Oxidation potentials of flavonoids determined by flow-through column electrolysis

Abstract

The first oxidation potentials of a series of structurally related flavonoids, mostly having hydroxyl substituents, were investigated by employing cyclic voltammetry and flow-through column electrolysis. The flow-through column packed with carbon fibers was useful for determining the half-wave potential (E1/2) of the flavonoids and the number of electrons (n-value) transferred in the oxidation process. The potentials of the flavonoids were shown to be strongly dependent on their structures. The important determinants for the ease of oxidation are: 1) ortho-trihydroxyl group (pyrogallol group); 2) ortho-dihydroxyl group (catechol group); 3) the presence of a 2,3-double bond in conjugation with 4-oxo and 3-hydroxyl groups; and 4) another addition of a resonance-effective substituent. The relation between the E1/2 values of the first oxidation step obtained from the hydrodynamic voltammograms and the antioxidant data (IC50) in biological systems obtained from references was examined to determine whether the E1/2 value was an indicator of the biological activity. It was found that more negative E1/2 values of flavonoids were usually associated with decreasing IC50 values. These findings suggest that the electrochemical properties of the flavonoids contribute to their antioxidant activity, and thus the E1/2 values of flavonoids can be used as indices of their antioxidant activities in biological systems.