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The biological activity of the ochratoxins : synthetic and structural investigations

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dc.contributor.advisor Steyn PS, Prof en
dc.contributor.advisor Oliver DW, Prof en
dc.contributor.author Payne BE en
dc.date.accessioned 2016-09-22T07:17:25Z
dc.date.available 2016-09-22T07:17:25Z
dc.date.submitted 1998 en
dc.identifier.uri http://hdl.handle.net/20.500.11892/11323
dc.description.abstract Ochratoxins A and B are important fungal secondary metabolites, mainly produced by Aspergillus ochraceus Wilh. and Penicillium verrucosum. Ochtatoxin B, the dechloro substance, is essentially nontoxic. Ochatoxin A, an environmental contaminant, occurs frequently in nature and is the cause of Danish Porcine Nephropathy and is implicated in the etiology of Balkan Endemic Nephropathy in the Balkan peninsula and Chronic Interstitial Nephropathy in Northern Africa. The objective of the thesis is to eventually obtain a better understanding of the structure activity relationships of the ochratoxins. The biological and toxicological data of the ochratoxins are summarized in Chapter one. The nephrotoxic ochratoxin A is also carcinogenic, teratogenic, immunosuppressive and only very slightly hepatotoxic. These data on ochratoxins A and B and their derivatives served as a guideline for the synthetic goals. Chapter two relates a critical evaluation of synthetic routes directed at the synthesis of functionalized dihydroisocoumarins. This background knowledge was instrumental in designing a facile approach to the synthesis of bromodihydroisocoumarins. Aromatic dehalogenation and halogenation are salient features in understanding the structural and biological properties of the ochratoxins; the current information directed the research on these aspects. Ochratoxin A contains L[beta]phenylalanine linked to the dihydroisocoumarin acid by an amide bond, therefore, much focus was directed to the peptide bond formation and it was decided to utilize the acid azide methodology so as to obtain a understanding of the reaction mechanism and optimal reaction conditions. Chapter three contains the discussion on the experimental results obtained in this study. Sufficient ochratoxin A was isolated from mouldered wheat for synthetic studies and the toxin was subjected to dehalogenation to yield ochratoxin B. The ochratoxin B was successfully converted into bromo and iodoochratoxin B this is a major first accomplishment. The linking of L[beta]phenylalanine to 5fluoro, 5chlorov 5bromo and 5iodosalicylic acid by converting the salicylic acids into the acid azides, was carefully investigated. A number of bases were investigated, triethylamine transpired to be the most suitable basic reagent. The coupling of L[beta]-phenylalanine to 5chlorosalicylic acid, in the presence of triethylamine to yield the (5chloro2hydroxyphenyl)carbonylLphenylalanine was fully studied. This also involved the isolation and characterization of the intermediary, 5chlorosalicylazide; the reporting of the 13C chemical shift value of the azide carbon atom is unique. The molecular modelling of ochratoxins A and B yielded valuable molecular properties of the toxins with reference to the lipophilicity, electrostatic potential and hydrogen donor/acceptor properties. The results of Bredenkamp et al., 1989 were confirmed in that energy minimization of ochratoxins A and B yielded structures identical to the reported crystal structures. Valuable insights into the conformation of ochratoxins A and B while docked in the active site of carboxypeptidase A (CPA) were gained, especially of the conformation and dihedral angle of the carbonyl amide bond. Furthermore, important information was gained about the interaction of the peptide residues implicated in the binding action of CPA with ochratoxins A and B. The interaction of the chloro and hydrogen substituents of ochratoxins A and B respectively, with tyrosine248 provided a possible explanation for the difference in hydrolysis rates of the toxins by CPA. The findings reported in this thesis have unlocked a number of research challenges: The availability of bromoochratoxin B and of iodoochratoxin B will enable several in vivo and in vitro studies leading to a better understanding of the role of halogens in ochratoxin biological activity; Expertise obtained on the optimal linking of L[beta]phenylalanine to the salicylic acids will lead to the facile linkage of substituted aromatic amino acids to dihydro-isocoumarin acids: These novel ochratoxins will similarly contribute to a better insight of structure activity relationships of this group of nephrotoxins. The study of the acylazide coupling reaction exposed the intricacies in this important reaction for peptide bond formation, thereby creating the need to adequately investigate the reaction kinetics and reaction dynamics of the reactions. The salicylic acid and the various halosalicylic acids coupled to L-phynylalanine, should provide valuable insights into the mechanism of action of ochratoxin A, as they share a large deal of structural similarity. These compounds are to be tested for their ability to mimic, for example, aspartame in alleviating the effects of OA. Their toxicity, as pertaining to lipid peroxidation, is also to be tested. The molecular modelling results will lead to advanced studies on the interaction of CPA with related ochratoxin A derivatives since the computational molecular properties of ochratoxins A and B are currently available, and modelling of the interaction with receptor/enzyme crystal structures is now possible. en
dc.language English en
dc.subject Chemistry en
dc.title The biological activity of the ochratoxins : synthetic and structural investigations en
dc.type Masters degree en
dc.description.degree MSc en

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