FLAVONOIDS AS ANTIOXIDANTS

Spectral, acid-base, and redox properties of the phenoxyl radicals der ived from 3,4-dihydroxybenzene derivatives and selected flavonoids wer e studied by pulse radiolysis of aqueous solutions. From the pH-depend ent changes in the phenoxyl spectra, the dissociation constants were d erived. The pK(a), values for the deprotonation of the 3'-OH group in the catechin (pK(a) = 4.6) and rutin (pK(a) = 4.3) radicals are simila r to the pK(a) value of the 3,4-dihydroxybenzoate radicals, pK(a) = 4. 2, which is expected from their similar electronic structures. Deproto nation of 5- and 7-OH in the catechin and rutin and of 5-OH in the hes peridin radicals has no effect on the radical spectra, which is explai ned by the inefficient coupling of the A-ring of the flavonoid radical s with the unpaired electron. Because of favorable reduction potential s of the phenoxyl radicals, E(7) = 0.56-0.7 V vs NHE, flavonoids may a ct as efficient antioxidants of alkylperoxyl and superoxide/hydroperox yl radicals. The ac kinetic conductivity method was developed for the measurements of the low reaction rate constants of the superoxide radi cal reactions with flavonoids and phenols in aqueous solutions at pH 1 0. The rates of the superoxide radical reactions with flavonoids, k = 3 X 10(2)-5.1 X 10(4) M(-1) s(-1), depend on the redox properties and the charge of the flavonoids. The highest rates are measured for the o xidation of quercetin and rutin, whereas the lowest are those for the B-ring monosubstituted derivatives, with substantially higher redox po tentials. Uncharged catechin at pH 7 reacts at k = 6.6 X 10(4) M(-1) s (-1), whereas the rate at pH 10, where catechin is doubly negatively c harged, is approximately 4 times lower, k = 1.8 x 10(4) M(-1) s(-1). T he activation parameters of the oxidation of rutin and trolox at pH 10 and methyl gallate at pH 7 were determined in an attempt to understan d why the rates of the superoxide reactions are low despite high drivi ng forces of Delta E greater than or equal to 0.4 V. Low activation en thalpies, Delta H-double dagger = 2.3-3.6 kcal/mol, and negative activ ation entropies, Delta S-double dagger = -25-28 cal/(mol K), point to an inner-sphere electron-transfer mechanism.