| Popis: |
Oxidative coupling between plant phenols and plant proteins is of interest because of its significance in the study of soil humus, and possible nutritional damage in stored foodstuffs. One aspect which has received little attention in the literature is the interaction of amino-acids, peptides or proteins with lignin and lignin precursors in oxidative conditions. The work described in this thesis was initiated to investigate the interaction of coniferyl alcohol (3-methoxy-4-hydroxy cinnamyl alcohol), a fundamental lignin precursor, with amino-acid and peptide esters under oxidative conditions. The work was continued with a study of the oxidation of feruloyl glycine, methyl ester, a derivative of a compound of interest in the formation of lignin and regulation of protein biosynthesis in the barley seed. Finally the oxidation of tconiferyl alcohol was again investigated, but this time in combination with sortho-dihydroxy benzenes. This was specifically of interest because an oxidative F route involving such a coupling had been suggested for the biosynthesis of silybin - a flavano-lignan showing antihepatotoxic activity, isolated from the milk thistle (Silybum marianum, Gaertn). Strong evidence was found for phenol-amine coupling when mixtures of coniferyl alcohol and amino-acid or peptide esters were oxidised, although no E pure compound was isolated. The phenol-amine link appeared to be exclusively between the primary amino group and y-carbon of the propenyl chain, probably from nucleophilic attack of the amine on a quinone methide radical intermediate of coniferyl alcohol. Although under optimum conditions at least 40% of the product involved phenol-amine coupling, it was thought that in future work amino-acids such as proline, lysine and cysteine, known to show greater interaction with ortho-quinones would help in optimising product formation. Some method of stabilising the product mixture prior to analysis (such as acetylation or hydrogenation) was also thought to be desirable. With feruloyl glycine, methyl ester the oxidation products appeared to be exclusively from phenol-phenol coupling. Some earlier work by Afzal (internal report, University College of Swansea; 1975) had given a product which appeared to involve coupling of the amide nitrogen, but it is the only evidence for participation of this group in intermolecular coupling during the oxidation. Combined oxidation of feruloyl glycine, methyl ester with amino-acid or peptide derivatives was not attempted, but might be considered in future work. From the coupling of coniferyl alcohol and simple ortho-dihydroxy benzenes under oxidative conditions, benzodioxanes were isolated in good yield. After comparison of these benzodioxanes with model compounds for confirmation of their structure, an oxidation of coniferyl alcohol with 3,5,7-tri-0-methyl quercitin was attempted, giving a product which appeared to be of the same conformation as an authentic dehydrosilybin derivative on H n.m.r. comparison. Thus strong evidence was obtained for the existence of this oxidative step in the biosynthesis of silybin. |
