ANTIOXIDANT POTENTIAL OF GALLOCATECHINS - A PULSE-RADIOLYSIS AND LASER PHOTOLYSIS STUDY

Gallocatechins and catechins, which are constituents of green tea, and related, simpler single-ring model compounds undergo one-electron oxi dation by the azidyl radical (k = (1.4-4.8) x 10(9) M(-1) s(-1)), whic h was used as a model one-electron, rapid oxidant. The initial oxidati on leads to the formation of a mixture of A- and B- (or C-) ring pheno xyl radicals. This finding was confirmed by comparison with the spectr a of 3,5-dihydroxyanisole (the model for A ring) and methyl gallate (t he model for B or C ring) radicals and by photoionization experiments in which only the B-ring radical of epigallocatechin was generated, as expected from its lower ionization potential. The A-ring phenoxyl rad ical is converted to the B- (or C-) ring phenoxyl radical by inter- an d intramolecular electron and proton transfer. The activation paramete rs clearly indicate solvent-assisted intermolecular electron and proto n transfer, whereas intramolecular transfer in epigallocatechin gallat e radicals is suggested to proceed through an intermediate molecular c omplex formation. Acid-base equilibria of parent gallocatechins (pK(al ) > 8.0) are significantly altered in the corresponding phenoxyl radic als (pK(rl) = 4.4-5.5). The low reduction potentials of gallocatechin radicals, E(7) = 0.42 V (which is lower than that of vitamin E radical s, E(7) = 0.48 V), are responsible for their antioxidant efficacy, whi ch may include the repair of vitamin E radicals. These low reduction p otentials also imply high susceptibility of parent gallocatechins to r apid oxidation in aerated aqueous media. The reactivity of epigallocat echin gallate with superoxide radical at pH 7, k = 7.3 x 10(5) M(-1) s (-1) is one of the highest measured rates of reduction of superoxide r adical by any chemical antioxidant. In this reaction, superoxide is co nverted to hydrogen peroxide, thus eliminating the redox cycling that may be involved in the corresponding oxidation reaction. The high rate s of quenching of singlet oxygen by gallocatechins in acetonitrile, k = (1.1-2.2) x 10(8) M(-1) s(-1), are comparable to quenching by vitami n E, k = 5 x 10(8) M(-1) s(-1).