Redox reactions of quercetin and quercetin-5′-sulfonic acid with Fe3+ and Cu2+ ions and with H2O2

Redox reactions of quercetin and quercetin-5'-sulfonic acid with Fe3+ and Cu2+ ions and with H2O2 were studied spectrophotometrically. Oxidation of the flavonoids occurs at the 3-OH and 4-OH groups. The redox reactions are largely influenced by pH. With Fe3+ ions, oxidation occurs in strongly acidic (pH 1-2), and with Cu2+ ions, in weakly acidic (pH 4-5) solutions. Oxidation of quercetin and quercetin-5'-sulfonic acid with Fe3+ and Cu2+ ions is accompanied by complexation. Hydrogen peroxide oxidizes the flavonoids at pH 1-3.5, and at pH > 4 oxidation is insignificant.