Chrysin and (±)-taxifolin electrochemical oxidation mechanisms

The electrochemical behaviour of the flavone chrysin and the dihydroflavonol (+/-)-taxifolin was investigated by different electrochemical techniques at different pH values. Dihydroflavonol (+/-)-taxifolin presented two oxidation peaks. The first electrooxidation reaction of (+/-)-taxifolin is reversible over the whole pH range. The dependence of peak current on pH showed a maximum around neutral pH values with a decrease in acidic and alkaline media. The influence of the catechol group deprotonation on ring B is related to the electron/proton donating capacity in (+/-)-taxifolin and to its radical scavenging antioxidant activity. In chrysin, where the ring B has an unsubstituted phenyl, the antioxidant active moiety is in ring A. Chrysin presents an irreversible oxidation peak at higher oxidation potentials than those usually obtained for the catechol moiety ion in ring B and a pH dependent behaviour. The -OH groups on ring A are affected by the 4-keto group as shown by the lower pK(a), value of the 7-OH, indicating that the 7-OH group is the more acidic. The 5-OH group is less acid and may be explained by the acidic weakening effect of an intramolecular H-bond between the 5-OH and 4-keto group of taxifolin and chrysin.