Biocatalytic studies of phenol oxidases producing antioxidants

Doctoral Thesis


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University of Cape Town

In recent years there has been increasing interest in the production of oligomers and polymers of economic importance using biocatalysts; the application of enzymes in dimerisation or polymerisation processes can lead to the synthesis of unique compounds, with novel properties, that could not be easily achieved by conventional methods. Further, the application of biocatalysts in polymerization processes can be exploited in development of bioremediation systems, and there is a demand for new technologies that can be utilized in the removal of organic pollutants such as phenolics from contaminated environment. This study reports on the potential application of laccase, obtained from the white rot fungi Trametes pubescens, in the synthesis of organic compounds which are dimers or polymers, and in the development of bioprocesses of potential economic importance. The focus of this study is, particularly, on the effect of organic solvents and the structure of the substrates on the nature of products formed. The thesis also gives some insight into the relationship between the structure of laccase products and their biological (antioxidant and antimalarial) activity. The compounds tyrosol, hydroxytyrosol, 8-hydroxyquinoline, and totarol were selected as the model compounds for laccase reactions. Tyrosol was oxidised by laccase, yielding dimeric and polymeric compounds which were identified by LC-MS and IH-NMR. As a comparative study, hydroxytyrosol was also oxidised by laccase yielding dimeric, trimeric and polymeric compounds which were identified by LC-MS. Manipulation of this biocatalytic system resulted in development of an efficient process that allows for selectivity with respect to the products. A system was then developed whereby oxidation of hydroxytyrosol by laccase would selectively yield either dimers or oligomeric products. Thus, use of 50 % acetone in the reaction medium favoured the synthesis of dimeric products, and 20 % methanol resulted in the formation of a polymeric product. These results showed that hydroxytyrosol-laccase reactions were more readily controlled than tyrosol-laccase reactions, and this difference was attributed to structural configuration of these substrates.

Includes bibliographical references (leaves 172-198).