Interaction of iron with polyphenolic compounds: application to antioxidant characterization

Antioxidant action of flavonoids and polyphenolics was analyzed in relation to iron coordination. Flavonoids including baicalein, baicalin, quercetin, and rutin showed little or no reducing activity, and enhanced the autooxidation of Fe2+ ion. Ascorbate-mediated reduction of iron was rather inhibited by flavonoids. Nonflavonoid polyphenolics such as protocatechuic acid and chlorogenic acid, on the contrary, showed a potent iron-reducing ability, and protected Fe2+ ion from autooxidation completely. Both flavonoids and nonflavonoids effectively inhibited the formation of thiobarbituric acid-reactive substances as a marker of lipid peroxidation of microsomes from rat liver. Flavonoids act as an antioxidant by inhibiting the oxygen radical formation through the enhanced oxidation of Fe2+ ion as the prooxidant. Antioxidant properties of nonflavonoids can be explained by the formation of inactive Fe(2+)-polyphenol complex, which cannot react with oxygen, and/or can scavenge reactive oxygen species. Analysis of the action of polyphenolics on the iron redox state may be useful for characterization of the antioxidant effect.