adipic acid. However, an increase in the metal loading exhibited a significant diminution in both cyclohexane percentage conversion and product selectivity toward adipic acid. This could be attributed to an increase in particle size which subsequently renders the efficiency of the surface catalyzed reaction. In an attempt to elucidate a plausible mechanism toward the formation of the products the authors used hydroquinone free radical scavenger. In this way, the cyclohexane percentage conversion exhibited an extreme decrease in the presence of hydroquinone under otherwise similar conditions from 89.9 to 2.5%. This could be attributed to the strong oxidizable aptitude of hydroquinone which subsequently results to free radical transfer from cyclohexane to hydroquinone to form quinone. This phenomenon further confirmed the free radical pathway already discussed in Eq. (3.1) to Eq. (3.8). Based on electron distribution of the highest occupied molecular orbital (HOMO) of the transition states of the AgW-SBA-15 catalyst the role of Ag and W metals in AgW-SBA-15 is toward the formation of the adipic acid from ring opening of the xepane2,7-dione was probed. Typically, one carbonyl oxygen coordinate with Ag atom. Subsequently, the electron cloud over the oxygen and carbonyl carbon was dragged by Ag atom which resulted to the polarization of the of the C-C bond electron of the two carbonyl carbons. This sensation possessed a colossal effect on weakening the C–C bond, which endorsed the xepane-2,7-dione attack by H2O2 and ring cleavage to form adipic acid. In another study, Li and co-workers [120] studied the effect of the boron and fluorine-doped C3N4 as the solid activator for photo assisted Fenton oxidation of cyclohexane catalyzed by 8-quinolinolato Fe3+ complexes ((Fe3+Qb)3) under visible light irradiation. In this case, the percentage conversion of 25.87% was obtained. The role of the solid activator and the catalysts in enhancing process efficiency was explained. The Fe-O bond of Fe3+(Qb)3 complex was cleaved by H2O2 to produce more stable pentadentate Fe3+ peroxide species with suspended hydroxide specie (complex 2). Subsequently, the complex 2 underwent homolytic cleavage of the Fe-OOH bond to form Fe3+Qc3–O• radical specie (complex 3) •OH radical (Figure 3.5). In general, the H2O2

Catalyst used

Tungsten trioxide nanosheets-nitrogendoped-carbon dots composite (NC-WO)

Titanium dioxide/ reduced graphene oxide hybrid photocatalysts (TiO/rGO)

AOP process

Photocatalysis

Photocatalysis

Solvent free system using O under UV irradiation

Solvent free system using air under visible light irradiation The NC-WO composite enhances the separation of the photo-generated charge carriers by transferring the electron to the nitrogen-doped carbon dots (NC). TiO/rGO hybrid photocatalysts under UV light irradiation enhance cyclohexane photocatlysis to cyclohexanone. The enhanced catalytic activity could be attributed to photgenerated charge separation by the transfer of the conduction band electrons of the photoactivated TiO to rGO. The electrons trapped on rGO promotes instigation of the O reduction pathway, which subsequently circumvents the formation of O•–. This phenomenon supresses the further oxidation of the cyclohexanone. These TiO/rGO properties result in successful production of CHA-one.

Reaction conditions Major findings

Table .: Some of the cyclohexane oxidation processes extracted from literature.

.

.

.

.

.

Cyclohexanol Cyclohexanone

% Selectivity

.

% Conversion

[]

[]

Reference

3.3 Heterogeneous Fe Fenton catalysts

45

Catalyst used

Vanadium-doped titanium oxide (VO@TiO)

C–N dual-doped CrO visible light derived from metalorganic framework (MOF)

Anatase TiO

AOP process

Photocatalysis

Photocatalysis

Photocatalysis

Table .: (continued)

HO and acetonitrile Water is oxidised by photousing O under solar generated holes to H+ and •OH. light irradiation. The V+ captures photogenerated electrons to form V+. The V+ transferres the electron to O to form O•Solvent free system The XPS detected only Cr+ in CrO whereas, Cr+ and Cr+ using O under UV irradiation at  °C. were detected in N-C-doped CrO. The CrO catalyst exhibited no catalytic activity however, N-C-doped CrO catalyst exhibited superior activity. The authors suggest that Cr+ is capable of trapping the holes therefore, preventing electron-hole recombination. Water as the solvent TiO–OH groups play a profound using O and UV role in enhancing catalytic aclight irradiation at tivity of TiO. The increase in room temperature. crystalline size of the anatase TiO catalysts increases the moles of cyclohexanone formed per minute per OH-site and the rate of cyclohexanone formed. The photocatalytic oxidation of

Reaction conditions Major findings

.

.

.

.

.

.

.

.

Cyclohexanol Cyclohexanone

% Selectivity

.

% Conversion

[]

[]

[]

Reference

46 3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

Gold loaded titanium dioxide–carbon nanotube composites (Au/TiO–CN)

Protonated Fecontaining mesoporous silica nanoparticle (H/Fe-MSN)

Zeolites doped iron catalysts (Fe-ZSM-)

Photo-Fenton oxidation recation

Fenton oxidation reaction

Photo-Fenton oxidation reaction Fenton oxidation reaction

Gold nanoparticles supported on carbon materials (Au/CNT)

Catalyst used

AOP process

Table .: (continued)

HO transpires on the photogenerated hole to produce •OH which subsequently enhances cyclohexane percentage conversion. Solvent free system HO is adsorbed at the surface using % HO. of the catalyst followed by decomposition by homolytic cleavage to product to two •OH radical species induced UV irradiation. Water as the solvent. HO is activated by tetrahedral Cyclohexane oxida- Fe+ in H/Fe-MSN catalyst via the formation of the bi-radical tion exploited at Fe-peroxy complex. The formed  K using % radicals initiated H-abstraction HO based cyclohexane oxidation pathway. Solvent free system The •OH radical is produced using % HO at from hmolytic cleavage of HO room temperature. and oxidation of Fe+. Water solvent under HO decomposes to produce mild reaction condi- oxygen centred •OH and •OOH radicals at the surface of the Auo tions using % and Au catalysts respectively. HO

Reaction conditions Major findings

–



.

.

.

.

.

.

.



.

Cyclohexanol Cyclohexanone

% Selectivity

.

% Conversion

[]

[]

[]

[]

Reference

3.3 Heterogeneous Fe Fenton catalysts

47

Catalyst used

Discrete nanosheets of Acetonitrile solvent CuO@SAPO‐ system, at room catalyst temperature.

Indirect ozonation reaction

Solvent free system at  K.

X zeolite supported cobalt oxides (Co/X zeolite)

Ether solvent at room temperature

The ozone directly reacts with cyclohexane via hydrogen abstraction to produce cyclohexyl radical and hydrogen trioxide radical. The hydrogen trioxide radical further abstract the hydrogen from ether to produce the alkyl ether radical. The alkyl ether radical subsequently abstract the hydrogen from cyclohexane to produce cyclohexyl radical therefore, enhancing process efficiency. The ozone was activated through decomposition at the surface of the Co/X zeolite) catalyst to produce mono oxide radical. Ozone is adsorbed at the surface of the catalyst followed by subsequent activation by decomposition. The singlet oxygen produced from ozone decomposition instigate cyclohexane oxidation through H-abstraction.

Reaction conditions Major findings

Indirect ozonation reaction

Direct ozonation No catalyst reaction

AOP process

Table .: (continued)

.

.



–

.

.

.

Cyclohexanol Cyclohexanone

% Selectivity

.

.

% Conversion

[]

[]

[]

Reference

48 3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

3.3 Heterogeneous Fe Fenton catalysts

49

activation is non-spontaneous and is often termed as the rate determining step of the Fenton process, therefore it requires few kcal/mole of energy. Nonetheless, this step was accelerated by structural alteration of the Fe3+(Qb)3 complex persuaded by π–π orbital stacking action of C3N4BF. In addition, this acceleration phenomenon was further endorsed by visible light irradiation which resulted to a transfer of the photogenerated electrons of C3N4BF to Fe3+(Qb)3, causing the weakening of the Fe3+(Qb)3 coordination bond.

3.3.7 Ozonation Ozonation is one of the promising technologies for production of fine oxygenated organic compounds due to its efficiency at ambient reaction conditions [121–133]. Ozone is a strong reducing agent, which can directly or indirectly react with organic compounds. The direct reaction of ozone with organic compounds transpires through one of the (a) 1,3 dipolar insertion (b) step-wise ionic hydride transfer and (c) step-wise radical H-abstraction (Scheme 3.2). However, 1, 3 dipolar insertion is energetically unfavorable, whereas ionic hydride transfer is favored on polar substrate. This suggest that the ozonation of polar organic compounds transpires through ionic hydride transfer and H-abstraction. On the other hand, the oxidation of the non-polar organic

Figure 3.5: Proposed reaction pathway for the Fe3+(Qb)3-photo-catalyzed oxygenation of cyclohexane by H2O2 with the help of solid additive [120].

50

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

Scheme 3.2: Possible pathways for ozone initiated oxidation of hydrocarbons [123].

compounds such as hydrocarbons transpires through H-abstraction. The H-abstraction results in the production of alkyl radical [119, 120, 123]. The indirect ozonation of organic compounds encompasses the decomposition of ozone to •OH specie as the main oxidant induced by radical initiators such as –OH, H2S, Fe2+, HO2−, HCOO−, H2O2 and UV in aqueous media (Eq. (3.17)) [123]. • O3 + OH− → O•− 3 + HO

(3.17)

However, some organic and inorganic molecules are proficient on reinforcing the generation of the O2− superoxide which subsequently promote ozone decomposition. These compounds are termed as radical promoters. Such compounds include primary alcohols, aryl groups, glyoxylic acid, formic acid, humic acids and inorganic compounds of phosphate species [124]. In general, the hydroxide radical (2.80 V) provides high oxidation potential than ozone (2.08 V) [125]. For this reason, indirect ozonation is more virtuous than direct ozonation. Certainly, most of the previous ozonation interrelated studies have been directed toward the utilization of the combined AOPs such as O3/H2O2, O3/UV/H2O2, UV/O3, O3/UV/TiO2, O3/HO− etc. for optimum efficiency [19, 126, 127]. Typically, Lu and co-workers [126] studied the decomposition of O3 in the presence of H2O2 for oxidation of chlorophenol. The authors reported that H2O2 enhances O3 decomposition through electron transfer by a reaction that can be implicit as H2O2 activation by O3 (Eq. (3.18)) [127]. H2 O2 + O3 → •OH + HO− + O2

(3.18)

In contrast, O3/UV/TiO2 reaction system entails the generation of the TiO2 conduction band electrons and valence band holes induced by UV irradiation (Eq. (3.19)). In the presence of an appropriate scavenger such as H2O, the photo induced TiO2 (h+)

3.3 Heterogeneous Fe Fenton catalysts

51

instigates the oxidative pathway to produce •OH radical and the hydrogen cation (H+) (Eq. (3.20)). Similarly, TiO2 (e−) instigates the reductive pathway to produce ozonide (O3•−) radical specie (Eq. (3.21)). The H+ produced from oxidation pathway reacts with O3•− radical liberated from reductive pathway to form hydrogen trioxide radical (HO3•) (Eq. (3.22)). Subsequently, the HO3• further reacts with H+ and the TiO2 (e−) to produce hydrogen peroxide (H2O2) and TiO2 (Eq. (3.23)). The H2O2 liberated consequently reacts with photo generated TiO2 (e−) to produce •OH and −OH (Eq. (3.24)). The HO− further participate in the production of •OH radical specie through Eq. (3.17) [72, 128]. However, the high band gap of the conventional TiO2 renders the process efficiency [20, 72, 128]. Consequently, a lot of efforts have been made to promote the energy efficiencies of this process by extending the TiO2 photo-response toward visible light region [20, 24, 25, 129]. Typically, numerous studies have achieved great viewpoint in attaining visible light TiO2 activated photo-response by non-metals (such as, C, N, F and S) doping and metals (such as V, Co, Cr and Fe) doping [25, 72]. Other semiconductor materials capable of producing photo induced electrons and holes under visible light irradiation have been reported. Such materials include surface mediated semiconductors, metals and metal oxides supported materials. Surface mediated and metal supported semiconductors are more prominent due to their extraordinary chemical and physical properties. Typically, supported atoms possesses high electron density therefore, they donate electrons to the conduction band of the support. This prodigy results in a decrease in electron density of the supported metal and the increase in surface charge of the semiconductor [130, 131]. Subsequently, the surface charge induces the decomposition of ozone. In contrast, surface mediating agents are also capable of inducing semiconductor surface charge by confining the crystal growth of the single crystal under kinetically controlled conditions. In this manner, the surface of the semiconductor becomes occupied by high energy atoms with large density of dangling bonds and induced surface charge [64, 133]. TiO2 + hv → e− + h+

(3.19)

TiO2 (h+ ) + H2 O → TiO2 + •OH + H+

(3.20)

O3 + TiO2 (e− ) → O•− 3 + TiO2

(3.21)

• + O•− 3 + H → HO3

(3.22)

2HO•3 + 4H+ + TiO2 (e− ) → 3H2 O2 + TiO2

(3.23)

H2 O2 + TiO2 (e− ) → •OH + HO− + TiO2

(3.24)

The production of the radical species from the O3/UV/H2O2 reaction system entails homolytic cleavage of H2O2 induced by UV irradiation to produce two •OH radical species (Eq. (3.25)). The •OH radical then reacts with O3 to form hydrogen tetraoxide radical (HO4•) (Eq. (3.26)) [19].

