Unusually large numbers of electrons for the oxidation of polyphenolic antioxidants

Reaction mechanisms of polyphenolic antioxidants were studied using electro chemical methods (flow column electrolysis and cyclic voltammetry). In flow column electrolysis, the numbers (ns) of electrons involved in the oxidati on of catechols (chlorogenic acid and caffeic acid) became larger than two (i.e. the number of -OH moieties) at pH > 7; the n-values finally reached c a. 4 at pH 10. Other polyphenols including catechin, ellagic acid, and curc umin exhibited higher n-values than the numbers of-OH moieties in the whole pH range studied (4 < pH < 10). Such unusually large n-values for polyphen ols were found to correlate to their irreversible behavior in cyclic voltam metry. A digital simulation analysis of the voltammograms of chlorogenic ac id clearly showed that the electrode reaction at higher pHs can be elucidat ed in terms of a quasi-reversible electron transfer followed by a chemical reaction and also suggested that the chemical reaction is of second order t o the concentration of chlorogenic acid, i.e. a dimerization reaction. In a similar manner, polyphenolic antioxidants generally undergo certain chemic al reactions on the occasion of their oxidation. As a result, some oxidizab le, phenolic -OH moieties are reproduced in the polymeric products. The unu sually large n-values of polyphenols and thus their higher radical scavengi ng activities may be ascribed to such reproduction of-OH moieties by oxidat ive polymerization. (C) 2001 Elsevier Science B.V. All rights reserved.