Flavonoids as antioxidants

Article abstract of DOI:10.1021/ja00090a032

Spectral, acid-base, and redox properties of the phenoxyl radicals derived from 3,4-dihydroxybenzene derivatives and selected flavonoids were studied by pulse radiolysis of aqueous solutions. From the pH-dependent changes in the phenoxyl spectra, the dissociation constants were derived. 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 similar to the pK(a) value of the 3,4-dihydroxybenzoate radicals, pK(a) = 4.2, which is expected from their similar electronic structures. Deprotonation of 5- and 7-OH in the catechin and rutin and of 5-OH in the hesperidin radicals has no effect on the radical spectra, which is explained by the inefficient coupling of the A-ring of the flavonoid radicals with the unpaired electron. Because of favorable reduction potentials of the phenoxyl radicals, E₇ = 0.56-0.7 V vs NHE, flavonoids may act as efficient antioxidants of alkylperoxyl and superoxide/hydroperoxyl radicals. The ac kinetic conductivity method was developed for the measurements of the low reaction rate constants of the superoxide radical reactions with flavonoids and phenols in aqueous solutions at pH 10. The rates of the superoxide radical reactions with flavonoids, k = 3 x 10²-5.1 x 10⁴ M^-1 s^-1, depend on the redox properties and the charge of the flavonoids. The highest rates are measured for the oxidation of quercetin and rutin, whereas the lowest are those for the B-ring monosubstituted derivatives, with substantially higher redox potentials. Uncharged catechin at pH 7 reacts at k = 6.6 x 10⁴ M^-1 s^-1, whereas the rate at pH 10, where catechin is doubly negatively charged, is approximately 4 times lower, k = 1.8 x 10⁴ M^-1 s^-1. The activation parameters of the oxidation of rutin and trolox at pH 10 and methylgallate at pH 7 were determined in an attempt to understand why the rates of the superoxide reactions are low despite high driving forces of ΔE ≥ 0.4 V. Low activation enthalpies, ΔH(paragraph) 2.3-3.6 kcal/mol, and negative activation entropies, ΔS(paragraph) = -25-28 cal/(mol K), point to an inner-sphere electron-transfer mechanism.