52

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

H2 O2 + hv → 2 •OH

(3.25)

OH + O3 → HO•4

(3.26)

However, obtaining the required quality in terms of percentage selectivity toward the desired products in degradation of organic compounds to value added products, partial and selective ozonation processes present a massive challenge [72]. This is because hydrocarbons’ oxidation products are highly reactive than their corresponding hydrocarbon starting materials. In this manner, they readily react with •OH specie to produce copious amount unwanted by-products which renders the economic and environmental efficiency of the process [126]. This is the major drawback of the ozonation processes. Nonetheless, numerous studies have shown that metal and metal oxides do not only instigate ozone decomposition but also possess tremendous product selectivity ability toward the desired products [133–137]. For this reason, heterogeneous catalysts based ozonation processes are the most comprehensively studied ozonation processes [3, 16, 44, 124, 127].

3.3.8 Ozonation of cyclohexane Ozonation of cyclohexane has been cited as one of the great platforms for production of cyclohexanol and cyclohexanone at low temperature and pressure conditions because of the comparable high oxidation potential of ozone (2.07 V) than other oxidants (oxygen, hydrogen peroxide and tert-butyl hydrogen peroxide) [72]. Low temperature ozonation of cyclohexane possesses some prolific advantages over conventional cyclohexane oxidation process, such as total control over KA oil percentage selectivity without compromising percentage conversion, short reaction hours and wide range of control of parameters to enhance percentage conversion [16]. In a study by Coote and co-workers [123] the mechanistic insight on the direct ozonation of cyclohexane was investigated using experimental and computational approaches. In the first step of the process (shown in Scheme 3.3), a cage followed by a pre-complex are formed (Int-1). From the Int-1 complex, both stepwise and resolute mechanisms are possible, with the former being more energetically favored. In this manner, the H-abstraction is a stepwise process that involves the formation of a pre-complex. After ozone initiated H-abstraction, the radical pair (cyclohexyl radical and hydrogen trioxide radical) (TS-2) could exist in resonance with the ionic pair (cyclohexyl cation and hydrogen trioxide anion) (TS-1). Nonetheless, the DFT calculations performed exhibited that the radical pair is more energetically feasible by about 100 kcal/mol than its ionic pair analogue. The cyclohexyl radical can further react with ozone and subsequently cyclohexane through equations (Scheme 3.3) to form cyclohexyl hydrogen trioxide (TS-3) or cyclohexyl-2-oxo hydrogen trioxide (TS-4). However, the formation of the cyclohexyl2-oxo hydrogen trioxide is stabilized by polar solvents. In this manner, the formation of cyclohexyl hydrogen trioxide is predominant in solvent free reaction system. The

3.3 Heterogeneous Fe Fenton catalysts

53

Scheme 3: Mechanistic insight in to ozone initiated oxidation of cyclohexane based on experimental and simulations studies [123].

cyclohexyl hydrogen trioxide can undergo RO———OOH or ROO———OH cleavage as outlined in Scheme 3.3. Based on the free energies of the resulting radical fragments, the RO———OOH cleavage is more feasible by about 3 kcal/mol than ROO—H fragmentation. The alternative plausible mechanism for the formation of cyclohexanol is by the combination of hydroxide radical and cyclohexyl radical. In addition, the authors also suggested that the hydroxide radical is also produced from dissociation of hydrogen trioxide (HO3• → HO• + 1O2). However, cyclohexanone could be formed according to Eqs. (3.7) and (3.8). In another study, Einaga and Futamura [133] studied the catalytic oxidation of cyclohexane over alumina supported manganese oxide catalysts using ozone. In this study, the cyclohexane percentage conversion was enormously dependent on the presence of the catalyst and the amount of manganese oxide loaded on alumina. For

54

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

examples, the minimal percentage conversion of 4.2% was obtained in blank oxidation. When, Al2O3 was used as a catalyst, a marginal increase in percentage conversion 5.7% was observed. Nonetheless when 5 wt% MnO/Al2O3 catalyst was used an upsurge in cyclohexane percentage conversion to 11.8% was observed. The increase in cyclohexane percentage conversion in the presence of the catalyst was attributed to the ozone decomposition at the surface of the catalyst to produce molecular oxygen highly reactive singlet oxide radical species. In addition to the decomposition of the ozone, the upsurge in cyclohexane percentage conversion was also attributed to C–H bond cleavage by the unsaturated manganese oxide centres. In this manner, the presence of the catalyst instigates the ozone decomposition and directly initiate the radical stepwise oxidation of cyclohexane. In a recent study by Mkhondwane and Pullabhotla [134], the pH dependent ozonation of cyclohexane using manganese loaded gamma alumina, Mn/γ-Al2O3, catalysts was also studied. The study was conducted at pH = 3, 7 and 11. The content of the Mn loaded to γ-Al2O3 was varied from 2.5 to 15 wt% in a 2.5 wt% intervals. The minimum cyclohexane percentage conversion 11% was obtained using γ-Al2O3γ and 15% Mn/γ-Al2O3 catalysts. The authors found that both the pH of the solution and the content of the Mn loaded to γ-Al2O3 profoundly affect the efficiency of the ozonation process without compromising the selectivity toward KA oil. Typically, the cyclohexane percentage conversion increases with pH value of the solution. In contrast, the loading of the Mn to γ-Al2O3 support enhances the efficiency of the process, however, further increase in the Mn content loaded to γ-Al2O3 sharply renders the cyclohexane percentage conversion. Based on these elucidations the authors proposed a plausible mechanism toward the formation of the products. Typically, the increase in the pH value increases the concentration of hydroxide ions present in the solution. Subsequently, the hydroxide ions instigate ozone decomposition to produce hydroxide radical specie, which eventually initiate the cyclohexane oxidation via H-abstraction. In contrast, the Mn loading induces the surface charge of the Mn/γ-Al2O3 catalyst by donating the electrons to the conduction band of the γ-Al2O3 support. The γ-Al2O3 conduction band electrons also initiate cyclohexane H-abstraction to produce cyclohexyl radical. In addition the Mn/γ-Al2O3 catalyst also play a massive role in decomposition of the ozone to produce a singlet oxygen radical which also initiate H-abstraction from cyclohexane. However, at high metal loading, the interaction between the Mn metal and γ-Al2O3 metal oxide is relatively weak, which result to the leaching of the metal from the support to the reaction solution and the formation of the sludge. In addition, the high Mn content results to a blockage of γ-Al2O3 pores which subsequently limit the diffusion of both ozone and the cyclohexane to the inner surface sites. This phenomenon rationalizes the drastic decrease in the catalytic activity of the 15 wt% Mn/γ-Al2O3. This was confirmed by BET surface area analysis. Based on the transmission electron microscopy (TEM) and X-ray diffraction (XRD) results, the particle size of the γ-Al2O3 exhibited a significant decrease upon Mn loading. For this reason, the small size effect for surface

3.4 Conclusion and outlook

55

catalyzed reactions was also a contributing element to the high activity of the Mn/γ-Al2O3 catalysts. The computational methods are also in the forefront as far as elucidating the origin of the catalysts and validating the reaction mechanism are concerned. For example, Liu and co-workers [145] used density functional theory (DFT) to probe the origin of the In2O3/N–TiO2 in photoelectrocatalytic oxidation of cyclohexane. Accordingly, DFT and photoelectrocatalytic test revealed that the photo-catalytic activity of the In2O3/N–TiO2 is due to enhanced visible light harvesting aptitude which is endorsed by N-doping and fast separation of the photogenerated electron-hole pair. In another study [146], density functional theory and multireference CASSCF-NEVPT2 methods were used to further elucidate the electronic structure during ozonation of cyclohexane. In this case it was described that the formation of the ozone-cyclohexane complex reported by Coote and co-workers is nearly energetically neutral with ΔH = −0.8 kcal mol−1. However, the transition state where ozone abstracts hydrogen from cyclohexane was identified at 16.9 kcal mol−1. The transition state exhibited elongation of 1.47 Å of the ozone bond between central oxygen and the oxygen receiving the hydrogen atom. Furthermore, the oxygen–hydrogen distance was identified to be 1.165 Å, which indicates that the O–H bond has not completely formed. The hydrogen-carbon was elongated suggesting the weakening of the C–H bond which subsequent transferred to ozone. These computational methods provided an insight in both the phenomenon of hydrogen transfer on the singlet surface and the ozone structure.

3.4 Conclusion and outlook This review provides a critical summary on advanced oxidation processes for cyclohexane oxidation over metals and metal oxides as catalysts and the mechanistic overview on each advanced oxidation process on the basis of its fundamentals and cyclohexane oxidation. Undoubtedly, oxidation of cyclohexane over metals and metal oxides catalysts using advanced oxidation processes have been one of the most prolific events in synthetic chemistry research over the past decades. In comparison, to conventional processes AOPs are more effective because of the production of the radical species. Ideally, the continuous development of these processes by implementation and alteration of the important parameters such as pH, photo inducers, size, shape and composition of the catalysts, will not only deepen the better understanding of the effects of these parameters in process efficiency, but will also accelerate future developments. The main disadvantage of the photocalytic process is the recombination of the photogenerated charge carriers. Nonetheless, numerous studies have reported that metal and metal oxide catalysts are capable of separating the photogenerated charge carriers, in this manner inducing the efficiency of the photocatalytic process by

56

3 Cyclohexane oxidation using AOPs with metal & metal oxide catalysts

extending their lifetime in the reaction solution. Fabrication of the photocatalysts with high photo-generated exciton separating aptitude mainly focusing on shape and composition have shown their potential as one of the potential future photocatalysts. For example, the activity of the Au/CQs catalyst outlined in this review emanated from synergic effect of both Au and CQs. The CQs plays a role of extending light absorption toward longer wavelength whereas the Au function as an electron acceptor therefore quenching fluorescence. This phenomenon results to the separation of photo-generated charge carriers which subsequently accelerate redox pathways because more electrons and holes are participating. Similarly, the catalytic efficiency of BiOI resulted from the synergic effect of the (110)/(001) facets. Typically, the (110) facet is responsible for the production of the HO• which accelerate cyclohexane hydrogen abstraction. On the other hand, the main disadvantages of the Fenton oxidation process are the necessity to regulate the pH of the solution at a value of 3.0, the increase in pH of the solution with time on stream induced by the hydroxide ions produced from decomposition of the H2O2 and the production of the slurry from Fe based catalysts. For these reasons Fenton oxidation process may become less popular in the near future. In this manner, a lot of effort needs to be done on modified Fenton process such as synthesis of the Fe free highly active catalysts for photo induced Fenton oxidation process. However, numerous tremendous catalysts in particular metal supported catalysts with synergic metal support effect have been reported. In addition, some metal oxides as supports have also been reported to reduce the production of slurry from Fe by controlling the extent of Fe leaching to reaction solution. Nonetheless, ozonation could also be a good substitute for Fenton based oxidation of cyclohexane owing to its high oxidation potential and efficiency.

