Abstract
An exploratory study of those reactions of catechin which lead to polymer formation has enabled the conditions for autoxidation to be defined. The formation of polymers and hydrogen peroxide during the autoxidation at 35 and pH6-8 of catechin and of related 3': 4'-dihydroxyflavans has been studied by measurement of oxygen uptake, and by absorption spectra, colour reactions and chromatographic properties of the polymers, and by elementary analyses of the electrodialysed polymers (1). The evidence obtained together with that provided by spectroscopic study of intermediates produced by silver oxide oxidation (2) support the theory of quinone polymerisation for the autoxidation of catechin. Whereas quinone polymerisation of catechin and 5:7:3':4'-tetrahydroxyflavan involves the phloroglucinol residue, oxidative coupling of 5:7-di-0-methylcatechin and 3':4'-dihydroxyflavan resembles that of catechol. Further evidence for head-to-tail quinone polymerisation of catechin has been found in studies of the autoxidation of mixed phenolic substrates. Enzymic oxidation by mushroom, potato and tobacco polyphenoloxidases proceeds at a faster rate and lower temperature than the autoxidation of catechin, and gives a product precisely similar to the autoxidation polymer. The rate of oxygen uptake is proportional to the enzyme concentration The significance of the autoxidation and enzymic oxidation of catechin to the formation of phlobatannins is considered. Oxidation polymers of catechin are comparable with the tannins which have now been isolated in relatively large amounts from amongst tannin extracts of Acacia catechu and Uncaria gambir, plants valued for their phlobatannins (3). Special sections summarise the chemistry of catechin, the physical techniques on which much of this work depends, and the application of the theory of condensation polymerisation to the polymerisation of catechin. Three flavonols have been identified in extracts from Acacia catechu.