Structural determination of colourless and yellow dimers resulting from (+)-catechin coupling catalysed by grape polyphenoloxidase

(+)-Catechin oxidation was carried out in aqueous systems using grape polyphenoloxidase as catalyst. Eight fractions corresponding to the major products formed at pH 3 and 6 were purified using HPLC at the preparative and semi-preparative scale. Structural characterization using UV-visible detection and mass spectrometry indicated that they corresponded to (+)-catechin dimers including two yellow pigments. Mono- and bi-dimensional H-1 and C-13 NMR analyses provided structural hypotheses for five oxidation products whereas the other three fractions were mixtures of several isomers. Colourless products, with C-C or C-O interflavan linkages, were dehydrodicatechins of the B-type. One of the two pigments corresponds to dehydrodicatechin A, already identified in other oxidation models, and the other pigment is a new structure of the quinone-methide type.