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Ivayla Pantcheva*, Nikolay Petkov, Svetlana Simova, Rumyana Zhorova and Petar Dorkov

4 Alkaline-earth metal(II) complexes of salinomycin – spectral properties and antibacterial activity Abstract: In the present paper the synthesis and structural characterization of alkaline-earth metal(II) complexes of the polyether ionophorous antibiotic salinomycinic acid (SalH.H2O) are discussed. The complexes [M(Sal)2(H2O)2] (M = Mg2+, 1; Ca2+, 2; Sr2+, 3; Ba2+, 4) were obtained reacting salinomycinic acid and Et4NOH with the corresponding metal(II) salts at metal-to-ligand-to-base molar ratio of 1:1:1. The spectral properties of 1–4 were characterized using infrared spectroscopy, fast atom bombardment-mass spectrometry, nuclear magnetic resonance and elemental analysis data. The crystallinity degree and morphology of complex 2 were studied by X-ray powder diffraction and transmission electron microscopy. The biometal(II) salinomycinate complexes 1 and 2 possess an enhanced antimicrobial activity compared to the parent antibiotic against Gram-positive bacteria. The comparison between the effectiveness of the complexes, reported here, and the already known isostructural coordination species of salinomycin and monensin (MonH.H2O), revealed that magnesium(II) and calcium(II) monensinates appear to be promising antibacterial agents against Bacillus subtilis, Bacillus cereus and Micrococcus luteus. Keywords: alkaline-earth complexes; Gram-positive bacteria; salinomycin.

4.1 Introduction Since their discovery in 1960–1970s, the polyether ionophores (PI) are still the most widely used antibiotics in veterinary medicine [1–6]. Isolated from Streptomyces spp., these compounds show significant coccidiostatic activity and are of primary choice in

*Corresponding author: Ivayla Pantcheva, Faculty of Chemistry and Pharmacy, Sofia University “St. Kl. Ohridski, 1, J. Bourchier Blvd., Sofia 1164, Bulgaria, E-mail: [email protected]fia.bg Nikolay Petkov, Faculty of Chemistry and Pharmacy, Sofia University “St. Kl. Ohridski, 1, J. Bourchier Blvd., Sofia 1164, Bulgaria Svetlana Simova, Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, Acad. G. Bontchev Str., bl. 9, Sofia 1113, Bulgaria Rumyana Zhorova, Integrated Micro-Electronics BG EOOD, Industrial Zone Microelectronica, Botevgrad 2140, Bulgaria Petar Dorkov, Research and Development Department, Biovet Ltd., 39, P. Rakov Str., Peshtera 4550, Bulgaria As per De Gruyter’s policy this article has previously been published in the journal Physical Sciences Reviews. Please cite as: I. Pantcheva, N. Petkov, S. Simova, R. Zhorova and P. Dorkov “Alkaline-earth metal(II) complexes of salinomycin – spectral properties and antibacterial activity” Physical Sciences Reviews [Online] 2022. DOI: 10.1515/psr-2021-0201 | https://doi.org/ 10.1515/9783110783629-004

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the case of parasitic infections caused by Eimeria spp. [7–9]. PI are capable to transfer certain metal cations through the cell membranes leading to electroneutral or electrogenic ion fluxes [10] which subsequently disturb the metal homeostasis inside the cells, activate energy-depending processes and finally result in cell death. Besides their pronounced anticoccidial effectiveness, PI possess also an activity against infections caused by different microorganisms, including drug resistant bacteria and parasites [11–13]. In 2009 salinomycin emerged as a selective inhibitor of the human cancer stem cells [14]. Since then, numerous experiments on its antiproliferation ability against various tumor cells were performed searching for an effective and (if possible) highly selective chemotherapeutics in cancer treatment [15]. A strategy that can be employed to potentiate the effect of biologically active compounds is their chemical modification, pointed to the introducing of new substituents into the original structure of the target molecules. In the field of salinomycin, such an approach is widely used, resulting in a number of new products, bearing various organic functionalities. In some cases, an enhanced bioactivity of the derivatives compared to that of the parent drug, was observed, thus obtaining more potent and selective medications [16–21]. The most common polyether ionophores (monensin, salinomycin, maduramycin) are known to act as monovalent metal ion carriers, but their biological properties depend to high extent on the presence of neighbor metal(II) environment. In the past decade, we extensively studied the interaction of these veterinary therapeutics with divalent cations to obtain new metal(II)-based derivatives of various composition and structure, depending on the origin of both the antibiotics and the corresponding metal(II) salts [22–25]. The structural modification of natural compounds by complex formation is an another approach in medicinal chemistry, which can be used to develop new pharmaceutical agents, that are involved in metal-dependent processes [26, 27]. It was found, that most of the biometal(II) complexes of monensin and salinomycin exert superior cytotoxicity compared to the parent ligands against Grampositive bacteria strains and tumor cell lines of human and animal origin [28–30]. These findings confirm the general potency of metallodrugs and are of great interest in the search for more effective biologically active candidates. With respect to the above mentioned, we focused our efforts towards the ability of salinomycinic acid to bind alkaline-earth metal(II) cations and evaluated the antibacterial activity of the biometal(II) complexes characterized. In the present study we report the synthesis and structural elucidation of novel bissalinomycinate complexes bearing ions of Mg(II), Ca(II), Sr(II) and Ba(II). The results revealed that Mg(II) and Ca(II) coordination compounds can be considered as biologically active species, while the binding of Sr(II) and Ba(II) cations shed light on the antidote potential of salinomycin in case of acute/chronic intoxications whose occurrence in the stock farming is possible. The inhibitory effect of new salinomycinate complexes with ions of Mg(II) and Ca(II) on the growth of a set of three Gram-positive

4.2 Experimental

67

bacteria is compared to the properties of the corresponding isostructural biometal(II) complexes of salinomycin and monensin.

4.2 Experimental 4.2.1 Materials Chemically pure sodium salinomycin (SalNa) was obtained from BIOVET Ltd. (Peshtera, Bulgaria). Salinomycinic acid monohydrate (SalH.H2O) was prepared as previously described [25]. Metal(II) salts (MgCl2.6H2O, CaCl2, Sr(NO3)2, Ba(NO3)2), Et4NOH (40% in H2O), solvents of analytical grade and meat peptone agar (MPA) were purchased from local suppliers.

4.2.2 Preparation of complexes 1–4 Compounds 1–4 were prepared using the following general procedure: a solution of SalH.H2O (0.5 mmol, 375.5 mg in MeCN/MeOH) was treated with Et4NOH (40% in H2O, 0.5 mmol, 180 µL) for 15 min to ensure the deprotonation of the carboxylic function. Next, a solution of the metal(II) salt (0.5 mmol in H2O) was added and the corresponding reaction mixtures were stirred for 2 h. Slow concentration of the solutions (24–48 h, at ambient conditions) afforded the precipitation of 1–4 in the form of white amorphous solids. The precipitates were filtered off, washed with MeOH and dried over P2O5. All coordination species possess limited solubility in most of the solvents. The efforts to obtain single crystals of 1–4 suitable for X-ray diffraction using various crystallization procedures felt down, confirming that salinomycin and its metal(II)-bearing derivatives do not easily crystalize [31]. The elemental analysis data and the corresponding yields are presented in Table 4.1.

4.2.3 Methods The infrared (IR) spectral analysis was performed on a Nicolet 6700 FT-IR spectrophotometer (Thermo Scientific, 4000–400 cm−1). Fast atom bombardment-mass spectrometry (FAB-MS) data were obtained using JEOL JMS-700. Solid state nuclear magnetic resonance (NMR) spectra (13C CPMAS) were acquired on a Bruker HD (1H, 500.13 MHz; 13C, 125.76 MHz) with 2.5 mm rotors. The X-ray powder diffraction spectra were measured using Xeuss 3.0 HR Q-Xoom system (Xenocs SA), equipped with a micro-focus Table .: Elemental analysis data and yield of complexes –. Compound Composition MW C, % H, % M, % Yield a

[Mg(Sal)(HO)], 

[Ca(Sal)(HO)], 

[Sr(Sal)(HO)], 

[Ba(Sal)(HO)], 

CHOMg . .b .a a . .b .a .b %

CHOCa . .a .b a . .b .a .b %

CHOSr . .a .b a . .b .a .b %

CHOBa . .a .b a . .b .a .b %

Calculated elemental analysis data; bexperimentally found.

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Genix 3D X-ray source with λ = 1.54 Å (CuKα) and a two-dimensional Eiger2 R 4M detector (Dectris Ltd). The sample was loaded in a powder holder and measured at room temperature (23 ± 1 °C) and in vacuum (pressure below 0.1 mbar), at a sample-to-detector distance of 300 mm, and exposure time of 20 min. An empty cell was used to collect the background data for subtraction. The SAXS data reduction (radial averaging, background subtraction, absolute intensity scaling) was performed using the XSACT software. Transition electron microscopy (TEM) was performed on JEOL JEM-2100 with EDS detection (Oxford Instruments X-max 80T). Microanalysis (C, H) was carried out with a VarioEL V5.18.0 Elemental Analyzer. The metal content of the complexes was determined by AAS on a Perkin Elmer 1100B after sample decomposition with conc. HNО3. The standard stock solution (Merck, 5.5 mM) was used for the preparation of the corresponding working reference solutions after a suitable dilution. The structure of calcium salinomycinate was modelled with Hyperchem software using the AMBER99 force field for an initial geometry optimization [32, 33]. Atomic charges were calculated with the PM3 semi-empirical method [34].

4.2.4 Antimicrobial activity assay The microorganisms Bacillus subtilis (NBIMCC 1709, ATCC 6633), Bacillus cereus (NBIMCC 1085, FDA strain PCI 213) and Micrococcus luteus (NBIMCC 159, FDA strain PCI 1001) were obtained from the Bulgarian National Bank for Industrial Microorganisms and Cell Cultures. The antibacterial properties of SalH.H2O and biometal(II) complexes 1–2 were studied by double layer agar hole diffusion method [35]. The ability of the tested compounds to inhibit the visible growth of target cultures was evaluated as their minimum inhibitory concentration (MIC, μM). The effect of the corresponding magnesium and calcium salts was previously reported [23]. The tested compounds were studied as ethanol solutions in the concentration range from 0.25 μg/mL to 1 mg/mL. The antibacterial assay conditions were as follows: 90 mm petri dishes; 10 mL sterile MPA (first layer); 10 mL inoculated MPA (second layer, 1.5% inoculum adjusted to McFarland 3); 6 mm hole diameter; 20 µL of the studied solutions per hole; control – ethanol. The diameter of the inhibition zones was measured on the 24th hour after an inoculation at 30 °C. All determinations were performed in triplicate and confirmed by three separate experiments. All equipment and culture media were sterile.

4.3 Results and discussion Salinomycinic acid monohydrate (SalH.H2O, Figure 4.1) reacts with alkaline-earth(II) cations in the presence of Et4NOH to form neutral complexes of composition [M(Sal)2(H2O)2] (M = Mg2+, 1; Ca2+, 2; Sr2+, 3; Ba2+, 4). The reactions take place at metalto-ligand molar ratio of 1:1. The complexes 1–4 can also be obtained using sodium salinomycin (SalNa) but in the absence of a weak base. The elemental analysis data confirm that an extraction of sodium ions occurs during the complexation reaction of SalNa with alkaline-earth metal(II) ions [25].

4.3 Results and discussion

69

Figure 4.1: Chemical structure of SalH and carbon numbering.

4.3.1 Spectral properties of complexes 1–4 In the IR spectrum of SalH.H2O (Figure 4.2a) attention should be paid to two particular bands – those at 3500 and 1710 cm−1, respectively. The first one is assigned to ν(OH) and originates from the bound water and the hydroxyl groups of the ligand. The lower energy band is associated with ν(C=O) arising from the carboxylic and the carbonyl functionalities of salinomycinic acid. In the spectra of complexes 1–4 (Figure 4.2b), the stretching vibration of the OH-groups shifts to 3410 cm−1 due to the coordination of water molecules and hydroxyl group(s) to the alkaline-earth metal(II) centers. The band at 1710 cm−1 decreases in intensity compared to the spectrum of the parent antibiotic and is assigned to the unaffected carbonyl group of the ligand. The deprotonation of the carboxylic function is evidenced by the appearance of two new bands at 1560 and 1410 cm−1, assigned to

Figure 4.2: IR spectra of (a) salinomycinic acid monohydrate; (b) complex 1 (taken as a representative).

