Enzymatic oxidation of gallocatechin and epigallocatechin: Effects of C-ring configuration on the reaction products

Tea leaf is rich in pyrogallol-type catechins, and their oxidation is important in the generation of black tea polyphenols. In the present study, the enzymatic oxidation of three pyrogallol-type catechins, (+)- and (-)-gallocatechins and (-)-epigallocatechin, was compared. The reactions yielded unstable quitione products, which were trapped as condensation products with o-phenylenediamine. The oxidation of' (+)-gallocatechin proceeded very slowly compared to the reaction of (-)-epigallocatechiii, and yielded a proepi t hea fl agallin- type dimer as the major product, though oxidation of (-)-epigallocatechin gave predominantly dehydrotheasinensin C. The cis-con figuration of' the C-3 hydroxyl group and the B-ring of (-)-epigallocatechin was apparently crucial for rapid and selective production of dehydrotheasinensin C. Oxidation of (-)-gallocatechin proceeded in a manner similar to that of (+)-gallocatechin, and produced an enantiomer of the (+)-gallocatechin product. The results suggest that enzymes catalyze oxidation of the pyrogallol B-ring to the o-quinone, with subsequent non-enzymatic coupling reactions proceed under highly steric control. (c) 2007 Elsevier Ltd. All rights reserved.