Antioxidant chemistry of green tea catechins. New oxidation products of (-)-epigallocatechin gallate and (-)-epigallocatechin from their reactions with peroxyl radicals.

Abstract

The green tea catechins (-)-epigallocatechin gallate (EGCG) and (-)-epigallocatechin (EGC) react with peroxyl radicals generated by thermolysis of the azo initiator 2,2'-azobis(2, 4-dimethylvaleronitrile) (AMVN) to produce several oxidation products. Structure elucidation of these products can provide insights into specific mechanisms of antioxidant reactions. We isolated and identified a previously unreported reaction product of EGCG and three reaction products of EGC. In the EGCG product, the B-ring was transformed into a ring-opened unsaturated dicarboxylic acid moiety. The EGC products include a seven-membered B-ring anhydride and a symmetrical EGC dimer, both analogues of previously described EGCG oxidation products. The third EGC product was an unsymmetrical dimer. In all identified products, changes occurred solely in the B-ring of EGCG or EGC. This confirmed our previous observation that the principal site of antioxidant reactions in EGCG and EGC is the trihydroxyphenyl B-ring, regardless of the presence of a 3-galloyl moiety. A stoichiometric factor n of 4.16 +/- 0.51 was measured for EGCG, whereas factors of 2.20 +/- 0.26 was found for EGC and 2.33 +/- 0.18 measured for methyl gallate. These values represent the net peroxyl radical trapping per catechin molecule by several competing reactions. EGCG and EGC oxidation involves addition of oxygen, which is not derived from water, but most likely from atmospheric oxygen via peroxyl radicals. Characteristic oxidation products may be useful markers for antioxidant actions in living systems.