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νas(COO−) and νs(COO−) of the carboxylate anion. The value of Δν = νas(COO−) – νs(COO−) = 150 cm−1 in the spectra of the complexes refers to a monodentate coordination manner of the carboxylate function to the metal(II) ions [36]. No other significant changes were observed in the IR spectra of 1–4 in the range between 1100 and 500 cm−1. The main molecular ions detected in the FAB-MS spectra of coordination species 1–4 relate to the presence of [M(Sal)]+ ions (1 – 773.5; 2 – 789.5; 3 – 837.4; 4 – 887.4 m/z). In addition, the existence of ions of [M(Sal)2Na]+ (1 – 1546.9; 2 – 1562.9 m/z) and [M2(C42H68O11)2H]+ (2 – 1578.9; 3 – 1674.8; 4 – 1774.8 m/z) at higher m/z ratio confirms the inclusion of alkaline-earth metal cations into the structure of the ligand. Due to the limited solubility of compounds 1–4, their NMR properties were studied in solid state. The comparison of the positions of the carbon signals of 1–4 to those of the parent ligand confirms the deprotonation of the carboxylic function of salinomycin in its alkaline-earth derivatives. The 1C-signal of SalH, observed at 177.6 ppm is shifted downfield to ca. 183 ppm in the spectra of 1–4, while the positions of all other carbon signals remain almost intact (Table 4.2, Figure 4.3). These findings are in good agreement with the data available for the divalent metal(II) complex of salinomycin with Zn(II) ions [25]. Despite the solid state 13C-signals are quite broad in both the spectra of the non-coordinated ligand and its metal(II) complexes, the NMR data corroborate well the IR properties of the studied compounds. From structural point of view, the most interesting of all newly isolated complexes is the Ca(II) salinomycinate 2 since only this compound showed any signs of crystallization during our numerous attempts to grow single crystals, appropriate for X-ray diffraction studies. During the process of slow concentration and when the whole

Table .: Typical C NMR chemical shifts of salinomycinic acid and complexes –. 

C

C CH= CH= *C *C *C CH *C CH CH CH CH CH CH

SalH









. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

4.3 Results and discussion

Figure 4.3:

71

13

C CPMAS of SalH and complexes 1–4.

volume of the solvent was evaporated, the complex vitrified which is a sign for possible formation of crystalline particles in the studied coordination species. To gain deeper insight into the structural properties of complex 2 detailed X-ray powder diffraction and electron microscopy evaluations were performed. The sample exhibits a negligible degree of crystallinity as can be seen from the characteristic diffraction pattern, containing only two peaks of low intensity (Figure 4.4). The electron microscopy of 2 revealed that the complex agglomerates and has almost regular shape tending to form spherical particles with an average size of 198 nm (Figure 4.5) and uniform element distribution.

Figure 4.4: X-ray powder diffraction pattern of complex 2.

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4 Alkaline-earth metal(II) complexes of salinomycin

Figure 4.5: TEM pictures of complex 2 at different magnification levels.

4.3.2 Structure of complexes 1–4 Based on the spectral properties observed for compounds 1–4, the elemental analysis data (Table 4.1) and their composition [M(Sal)2(H2O)2], we assume that alkaline-earth metal(II) complexes of salinomycin are isostructural with those previously reported for Co(II), Ni(II), Cu(II) and Zn(II) coordination species [25]. The metal(II) cations are ligated by two bidentate cis-oriented salinomycinate anions, forming mononuclear complexes. The main coordination sites of the antibiotic are its monodentately bound carboxylate function and the hydroxyl group attached to 28C, both placed at the opposite ends of the antibiotic molecule. The divalent metal ion does not suit the hydrophilic inside of salinomycin. Instead, each ligand cavity is occupied by a water molecule, which coordinates to the metal center and most probably stabilizes the “head-to-tail” folded polyether bone by H-bonds of various origin. The coordination core of 1–4 consists of a [MO6] unit, with a metal center participating in two ionic and four donor-acceptor bonds. The geometry of the internal coordination sphere can be described as a distorted octahedron, where the antibiotic anions are placed in the equatorial plane, and the water molecules occupy the additional axial positions. The proposed general arrangement of the donor atoms in 1–4 (Figure 4.6) resembles that observed for already reported monensinate and salinomycinate coordination species with divalent metal cations [23–25].

Figure 4.6: Structure of bis-salinomycinate alkaline-earth complexes 1–4. Colour scheme: C – grey, O – red, alkaline-earth metal ion – violet (hydrogens are omitted for clarity).

4.3 Results and discussion

73

To get a deeper insight into the structural features of 1–4, we constructed a model of [Ca(Sal)2(H2O)2] based on the crystallographic data available for monensin complexes with Mg(II), Ca(II), Ni(II) and Zn(II) ions [23, 24]. A number of restrictions were imposed knowing the monensin complexation pattern. The crystal structure of sodium salinomycinate served as a source for the initial antibiotic conformation [31]. The geometry of the proposed structure of the calcium complex was optimized with AMBER99 force field [33]. Possibly, the water ligands participate in the formation of donor-acceptor bonds with the metal(II) center by the lone electron pairs of the oxygen atoms. In addition, the most probable hydrogen bonding pattern includes water engagement in intramolecular H-bonds with the carbonyl group at position 11C and with the hydroxyl group attached to 20C into the molecule of salinomycin. It seems that the origin of the hydrogen bonds formed affects significantly the model stability, since other possible constructs lead to unfolding of the ligand and exclude the binding of water to the metal(II) center. The salinomycin carboxylates tend to deviate from the equatorial plane of the modelled calcium complex compared to the isostructural monensinates, bearing divalent metal ions. This observation could be explained as a consequence of the absence of a primary alcohol group in salinomycin, and in such a way the tertiary alcohol substituent at 28C remains the only possibility for complex formation together with the carboxylate function at the opposite end of the ligand. On the other hand, the contribution of the spiro ring system of salinomycin along with its long carbon backbone cannot be ignored due to the possible impact on the ligand flexibility. Thus, without any reliable crystallographic information on salinomycinic acid monohydrate or its bis-complexes, it is highly speculative to define the exact conformation of salinomycin in its coordination species with alkaline-earth metal(II) cations.

4.3.3 Antimicrobial activity of complexes 1–2 The antibacterial properties of SalH.H2O and the biometal(II)-containing derivatives 1–2 were evaluated towards a panel of three Gram-positive bacteria, namely B. subtilis, B. cereus and M. luteus. The activity of the tested compounds is expressed in terms of the minimum inhibitory concentration (MIC), which is defined as the lowest concentration, leading to a visible inhibition of the bacterial growth. At MIC levels the size of the inhibition zone diameter was 6.9 ± 0.9 mm compared to the control (6.0 mm), where no activity is present. The data reveal that salinomycinic acid is a relatively non-toxic agent with values of MIC 42.6 µM (B. subtilis, M. luteus) and 21.3 µM (B. cereus). Both complexes 1 and 2 are more effective, inhibiting the visible growth of the microorganisms at 10 µM (B. subtilis, B. cereus) and 5 µM (M. luteus). The coordination of biometal Mg(II) and Ca(II) ions leads to a 2–8-fold improvement of the antibacterial activity of the parent antibiotic depending on the treated microorganism. Looking to the set of the isostructural

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4 Alkaline-earth metal(II) complexes of salinomycin

biometal(II) complexes of monensin and salinomycin [23–25] (Figure 4.7), it can be concluded, that monensic acid is more toxic compared to salinomycinic acid, and Mg(II) and Ca(II) monensinates are the most promising therapeutics against the tested Gram-positive microbial strains with MIC of 0.7 µM (B. cereus) and 1.4 µM (B. subtilis, M. luteus). In summary, the inclusion of biometal(II) ions into the structure of polyether ionophores affects the activity of the parent compound differently. The efficacy of the complexes, obviously, depends on various factors like the origin of the target cultures, the cation-ligand combination, the inhibitory properties of the particular antibiotic, etc. Тhe overview of the antibacterial potential of the metal(II)-bearing polyether ionophores reveals that, based on a limited number of biological tests, the tendencies in their activity cannot be simply outlined. To derive a plausible conclusion on the intimate mechanism of action of biometal(II) coordination species of monensin and salinomycin, a comprehensive microbiological study has to be designed.

Figure 4.7: Minimum inhibitory concentration (MIC) of polyether ionophores and their biometal(II) complexes (upper – salinomycin, bottom – monensin).

References

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The Sr(II) and Ba(II) coordination species 3–4 were not included into the antibacterial assay against Gram-positive microorganisms. The main reason relates to the fact that in animals Sr(II) can replace Ca(II) (much easier in young individuals) and Ba(II) is known for its renal toxicity [37, 38]. Due to the negative effect of these metal cations in hosts, their complexes cannot be treated as biologically active and the evaluation of the antimicrobial potency does not possess any practical merits. However, the binding of these cations as neutral complex species is more intriguing, because this finding points out possible application of salinomycin as an antidote, and may lay ground for future studies in the field.

4.4 Conclusions New bis-salinomycinates of alkaline-earth metal ions were prepared and their structure was characterized with a number of spectroscopic methods. The complexes are of general composition [M(Sal)2(H2O)2] (M = Mg2+, Ca2+, Sr2+, Ba2+) with a metal(II) central ion positioned in an octahedral environment. The Mg(II) and Ca(II) complexes improve the antibacterial activity of the antibiotic, while the coordination of Sr(II) and Ba(II) ions may attract the attention in the case of heavy metal intoxications in stock farming. The reported findings will be useful for further studies on the coordination behavior of alkaline-earth metal(II) cations and their potential effect on the structural diversity and biological efficacy of metal(II)-based modifications of clinically used veterinary therapeutics.

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10. Antonenko YN, Rokitskaya TI, Huczyński A. Electrogenic and nonelectrogenic ion fluxes across lipid and mitochondrial membranes mediated by monensin and monensin ethyl ester. Biochim Biophys Acta – Biomembr 2015;1848:995–1004. 11. Kevin DA II, Meujo DA, Hamann MT. Polyether ionophores: broad-spectrum and promising biologically active molecules for the control of drug resistant bacteria and parasites. Expet Opin Drug Discov 2009;4:109–46. 12. Rutkowski J, Brzezinski B. Structures and properties of naturally occurring polyether antibiotics. BioMed Res Int 2013;2013:162513. 13. D’Alessandro S, Corbett Y, Ilboudo DP, Misiano P, Dahiya N, Abay SM, et al. Salinomycin and other ionophores as a new class of antimalarial drugs with transmission-blocking activity. Antimicrob Agents Chemother 2015;59:5135–44. 14. Gupta P, Onder TT, Jiang G, Tao K, Kuperwasser C, Weinberg RA, et al. Identification of selective inhibitors of cancer stem cells by high-throughput screening. Cell 2009;138:645–59. 15. Dewangan J, Srivastava S, Rath SK. Salinomycin: a new paradigm in cancer therapy. Tumor Biol 2017;39:1–12. 16. Huczyński A, Janczak J, Antoszczak M, Wietrzyk J, Maj E, Brzezinski B. Antiproliferative activity of salinomycin and its derivatives. Bioorg Med Chem Lett 2012;22:7146–50. 17. Huczyński A, Antoszczak M, Kleczewska N, Lewowska M, Maj E, Stefańska J, et al. Synthesis and biological activity of salinomycin conjugates with floxuridine. Eur J Med Chem 2015;93:33–41. 18. Steverding D, Antoszczak M, Huczyński A. In vitro activity of salinomycin and monensin derivatives against Trypanosoma brucei. Parasites Vectors 2016;9:409. 19. Antoszczak M, Huczyński A. Bioconjugation of ionophore antibiotics: a way to obtain hybrids with potent biological activity. Mini-Reviews Org Chem 2017;14:258–71. 20. Antoszczak M, Urbaniak A, Delgado M, Maj E, Borgström B, Wietrzyk J, et al. Biological activity of doubly modified salinomycin analogs – evaluation in vitro and ex vivo. Eur J Med Chem 2018;156: 510–23. 21. Antoszczak M, Huczyński A. Salinomycin and its derivatives – a new class of multiple-targeted “magic bullets”. Eur J Med Chem 2019;176:208–27. 22. Dorkov P, Pantcheva IN, Sheldrick WS, Mayer-Figge H, Petrova R, Mitewa M. Synthesis, structure and antimicrobial activity of manganese(II) and cobalt(II) complexes of the polyether ionophore antibiotic sodium monensin A. J Inorg Biochem 2008;102:26–32. 23. Pantcheva IN, Zhorova R, Mitewa M, Simova S, Mayer-Figge H, Sheldrick WS. First solid state alkaline-earth complexes of monensic acid A (MonH): crystal structure of [M(Mon)2(H2O)2] (M = Mg, Ca), spectral properties and cytotoxicity against aerobic Gram-positive bacteria. Biometals 2010; 23:59–70. 24. Pantcheva IN, Ivanova J, Zhorova R, Mitewa M, Simova S, Mayer-Figge H, et al. Nickel(II) and zinc(II) dimonensinates: single crystal X-ray structure, spectral properties and bactericidal activity. Inorg Chim Acta 2010;363:1879–86. 25. Ivanova J, Pantcheva IN, Zhorova R, Momekov G, Simova S, Stoyanova R, et al. Synthesis, spectral properties, antibacterial and antitumor activity of salinomycin complexes with the transition metal ions Co(II), Ni(II), Cu(II) and Zn(II). J Chem Chem Eng 2012;6:551–62. 26. Franz KJ, Metzler-Nolte N. Introduction: metals in medicine. Chem Rev 2019;119:727–9. 27. Englinger B, Pirker C, Heffeter P, Terenzi A, Kowol CR, Keppler BK, et al. Metal drugs and the anticancer immune response. Chem Rev 2019;119:1519–624. 28. Alexandrova RI, Alexandrov M, Miloshev G, Georgieva M, Pantcheva IN, Mitewa MI. Cytostatic and cytotoxic properties of monensic acid and its biometal(II) complexes against human tumor/nontumor cell lines. Cent Eur J Chem 2012;10:1464–74.

