Antioxidant potential of gallocatechins. A pulse radiolysis and laser photolysis study

Article abstract of DOI:10.1021/ja00144a014

Gallocatechins and catechins, which are constituents of green tea, and related, simpler single-ring model compounds undergo one-electron oxidation by the azidyl radical (k=(1.4-4.8)x10⁹ M^-1 s^-1), which was used as a model one-electron, rapid oxidant. The initial oxidation leads to the formation of a mixture of A- and B- (or C-) ring phenoxyl radicals. This finding was confirmed by comparison with the spectra of 3,5-dihydroxyanisole (the model for A ring) and methyl gallate (the 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 radical is converted to the B- (or C-) ring phenoxyl radical by inter- and intramolecular electron and proton transfer. The activation parameters clearly indicate solvent-assisted intermolecular electron and proton transfer, whereas intramolecular transfer in epigallocatechin gallate radicals is suggested to proceed through an intermediate molecular complex formation. Acid-base equilibria of parent gallocatechins (pK(a1)>8.0) are significantly altered in the corresponding phenoxyl radicals (pK(r1)=4.4-5.5). The low reduction potentials of gallocatechin radicals, E₇=0.42 V (which is lower than that of vitamin E radicals, E₇=0.48 V), are responsible for their antioxidant efficacy, which may include the repair of vitamin E radicals. These low reduction potentials also imply high susceptibility of parent gallocatechins to rapid oxidation in aerated aqueous media. The reactivity of epigallocatechin gallate with superoxide radical at pH 7, k=7.3x10⁵ M^-1 s^-1, is one of the highest measured rates of reduction of superoxide radical by any chemical antioxidant. In this reaction, superoxide is converted to hydrogen peroxide, thus eliminating the redox cycling that may be involved in the corresponding oxidation reaction. The high rates of quenching of singlet oxygen by gallocatechins is acetonitrile, k=(1.1-2.2)x10⁸ M^-1 s^-1, are comparable to quenching by vitamin E, k=5x10⁸ M^-1 s^-1.