Oxidation potentials of flavonoids determined by flow-through column electrolysis

The first oxidation potentials of a series of structurally related flavonoi ds, mostly having hydroxyl substituents, were investigated by employing cyc lic voltammetry and now-through column electrolysis. The flow-through colum n packed with carbon fibers was useful for determining the half-wave potent ial (E-1 2) of the flavonoids and the number of electrons (n-value) transfe rred in the oxidation process. The potentials of the flavonoids were shown to be strongly dependent on their structures. The important determinants fo r the ease of oxidation are. 1) ortho-trihydroxyl group (pyrogallol group); 2) ortho-dihydroxyl group (catechol group); 3) the presence of a 2,3-doubl e bond in conjugation with 4-oxo and 3-hydroxyl groups; and 1) another addi tion of a resonance-effective substituent. The relation between the E-1,E- 2 values of the first oxidation step obtained from the hydrodynamic voltamm ograms and the antioxidant data (IC50) in biological systems obtained from references was examined to determine whether the E-1,E- 2 value was an indi cator of the biological activity. It was found that more negative E1 2 valu es of flavonoids were usually associated with decreasing IC50 values. These findings suggest that the electrochemical properties of the flavonoids con tribute to their antioxidant activity, and thus the E-1 2 values of flavono ids can be used as indices of their antioxidant activities in biological sy stems.