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Mutiu Sowemimo and Adeleke Adeniyi*

5 Synthesis and characterization of alkaloid derived hydrazones and their metal (II) complexes Abstract: Alkaloids have been known overtime to have medicinal uses. Exploring alkaloid derived hydrazones and their complexes as potential therapeutic agents with a view to improving the medicinal uses of alkaloids are imperative. 1,8-dichloroacridone hydrazone hydrochloride, 1-chloro pilocarpine nitrate-3-chlorophenyl hydrazone and 1-phenethyl-4-piperidone formyl hydrazone ligands were synthesized via Wolff–Kishner condensation reaction. Five metal (II) complexes of cobalt, nickel and manganese were prepared by stirring the ligands with the respective metal salts. The ligands and complexes were characterized using elemental analyses, molar conductivity, FTIR, 1H and 13C nmr, UV–Vis spectra, melting point and solubility. Antimicrobial activities of the ligands and their complexes were tested against four bacteria (Staphylococcus aureus, Streptococcus faecalis Escherichia coli, Salmonella paratyphimurium) and a fungus (Candida albican). The molar conductance values indicate that they are 1:1 and 1:2 type electrolytes while the elemental analyses of the complexes reveals a 1:1 metal to ligand stoichiometry. The relevant IR bands suggest coordination is through the C=N, C=O, N=C–O and C–N groups. Both 1H and 13C nmr corroborated the elemental analysis while the UV–Vis reveals intra-ligand charge transfer while the complexes exhibited the expected metal transitions bands. A proposed octahedral geometry is supported by spectral data. Only 1, 8-dichloroacridonehydrazone hydrochloride ligand was found to be active against all the organisms. Cobalt and nickel complexes of 1,8-dichloroacridonehydrazone hydrochloride were active against S. paratyphimurium and S. aureus, respectively, while cobalt complex of 1-chloropilocarpinenitrate-3-chlorophenylhydrazone was active against S. faecalis and S. paratyphimurium. The minimum inhibitory concentrations (MIC) were also recorded. Keywords: alkaloids; antimicrobial; cobalt; hydrazones; manganese; nickel.

*Corresponding author: Adeleke Adeniyi, Department of Chemistry, Lagos State University, Ojo, Lagos, Nigeria, E-mail: [email protected] Mutiu Sowemimo, Department of Chemistry, Lagos State University, Ojo, Lagos, Nigeria, E-mail: [email protected] As per De Gruyter’s policy this article has previously been published in the journal Physical Sciences Reviews. Please cite as: M. Sowemimo and A. Adeniyi “Synthesis and characterization of alkaloid derived hydrazones and their metal (II) complexes” Physical Sciences Reviews [Online] 2022. DOI: 10.1515/psr-2021-0163 | https://doi.org/10.1515/9783110783629-005

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5.1 Introduction Hydrazones like alkaloids are very useful medicine in treating ailments such as cancer, tuberculosis, malaria and eye defect. Hydrazones are related to ketones and aldehydes, they belong to a class of organic compounds with the structure, R1R2C=NNH2 [1]. Hydrazones have recorded a great deal of interest from researchers due to their various biological and pharmacological characteristics and their chelating ability. In addition, their structural flexibility and wide range of applications have also been investigated. Findings on hydrazone anti-proliferative behavior, its cytotoxic behaviour and receptor tyrosine kinase behavior were among the reported usefulness of hydrazones [2]. The alkaloids from which hydrazones were derived from in this study are acridone, 1-phenethyl-4-piperidone and pilocarpine alkaloids. Acridone is an aromatic biological compound containing carbonyl and imine groups attached at 9th and 10th positions respectively. It has a lot of pharmacological activities such as antimalaria, anticancer and antiviral [3]. Many natural synthetic acridone derivatives are known to possess potent antimalarial property against chloroquine-susceptible HB3 and the chloroquine resistant W2 clones of Plasmodium falciparum [4]. Acridone can be isolated from plants; a good example is Caledonian plants from which three acridone alkaloids (Geijera balansae, Sarcomelicopine glauca and Sarcomelicopine dogniensis) were reported to have been isolated [5]. Acridone was also reported to have been isolated from Hartley (Rutaceae) plant. The plant showed the broadest spectrum of in vivo antineoplastic activity. In recent times, chlorospermine B (an acridone alkaloid) was isolated from stem bark of Glycosmis. It was established that Chlorosperma possess significant inhibitory property against dual-specificity tyrosine phosphorylation regulated kinase 1A (DYRK1A) [5]. Pilocarpine is a parasympathomimetic alkaloid obtained from the leaves of tropical South American shrubs from the genus Pilocarpus [6, 7]. It is an imidazole alkaloid with useful pharmacological activity. It has been used for the treatment of glaucoma and dry mouth. It has also been reported to be useful in the activation of Muscarinic 3 Receptor (M3R). The effect of Muscarinic 3 Receptor (M3R) is experienced in certain areas of the nervous system and many endocrine and exocrine glands, performing a principal role in hormone secretion [8]. Its uses have been reported in the literature that pilocarpine affects the central nervous system. It is a direct cholinergic; stimulating the parasympathetic system and thereby affecting the bladder, tear ducts, salivary and sweat glands. It has been used for the treatment of xerostomy, mainly in patients who have been subjected to radiotherapy in the head and neck area [8–11]. Piperidones are a class of chemical compounds sharing the piperidine alkaloid skeleton. They are known to exhibit varied biological properties such as antimicrobial, antioxidant, antitumor, cytotoxic, analgesic, anticancer and anti-HIV [12].

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Hydrazone metal complexes are useful in the treatment of different diseases like tuberculosis, malaria, hypertension and cancer [13]. Hydrazone are known to form stable chelates with transition metals that are present in cells, this property is what is attributed to their biological activities. Literature survey reveals that alkaloid derived hydrazones and their complexes have recorded little attention [13], while there are none reported for acridone, 1-phenethyl-4-piperidone and pilocarpine alkaloid derived hydrazones respectively. The aim of the study is to synthesize and characterize cobalt, nickel and manganese complexes of acridone, piperidone and pilocarpine hydrazones and to investigate their antimicrobial activity against four bacteria (Staphylococcus aureus, Streptococcus faecalis, Escherichia coli, Salmonella paratyphimurium) and a fungus (Candida albican).

5.2 Materials and methods 5.2.1 Chemicals and reagents All the chemicals were purchased from Sigma-Aldrich.

5.2.2 Synthesis of ligands The alkaloids, acridone, pilocarpine and piperidone were condensed under reflux with 3-chlorophenyl hydrazine and formyl hydrazide using methanol medium in the presence of acetic acid (catalytic amount) to give the respective hydrazones [14, 15]. 5.2.2.1 1, 8-Dichloro acridone hydrazone hydrochloride (ACH): A total of 20 mmol of acridone was refluxed with 40 mmol of 3-cholorophenyl hydrochloride hydrazine for 72 h in methanol with 3 ml of acetic acid [2, 14–17]. The resulting light yellow precipitate was thoroughly washed with methanol and dried using silica gel in vacuo [14]. Its solubility was determined and recrystallized using tetrachloromethane [16]. The reaction is summarized as follows: C13 H9 NO + 2C6 H7 ClN2 .HCl → C13 H11 N5 Cl2 .HCl(ACH) + 2C6 H6 + H2 O

5.2.2.2 1-Chloro pilocarpine nitrate-3-chlorophenyl hydrazone (PILCH): Equimolar amounts (20 mmol) of pilocarpine nitrate and 3-cholorophenyl hydrochloride hydrazine were refluxed for 72 h in methanol with 3 ml of acetic acid [14–17]. The resulting light brown precipitate was thoroughly washed with methanol and dried in vacuo using silica gel [14]. Its solubility was determined and recrystallized using ethyl acetate [16]. The reaction is summarized as follows: C11 H16 N2 O2 .HNO3 + C6 H7 ClN2 .HCl → C17 H19 N4 Cl2 O.HNO3 (PILCH) + 2H2 O

5.2.2.3 1-Phenethylpiperidone formyl hydrazone (PIPF): 20 mmol of 1-phenethylpiperidone was refluxed with 10 mmol of formyl hydrazide for 72 h in methanol [14–16]. The resulting light brown

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precipitate was thoroughly washed with methanol and dried in vacuo using silica gel [15]. Its solubility was determined and recrystallized using ethyl acetate [16]. The reaction is summarized as follows: C13 H17 NO + HCONHNH2 → C14 H19 N3 O(PIPF) + H2 O

5.2.3 Synthesis of the metal complexes Equimolar amounts of the ligands were stirred with hydrated metal chloride salts (MnCl2.4H2O), (CoCl2.6H2O) and (NiCl2.6H2O), respectively, for ten hours in methanol, the resulting precipitate was washed with methanol, dried in vacuum over silica gel. The solubility of the synthesized complexes were determined and then recrystallized with dichloromethane as solvent [16–21]. The reactions are summarized as follows: MnCl2 .4H2 O + ACH → [Mn(ACH)(H2 O)]Cl2 .HCl. NiCl2 .6H2 O + ACH → [Ni(ACH)(Cl)(H2 O)]Cl.HCl. CoCl2 .6H2 O + ACH → [Co(ACH)(Cl)(H2 O)]Cl.HCl. CoCl2 .6H2 O + PILCH → [Co(PILCH)(Cl)(H2 O)]Cl.HNO3 . NiCl2 .6H2 O + PIPF → [Ni(PIPF)(H2 O)2]Cl2 .

5.2.4 Physicochemical measurements The melting points of the compounds were determined with a Griffin melting point device. Jenway 4510 conductivity meter and PG instrument T80 UV–Vis spectrometer at the Department of Chemistry, Lagos State University, Ojo, Nigeria, were used to measure the conductance and electronic data of the compounds respectively. FTIR, NMR and CHN analyses were carried at the School of Chemistry, University of Witwatersrand, South Africa. For CHN, Elementar, Vario EL cube model was used; while both 1H and 13 C spectra were recorded using a Bruker Avance 500 MHz spectrometer at 500 MHz for 1H and 126 MHz for 13C at 27 °C in DMSO-d6. Whereas, the FTIR spectra of the compounds were recorded neat using a Bruker TENSOR 27 single-channel infrared spectrometer.

5.2.5 Antimicrobial screening The synthesized ligands and complexes were tested against four bacteria (S. aureus, S. faecalis, E. coli and S. paratyphimurium) and a fungus (C. albican) using Agar well diffusion method using 3000 ppm stock solution of the respective compounds [22, 23]. The antimicrobial screening was done at the Department of Biological Sciences, Covenant University, Ota, Nigeria.

