How do phenolic compounds react toward superoxide ion? A simple electrochemical method for evaluating antioxidant capacity.

The reactivities of different phenols and polyphenols versus superoxide ion (O₂(•-)) were investigated as an easy-to-handle electrochemical method for evaluating antioxidant capacities. In view of this application, the O₂/O₂(•-) couple and associated reactions between O₂(•-) and polyphenols (or phenols) were examined in an aprotic solvent [dimethylformamide (DMF)] by cyclic voltammetry. Comparisons based on simple criteria (reversibility of the O₂ reduction in the presence of the phenolic compound, electron stoichiometry, or apparent kinetic constants) allow discriminations between the possible mechanistic pathways (acid-base or radical reaction type). The results highlight that the proton-transfer and radical-transfer pathways are both present for monophenols and polyphenols, with the relative contributions of the two pathways depending on the phenol structure. In agreement with the literature, polyphenols containing an o-diphenol ring (as in flavonoids) were found to present the highest reactivities.