5.3 Results and discussion The representative spectra for FTIR, UV–Vis, 1H NMR and 13C NMR of the compounds are shown Figures 5.1–5.4. ACH: Colour, light yellow; melting point, 230–233 °C; yield, 70%; IR (cm−1), 3428 m (N–H), 3272.17 m (ν OH),1633.03 m (ν C–N), 1547.52 s (ν C=N), 1343.65 s (δ N–N); 1H NMR (DMSO-d6, ppm), 7.7059 (2H, d, azomethine protons), 7.50–7.72 (5H, m, aromatic ring protons), 7.55 (1H, t, amide proton), 7.50 (2H, s, imine proton), 3.50 (1H, s, acridone

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83

Figure 5.1: IR spectrum of ACH.

Figure 5.2: UV–Vis spectrum of Co(PILCH).

proton); 13C NMR (DMSO-d6, ppm) 177.27 (azomethine carbon C=NNH2), 117–141.37 (aromatic carbon); UV–Vis (nm), 205, 980; conductance (Ω−1 cm2 mol−1), 33; elemental analysis, experimental (calcd.) %, C, 43.27 (40.00); H, 4.75 (3.90); N, 15.98 (18.27); antimicrobial test: active against all five microorganisms, MIC (ppm), 187.50–375.00. The compound has poor solubility in most organic solvents [17]. The conductance value shows that it is a non electrolyte in agreement with earlier reported conductivity values of some organic ligands [19, 24]. The electronic spectra of ACH shows bands in

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Figure 5.3: 1H-NMR spectrum of PIPF.

Figure 5.4:

13

C NMR spectrum of PILCH.

5.3 Results and discussion

85

the regions of 205 nm and 980 nm assigned to both σ → π* transition and charge transfer (CT) band. This agrees with earlier reported spectrum of octaaza macrocyclic hydrazone [13]. The significant bands in the FTIR spectrum of ACH showed a medium band at 3428 cm−1 due to ν N–H, 3272.17 cm−1 due to ν OH possibly of methanol. The medium band at 1633.03 cm−1 is assigned to C–N stretch, whereas, 1547.52 cm−1 is due to ν C=N and 1343.65 cm−1 is ascribed to δ N–N respectively [13, 24]. The 1H NMR spectrum of ACH exhibits signals due to a total of eleven protons which have been properly assigned. In addition, the 13C NMR spectrum of ACH displayed a total of thirteen carbon signals ascribed mainly to azomethine and aromatic carbons. These chemical shift patterns are similar to those of earlier reported hydrazones [2, 25].The observed signals corroborate the CHN results. The microorganisms were susceptible to ACH, with MIC values of 375 ppm for S. aureus, and 187.50 ppm for S. faecalis, E. coli, S. paratyphimurium and C. albican respectively. Mn(ACH): Colour: dark brown; melting point, 228–229 °C; yield, 60.40%; IR (cm−1), 3395 m, br (ν N-H), 3200 w (ν OH), 1625 s (ν C–N), 1538 sh (ν C=N), 1325 sh (δ N–N); UV– Vis (nm), 205, 250, 380, 400, 510, 820; conductance (Ω−1 cm2 mol−1 in methanol), 317; elemental analysis, experimental (calcd.) %, C, 39.45 (34.28); H, 4.48 (4.17); N, 15.13 (15.21); antimicrobial test, compound is inactive against all five organisms. Co(ACH): Colour: dark brown; melting point, 217–219 °C; yield, 36.45%; IR (cm−1) 3380 m (ν N–H), 2900 m (ν OH), 1630 w (ν C–N), 1540 w (ν C=N), 1330 w (δ N-N); UV–Vis (nm), 220, 280, 550, 600, 850; conductance (Ω−1 cm2 mol−1 in methanol), 85.60; elemental analysis: experimental (calcd.) %, C, 38.14 (33.98); H, 4.20 (4.57); N: 13.14 (15.25); antimicrobial test: active against S. paratyphimurium and C. albican with MIC of 187.50 ppm. Ni(ACH): Colour, light brown; melting point, 220–221 °C; yield, 40.24%; IR (cm−1) 3424 s (ν N–H), 2980 sh (ν OH), 1620 w (ν C–N), 1545 w (ν C=N), 1340 w (δ N–N); UV–Vis (nm), 220, 280, 420, 450, 700; conductance (Ω−1 cm2 mol−1 in methanol), 85.60; elemental analysis, experimental (calcd.) %, C, 37.45 (33.91); H, 4.30 (4.58); N, 14.05 (15.25); antimicrobial test: the compound is active, against S. aureus (MIC, 375 ppm). The colours of the ACH complexes are not unexpected of metal (II) transition metal compounds [26]. The CHN elemental analyses are in conformity with the molecular formulae proposed for the complexes. Similar metal to ligand stoichiometries have been reported before [14]. The molar conductance of the complexes in methanol suggest a 1:1 electrolyte for the cobalt and nickel complexes, while the manganese complex is 1:2 type electrolyte [12, 17]. The bands in the UV–Vis spectra in the region 205– 280 nm are for σ–π* and π–π* transitions respectively. Three visible absorption bands (nm), 400, 510, 820 were observed for Mn(ACH) which is ascribed to charge transfer [26]. Similarly three transitions were observed for Co(ACH), 4T1g (F) → T2g (F), 4T1g (F) → 4A2g (F) and 4T1g (F) → 4T1g (P); while the transitions observed for Ni(ACH) are 3 A2g (F) → 3T2g (F), 3A2g (F) → 3T1g (F) and 3A2g (F) → 3T1g (P) respectively. Similar visible transition spectral behaviors have been reported earlier [25, 27, 28].

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The IR spectra of ACH complexes show a negative shift of wave number of N–H and C–N groups compared to the ligand, ACH. This suggest the involvement these groups in ligand-metal bonding in the complexes [1, 15, 16, 21]. Co(ACH) was active towards the bacteria, S. paratyphimurium and fungus, C. albican whereas, the bacteria, S. aureus was found susceptible to Ni(ACH). The MIC values were 187.50 ppm for Co(ACH) and 375 ppm for Ni(ACH) respectively. These results suggest that ACH has a more broad-based potential as an antimicrobial agent [12, 21, 28]. PIPF: Colour, light brown; melting point, 120–123 °C; yield, 41.82%; IR (cm−1) 3441 m (ν N–H), 3030 m (ν OH), 1745 sh (ν C=O), 1641 s (ν C–N), 1459 s (ν C=N), 1357s (δ N-N); UV–Vis (nm), 198, 310, 400; conductance (Ω−1 cm2 mol−1 in methanol), 0.36; 1H NMR (DMSO-d6 ppm), 8.39 (1H, s, amide protons NCHO), 8.20 (1H, s, azomethine proton C=NNH), 7.17–7.29 (5H, m, aromatic ring), 2.58–2.76 (4H, m, ethyl protons in the piperidone ring), 2.36–2.46 (8H, m, piperidone ring protons); 13C NMR (DMSO-d6 ppm), 163 (amide carbon NHCHO),126–140 (azomethine carbon, C=NNH), 126–140 (aromatic carbons), 33.51–59.50 (piperidone ring carbon), 27.80 (aromatic ethyl carbons (CH2)2); elemental analysis, experimental (calcd.) %, C, 75.03 (68.01); H, 8.67 (8.50); N, 13.67 (17.04); antimicrobial test: PIPF was found to be inactive against the five organisms. The elemental analyses results were corroborated by the carbon-13 and proton nmr spectra. The bands in the UV–Vis spectrum (nm) of 198, 310 and 400 are ascribed to σ → π*, π → π* transitions and intra-ligand charge transfer respectively [14]. The conductance, 0.36 Ω−1 cm2 mol−1 shows it is a non-electrolyte [14, 18]. The IR spectrum of PIPF showed absorption bands, that are attributed to ν NH, ν OH, ν C=O, ν C–N, ν C=N and δ N–N groups respectively [14, 16]. The total protons recorded for the ligand was nineteen. In addition, the 13C NMR spectrum of PIPF displayed four characteristic peaks with a total of fourteen carbons recorded. The antimicrobial analysis result revealed that none of the organisms are susceptible to PIPF [12]. Ni(PIPF): Colour, dark brown; melting point, 192–195 °C; yield, 54.90%. IR (cm−1) 3363 m, br (ν N–H), 2950 m (ν OH), 1710 sh (ν C=O), 1652.86 s (ν C–N), 1451 m (ν C=N), 1413.46 m (δ N–N); UV–Vis (nm), 220, 300, 420, 480, 600; conductance (Ω−1 cm2 mol−1 in methanol), 196.70; elemental analysis, experimental (calcd.) %, C, 40.21 (40.75); H: 6.14 (6.16) N: 0.58 (7.04); Antimicrobial test, Inactive against the five microorganisms. The elemental analyses reveal the composition of the complex as 1:1 metal: ligand stoichiometry. The electronic spectrum shows bands (nm) at 220 and 300 are assigned to σ → π*, π → π* transitions. The electronic bands in the visible region are (nm), 420, 480 and 600 are assigned respectively to 3A2g (F) → 3T2g (F), 3A2g (F) → 3T1g (F) and 3A2g (F) → 3T1g (P) transitions. Nickel complexes are expected to show these three d → d transitions [27]. Conductance was 196.7 Ω−1 cm2 mol−1 revealing a 1:2 electrolyte ratio. This is in agreement with earlier reported hydrazone complexes [14, 19]. The assignments of the significant bands in the IR spectrum of the complex were made by comparison with those of the ligands and related compounds. The ν (N–H) and ν (C=O)

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bands at 3441 cm−1 and 1745 cm−1 respectively in the ligand were lowered in Ni(PIPF) complex owing to the probable formation of nickel–nitrogen and nickel–oxygen bonds. The other groups are uncoordinated in the complex [15, 16, 19]. The antimicrobial screening results revealed that none of the organisms are susceptible to Ni(PIPF). PILCH: Colour, light brown; melting point, 150–153 °C; yield, 77.17%; IR (cm−1), 3422 s (ν N–H), 3216 s (ν OH), 1768.14 s (ν C–N), 1491.72 s (ν C=N), 1391.05 s (δ N–N), 1570 sh (ν C–O–C); UV–Vis (nm), 195, 207, 720; conductance (Ω−1 cm2 mol−1 in methanol), 66; 1 H NMR (DMSO-d6 ppm), 6.93–7.59 (4H, m, aromatic protons), 3.99 (3H, m, imidazole methyl protons, NCH3), 3.79 (1H, s, azomethine proton, C=NNH), 1.0–4.25 (11H, m, furan protons),13C NMR (DMSO-d6 ppm), 163 (azomethine carbon, C=NNH), 113–145 (aromatic carbons), 38–70 (furan carbons), 18 (imidazole methyl carbon, NCH3), 15–27 (Furan ethyl carbon CH2CH3); Elemental analysis: experimental (calcd.) % C: 44.12 (46.80); H: 5.90 (5.97); N: 16.67 (16.09); antimicrobial test: the compound is inactive against the tested microorganisms. The conductance of PILCH in methanol reveals that it is a nonelectrolyte [13, 18]. UV–Vis spectrum bands are assigned respectively to σ → π*, π → π* transitions and intra-ligand charge transfer, which agrees with those reported for similar hydrazones [13]. The IR spectrum of PILCH showed absorption bands which are attributed to (ν N– H), (ν OH), (ν C=O), (ν C–N), (ν N=C–O), (ν C=N) and (δ N–N) groups respectively [25]. A total of nineteen protons were recorded for the ligand. In addition, the 13C NMR spectrum of PILCH displayed a total of seventeen carbon signals. This chemical shift pattern is similar to hydrazones reported earlier [25, 27]. The antimicrobial analysis result revealed that none of the organisms is susceptible to PILCH. Co(PILCH): Colour, dark brown, melting point, 105–107 °C; yield, 35.60%; IR (cm−1), 3380 m (ν N–H), 2900 m (ν OH), 1630 w (ν C–N), 1540 w (ν C=N), 1330 w (δ N–N); UV–Vis (nm), 205,280,570,610,850; conductance (Ω−1 cm2 mol−1 in methanol), 128; elemental analysis, experimental (calcd.) %, C, 28.62 (29.04); H, 6.16 (5.44); N, 9.50 (9.68); antimicrobial test, active only against, S. faecalis (MIC, 187.50 ppm). The elemental analyses revealed the composition of the complex as 1:1 metal: ligand stoichiometry [16]. The UV bands at 205 nm and 280 nm are assigned to σ → π*, π → π* transitions respectively. Nevertheless, the bands at the visible region (nm) at 570, 610 and 850 are assigned respectively to 4T1g (F) → 4T2g (F), 4T1g (F) → 4A2g (F) and 4 T1g (F) → 4T1g (P), these bands are as expected for cobalt (II) octahedral complexes [28]. The conductance reveals 1:2 electrolyte ratio [14, 19]. Similarly, the relevant IR bands recorded for the complex revealed coordination of the ligand with cobalt ion. The IR bands for ν N–H, ν C–N, ν C–O–C and δ N–N respectively were lowered to (cm−1) 3410, 1606.34, 1528 and 1350 in the complex, an indication of ligand to metal coordination [19, 25]. S. faecalis was found susceptible to Co(PILCH) with an MIC value of 187.50 ppm. The antimicrobial analysis revealed that four of the compounds were found active against one or more of the organism(s). One of the ligands (ACH) was found to

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be active against all the organisms (four bacteria (S. aureus, S. faecalis E. coli, S. paratyphimurium) and a fungus (C. albican); Ni(ACH) was found active against only S. aureus; while Co(PILCH) complex was observed to be active against S. faecalis however, its ligand (PILCH) was inactive against any of the organisms. Ni(ACH) complex was found to be more active than its ligand (ACH). The proposed structures of the ligands and complexes are shown Figures 5.5–5.10.

Figure 5.5: Proposed structure of ACH.

Figure 5.6: Proposed structure of the ACH complexes {a, (M = Mn); b, (M = Co, Ni)}.

5.3 Results and discussion

Figure 5.7: Proposed structure of PIPF.

Figure 5.8: Proposed structure of PIPF complex.

Figure 5.9: Proposed structure of PILCH.

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Figure 5.10: Proposed structure of PILCH complex.

5.4 Conclusions Three hydrazones ligands derived from acridone, pilocarpine and piperidone alkaloids and five metal complexes from the ligands were synthesized and characterized by analytical spectral data. Elemental analyses of the ligands and complexes reveal 1:1 metal to ligand stoichiometry. The shift in the stretching frequency bands of the C=N and C–N groups of the ligands to lower bands in the complexes supports the proposed structures of the complexes. Four of the compounds, ACH, Co(ACH), Ni(ACH) and Co(PILCH) were found active against one or more of the microorganisms used in this study (S. aureus, S. faecalis, E. coli, S. paratyphimurium, and C. albican). Ni(ACH) recorded the higher antibacterial activity against S. aureus when compared to its ligand, ACH. The minimum inhibitory concentrations of the active compounds were also recorded. Acknowledgements: The authors express their gratitude to the Department of Chemistry, Lagos State University for providing some of the facilities used for the study. The authors are also grateful to Dr. Olawale Raimi (University of Dundee, Scotland) and Dr. Abdullahi Sobola (Lagos State University, Ojo, Nigeria) for valuable assistance.

References 1. Singh N, Ranjana R, Kumari M, Kumar B. A review of biological activities of hydrazones derivatives. Int. J Pharm Clin Res. 2016;8:162–6. 2. Bhaskar RS, Ladole CA, Salunkhe NG, Barabde JM, Aswar AS. Synthesis, characterization and antimicrobial studies of novel ONO donor hydrazone Schiff base complexes with some divalent metal (II) ions. Arab J Chem 2020;13:6559–67.

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3. Yerragunta V, Reddy ES, Kishore M, Rao HOP, Sadia A, Saba A, et al. A review on acridone derivatives and its importance. Pharma Tutor 2015;3:27–9. 4. Basko LK, Mitaku S. Invitro activities of furoquinoline and acridone alkaloid against plasmodium falciparum. Antimicrob Agents Chemother 1994;38:1169–71. 5. Mandal T, Karmakar S, Kapat A, Dash J. Studies directed towards the synthesis of the acridone family of natural products: total synthesis of acronycines and atalaphyllidines. ACS Omega 2021; 6:27062–9. 6. Dewick PM. Medicinal natural products: a biosynthetic approach. New York: John Wiley & Sons; 1997. 7. Valdez IH, Wolff A, Atkinson JC, Macynski AA, Foz PC. Use of pilocarpine during head and neck radiation therapy to reduce xerostomia and salivary dysfunction. Cancer 1993;71:1848–51. 8. Pronin AN, Wang Q, Slepak VZ. Teaching an old drug new tricks: agonism, antagonism, and biased signaling of pilocarpine through M3 muscarinic acetylcholine receptor. Mol Pharmacol 2017;92: 601–12. 9. Jain N, Verma A, Jain N. Formulation and investigation of pilocarpine hydrochloride niosomal gels for the treatment of glaucoma: intraocular pressure measurement in white albino rabbits. Drug Deliv 2020;27:888–99. 10. Sawaya ACHF, Abreu IN, Andreazza NL, Eberlin MN, Mazzafera P. Pilocarpine and related alkaloids in pilocarpus vahl (Rutaceae). In: Cassiano NM, editor. Alkaloids: properties, applications and pharmacological effects. New York: Nova Science; 2010:63–80 pp. 11. Carlson AB, Kraus GP. Physiology cholinergic receptors. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing.; 2021. PMMID: 30252390. 12. Kumar A, Pavithra G, Renuka N. Piperidone analogs: synthesis and their diverse biological applications. Int Res J Pharmaceut Appl Sci 2012;2:145–54. 13. Sari N, Nartop D, Karci F, Disli A. Novel hydrazone derivatives and their tetracoordinated metal complexes. Asian J Chem 2008;20:1975–85. 14. Xavier AJM, Thakuri M, Marie JM. Synthesis and spectral characterisation of hydrazone based 14-membered octaaza macrocyclic Ni(II) complexes. J Chem Pharmaceut Res 2012;4:986–90. 15. Rai BK, Singh R, Anand P, Singh SK, Amit A. Synthesis, spectral and biocidal studies of Co(II),Ni(II) and Cu(II) complexes of hydrazone Orient. J Chem 2013;29:753–8. 16. Pouralimardan O, Chamayou AC, Janiak C, Hosseini-Monfared H. Hydrazone Schiff basemanganese (II) complexes: synthesis, crystal structure and catalytic reactivity. Inorg Chim Acta 2007;360:1599–608. 17. Adeniyi A, Okedeyi O, Aremu J, Oyedeji O, Sowemimo M. Infrared spectroscopic characterization of calcium and barium hydrazone complexes. Pak J Chem 2013;3:64–70. 18. Wang B, Yang Z, Zhang D, Wang Y. Synthesis, structure, infrared and fluorescence spectra of new rare earth complexes with 6-hydroxy chromone-3-carbaldehyde benzoyl hydrazone. Spectrochim Acta, Part A 2006;63:213–9. 19. Ali M, Wani A, Saleem K. Empirical formulae to molecular structures of metal complexes by molar conductance. Synth React Met-Org Nano-Met Chem 2013;43:1162–70. 20. Kaplanek R, Jakubek M, Rak J, Kejik Z, Havlik M, Dolensky B, et al. Caffeine–hydrazones as anticancer agents with pronounced selectivity toward T-lymphoblastic leukaemia cells. Bioorg Chem 2015;60:19–29. 21. Dhande VV, Badwaik VB, Aswar AS. Hydrazone as complexing agent: synthesis, structural characterization and biological studies of some complexes Rus. J Inorg Chem 2007;52:1206–10. 22. Massoud R, Saffari H, Massoud A, Moteian MY. Screening methods for assessment of antibacterial activity in nature. In: Proceedings of the 4th International Conference on Applied Researches in Science and Engineering, Institution of Engineering and Technology of London, Belgium, Oct. 19, 2019. Vrije Universiteit Brusel; 2020:1–11 pp.

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23. Balouiri M, Sadiki M, Ibnsouda SK. Methods for in vitro evaluating antimicrobial activity: a review. J Pharm Anal 2016;6:71–9. 24. Sathyadevi P, Krishnarmoorthy P, Alagesan M, Thanigaimani K, Muthiah PT, Dharmaraj N. Synthesis, crystal structure, electrochemistry and studies on protein binding, antioxidant and biocidal activities of Ni(II) and Co(II) hydrazone complexes. Polyhedron 2012;31:294–306. 25. Anitha C, Sumathi S, Tharmaraj P, Sheela C. Synthesis, characterization, and biological activity of some transition metal complexes derived from novel hydrazone azo Schiff base ligand Inter. J Inorg Chem 2011;2011:493942. 26. Lee JD. Concise inorganic chemistry, 5th ed. New Jersey: Blackwell Science; 1996. 27. Mandewale MC, Kokate S, Thorat B, Sawant S, Yamgar R. Zinc complexes of hydrazone derivatives bearing 3,4-dihydroquinolin-2(1H)-one nucleus as new anti-tubercular agents. Arab J Chem 2019; 12:4479–89. 28. Al-Qahtani SD, Alsoliemy A, Almehmadi SJ, Alkhamis K, Alrefaei AF, Zaky R, et al. Green synthesis for new Co(II), Ni(II), Cu(II) and Cd(II) hydrazone-based complexes; characterization, biological activity and electrical conductance of nano-sized copper sulphate. J Mol Struct 2021;1244:131238.

Joseph C. Oguegbulu*, Abedawn I. Khalaf, Colin J. Suckling, Margaret M. Harnett and William Harnett

6 Lead optimisation efforts on a molecular prototype of the immunomodulatory parasitic protein ES-62 Abstract: The immunomodulatory property of some parasitic helminths is well documented. The glycoprotein ES-62 from the nematode, acanthocheilonema viteae has been found to possess immunomodulatory properties. Two small molecule analogues (SMA’s) of ES-62 (S3 and S5) were found to mimic its immunomodulatory properties in vivo and were active in animal models of allergic, inflammatory and autoimmune diseases. In this work, new efforts were made to further optimise the activities of compound S3 by making small but directed structural changes. A variety of analogues based on the S3 prototype were simulated by making variations at one position and then screened in silico. The best compounds were selected based on predicted physiochemical properties and medicinal chemistry indices and synthesised. Structural elucidation was done via HNMR, LCMS, FTIR and HRESIMS. The predicted properties were evaluated by HPLC method. A total of 11 novel molecules were synthesised and characterised. Significant correlation was obtained between the predicted physicochemical properties and their HPLC retention times (RT) for eight of our novel compounds. This suggests that these compounds may behave in a physiological environment as closely as computationally predicted. This entails, lesser host toxicity while maintaining good or better activities compared to the earlier prototype. They hence provide a good opportunity for development of drugs for immune conditions such as asthma, inflammation and autoimmune diseases. Keywords: allergy; anti-inflammatory; ES-62; immune diseases; in silico screening; lead optimisation.

*Corresponding author: Joseph C. Oguegbulu, Department of Chemical Sciences, Bingham University, PMB 005, Karu, Nasarawa State, Nigeria, E-mail: [email protected]. https:// orcid.org/0000-0003-3787-4628 Abedawn I. Khalaf and Colin J. Suckling, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, UK Margaret M. Harnett, Institute of Infection, Immunity and Inflammation, University of Glasgow, Glasgow G12 8TA, UK William Harnett, Strathclyde Institute of Pharmacy and Biomedical Sciences, University of Strathclyde, Glasgow G4 0RE, UK As per De Gruyter’s policy this article has previously been published in the journal Physical Sciences Reviews. Please cite as: J. C. Oguegbulu, A. I. Khalaf, C. J. Suckling, M. M. Harnett and W. Harnett “Lead optimisation efforts on a molecular prototype of the immunomodulatory parasitic protein ES-62” Physical Sciences Reviews [Online] 2022. DOI: 10.1515/psr-2021-0235 | https://doi.org/10.1515/9783110783629-006

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6.1 Introduction The human immune system exists in a homoeostatic balance. Existing remedies for diseases of the immune system such as anti-inflammatory, anti-rheumatic drugs, corticosteroids, glucocorticoids, as well as immunosuppressing drugs generally aim to manage or treat symptoms of the diseases. Reported side effects associated with most existing immunomodulatory imide drugs (IMiDs) include anaemia, thrombocytopaenia, hepatotoxicity as well as recurrent lymphoma [1]. Furthermore, the approach of immune suppression in drug discovery comes with a high risk of further infection. However, some parasitic worms are able to survive in the blood streams and guts of humans without effect. Their ability to do this is thought to be by modulation of the human immune system such that they are not cleared by the host immune system [2, 3]. Interestingly, they do so without suppressing or leaving the host system vulnerable. One such example of such immunomodulatory worms is the rodent filarial nematode, Acanthocheilonema viteae. Despite the amount of existing knowledge, limited breakthrough has been made in the area of immunogenic drug discovery and a gap exists between the level of available information and available pharmaceutical remedies. The development of novel immunomodulatory drugs to successfully navigate the delicate homoeostatic balance of the immune system remains an area of keen research interest. The discovery and characterisation of the immunomodulatory protein, ES-62 (Figure 6.1) from the parasitic worm, Acanthocheilonema viteae has enabled in-depth understanding of the molecular aspects and mechanisms of immunomodulation of these parasites [3–11]. ES-62 is a tetrameric glycoprotein secreted by the rodent filarial nematode, Acanthocheilonema viteae. It has been found to have a range of immuno-activity including anti-inflammatory, anti-allergy, anti-arthritis, and anti-asthma as well as prophylactic properties [5, 9, 12–16]. The isolated protein is unsuitable as a drug candidate given its large molecular size (circa 240 kDa). Its phosphorylcholine (PC) moiety is found to be responsible for its immune-activity [6, 13]. This active moiety,

Figure 6.1: Representation of the parasitic protein ES-62 showing the immunogenic choline moiety, the zwitterionic phosphate and the bulky part of the protein.

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95

being a nematode specific post translational modification, makes it possible therefore to design and develop synthetic analogues with similar immuno-activity where it was impossible for other active nematode secreted products [12]. Based on this hypothesis, novel PC-based small molecule analogues (SMAs) of ES-62 have been successfully synthesised and evaluated for their ability to mimic the immunomodulatory activity of ES-62. In the development of synthetic analogues, a library of 116 SMAs of ES-62 were synthesised in our previous work [17]. We reported that phosphonates, sulfones, sulfonamides, and carboxamides were used to replace the highly labile PC ester group of ES-62 while a substituted benzene ring was used in place of the peptide backbone to make a library of different series of analogues. The biological activities of the different series of analogues on different biomarkers are well documented [17]. The sulphones and sulphonamides were better candidates than the phosphonates (Figure 6.2) due to the zwitterionic nature of the phosphonate group. The sulfone moiety is used widely in medicinal chemistry to different effects such as in peptidomimetics where they give a transition state by linking amino acid-like components [18], in organic synthesis as activators of alkenes in Michael addition reactions [19]. It has also been described in anti-inflammatory inhibitor molecules [20,21]. These design considerations yielded a generic structure in which there were three major possible points of variation, vis; the substituent on the aromatic ring (position X), the length of the methylene chain (position n), and the nature of the substituted amino group (position Z) (Figure 6.3). The most promising SMA from this library, SMA-11a (named S3) was found to be as effective as ES-62 in some immune conditions, and its mechanism of action was same or analogous to that of ES-62.

A) O X

O P

B) O

NR3

O

X

O S

NR2

O

Figure 6.2: (A) The choline phosphate series showing the zwitterionic phosphate group. (B) The sulphone series.

Y

( )n

Z

X

Figure 6.3: A generic structure showing the points of variation on the prototype.

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6 Molecular prototype of the immunomodulatory parasitic protein ES-62

Structurally, S3 (2-(4-bromobenzylsulfonyl)-N,N-dimethylethanamine) is a 4-bromo substituted aryl sulphone with a dimethyl amino group in position ‘z’ and a two-carbon methylene chain in position ‘n’. In the sulfone series, it was shown that compounds with a two-carbon methylene chain were generally more effective at inhibiting the release of pro-inflammatory cytokines than similar compounds with three-carbon methylene chains. A second promising molecule, SMA-12b (named S5) (Figure 6.4) which was a methyl substituted sulphone with a quaternary amine in position z was tested in experimental arthritis and showed good prophylactic activity by modifying inflammatory response genes [22]. Both S3 and S5 have shown curative and prophylactic activities in models of various immune diseases such as inflammation, chronic asthma, allergies, autoimmune disorders such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) [9, 12, 15, 23–25]. Unexpected findings in the previous work showed that certain SMAs promoted TLR-driven cytokine responses rather than inhibit them. Furthermore, these prototypes were unable to provide protection in in vivo models of inflammatory bowel disease, multiple sclerosis, type I diabetes – autoimmune diseases where other parasitic helminth secretion products are able to provide protection [26]. Although promising lead compounds for an array of immune conditions, there is still some way to go in the development of these PC-based synthetic analogues of ES-62 to credible drug prospects. Their PKPD properties must be improved in efforts to develop new clinical therapies. This, just like all lead optimisation efforts, would require iterative synthetic modifications and medicinal chemistry. In the present work, we aim to further optimise the molecular prototype, S3 in a bid to minimise cellular toxicity, off target activities and improve activity. We focus all modifications on the Z-position of the lead, S3. The use of one variable at a time enables the exhaustive study of the possible outcomes in order to create a clearer understanding of the competing factors. Our previous work showed that shorter chain lengths between the amine and the aromatic ring improved release of pro-

A)

O S

Br

O

B) N

O S

Me

N

O

Figure 6.4: (A) SMA-11a of ES-62 (termed S3) has a bromo substituent on the benzene ring (position X) and a tertiary amine in position Z. (B) SMA-12b of ES-62 (termed S5) has a methyl substituent on the aromatic ring (position X) and a quaternary amine in position Z. Both analogues have a two-carbon chain in position ‘n’ and are the best candidates from a library of ES-62 analogues.

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97

inflammatory cytokines. This was important information in the design of new variants; hence a two-carbon methylene chain in position ‘z’ was maintained, with little experimentation. We select suitable substituted amino groups for this position via in silico screening and synthesise novel S3 variants using the selected amino groups. Furthermore, using chromatographic methods, we attempt to show that synthesised novel compounds will behave in vivo as computationally predicted. The retention of compounds in reverse phase HPLC is governed by their lipophilicities. LogP, which is the logarithm of octanol–water partition coefficient of compounds, is a measure of lipophilicity of molecules [27, 28]. It is an all-important physicochemical parameter in medicinal chemistry which is widely used in modelling biological distribution, evaluating ADMET parameters and offering information about in vivo interactions [29]. Hence HPLC can be used to assess the lipophilicity of compounds, thereby providing relevant information about the compounds’ physicochemical, pharmacokinetic, pharmacodynamic and even toxicological profile [29, 30]. Standardisation is still an issue in this field and the use of biomimetic stationary phases may provide better insight for biological distribution processes. However, we here hypothesise that the extent to which the logP surrogate values of these molecules (obtained from reverse phase HPLC) correlate to the in silico predicted logP values, can provide us with a confidence level regarding their eventual in vivo behaviour in the current absence of biological evaluations.

6.2 Methods 6.2.1 In silico screening A library of 50 novel ES-62 analogues based on S3 prototype with varying amines in position “Z” were virtually screened to predict their physicochemical and ADMET properties using ChemDraw® and “pkCSM”® web respectively. This was done with a view to selecting only the most suitable compounds for synthesis and thus designing novel variants with better properties relative to S3. In silico screening parameters used included predicted lipophilicity (logP; 1–3), molecular weight (300–400 Da), aqueous solubility (logS: −2.0 to −5.0), pKa (4.0–10.0), enzyme metabolism (CYP450), toxicity (AMES, hepatotoxicity) as well as off-target activity (at hERG IC). All SMAs designed with corresponding amines were screened for drug-likeness based on acceptable medicinal chemistry indices [27, 31] and the best were selected. All amines used for simulation and screening are shown below (Figure 6.5).

6.2.2 Synthetic methods Compound 1a (1-bromo-4-((vinylsulfonyl)methyl)benzene) (Figure 6.6) was used as starting material for this work, being the direct precursor to the prototype of interest, S3 (2-(4-bromobenzylsulfonyl)N,N-dimethylethanamine) which was the most promising lead from a library of ES-62 synthetic analogues. The preparation of 1a was according to scheme 1 (Figure 6.7). It acts as the compound’s ‘left

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6 Molecular prototype of the immunomodulatory parasitic protein ES-62

1.

2.

8.

4.

3.

9. NO2

5.

10.

6.

11

7.

12.

13.

18.

19.

25.

26.

H N

15.

14.

20.

21.

27.

28.

41.

47.

17.

23.

22.

29.

35.

34.

16.

36.

48.

30.

37.

43.

42.

49.

24.

32.

31.

38.

39.

44.

45.

33.

40.

46.

50.

Figure 6.5: Fifty novel S3 variants were designed using these amines in position Z of the S3 precursor 1a and screened in silico for suitable physicochemical and PKPD properties.

Figure 6.6: Compound 1a (1-bromo-4-((vinylsulfonyl)methyl)benzene).

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99

hand side’ (LHS) in our design simulation while the amine group serves as the compound’s ‘right hand side’ (RHS). 6.2.2.1 Preparation of starting material (1a) [1-bromo-4-((vinylsulfonyl)methyl)benzene]: 1-Bromo-4((vinylsulfonyl)methyl)benzene was prepared according to scheme 1 (Figure 6.7) by mesylation of the corresponding sulfone precursor 2-(4-bromobenzylsulfonyl)ethanol which was obtained from the Department of Pure and Applied Chemistry Laboratory, University of Strathclyde, Glasgow. It was previously prepared by oxidation of the thioether precursor (2-(4-bromobenzylthio)ethanol), obtained by alkylation of 2-thioethanol using 1-bromo-4-(bromomethyl)benzene. As previously demonstrated [17], mesylation of the sulfone likely yielded a mixture of the vinyl sulfones (1a) and the mesylate (1ab) and this product was used for futher synthesis of SMAs without seperation. There was no need for separation because both 1a and 1ab yielded the same product in the subsequent steps under same or equivalent conditions. The use of a sulphone type molecule (1a) as starting material for synthesis with amines naturally lends itself to an adaptation of a Michael’s addition reaction, given that sulphones are good alkene activators in Michael additions [19]. O

O

S

OH O Br 2-(4-bromobenzylsulfonyl)ethanol

S

MsCl (3 mol. equ.), NEt3 DCM, 00C

OMs O Br 2-(4-bromobenzylsulfonyl)ethyl methanesulfonate

O S O Br Compound 1ab

OMs

+

Beta elimination

-MsOH

O S

O Br 1-bromo-4-((vinylsulfonyl)methyl)benzene Compound 1a

Figure 6.7: Synthesis of compound 1a (1-bromo-4-((vinylsulfonyl)methyl)benzene) which was used as starting material for synthesis of S3 variants. 6.2.2.2 Synthesis of analogues: A one pot method which was an adaptation of the Michael addition reaction was used in the synthesis of new S3 variants. The sulphone acts as the Michael acceptor rather than a conventional carbonyl, while the amine acts as the Michael donor. The general scheme of the reactions is shown in Figure 6.8. However, as with nucleophilic additions, various factors including the size of the substituents on the amino group would affect the speed or possibility of reaction due to steric interactions. Hence, depending on the nature and structure of the amine, the reaction conditions were varied and where necessary, modified. Dichloromethane (DCM) and acetonitrile (anhydrous) were generally used as solvents. While both solvents are aprotic, acetonitrile is more polar and hence was useful for reactions involving fairly polar amines, while DCM offered the advantage of being more volatile and easier to evaporate. Methanol which is a protic polar solvent was used for solubilising purposes in reactions involving otherwise insoluble polar amines.

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6 Molecular prototype of the immunomodulatory parasitic protein ES-62

Figure 6.8: Scheme of addition of selected amines (RHS) to compound 1a (LHS) follows a Michael addition.

6.2.3 HPLC method for LogP determination The HPLC method was used in lieu of biological evaluations, and a correlation between the compounds’ predicted lipophilicities (LogP) and their retention time (RT) in reversed-phase HPLC is a great indication of their corresponding in vivo behaviour. The lipophilicity of molecules measured as logP provides a good deal of pharmacokinetic and pharmacodynamic information [29, 30]. Reverse-phase HPLC RT was used to assess the lipophilicity of the compounds. An adaptation of the method of Giaginis and Tsantili-Kakoulidou (2008) [30] using PORO ammonium acetate method at absorbance 254 nm was used. Compound concentrations