Electrochemical monitoring of DPPH radical-scavenging reactions enables structural classification of antioxidant reaction mechanisms

DPPH radical (DPPH•) scavenging reactions by antioxidants were monitored using continuous cyclic voltammetry. Electrochemical responses of both DPPH• and the antioxidants were simultaneously recorded, and the reversibility of electron transfer was evaluated from changes in their respective concentrations. In reactions between DPPH• and antioxidants containing a p -dihydroxybenzene moiety, the electrochemical responses of both DPPH• and the antioxidants showed no significant changes over time. This indicates that both species underwent reversible electron transfer accompanied by H + transfer without side reactions. In contrast, for antioxidants containing an o -dihydroxybenzene moiety, DPPH• consistently showed reversible electron transfer, whereas the electrochemical response of the antioxidants gradually decreased during the reaction. This suggests that DPPH• underwent electron transfer with H + transfer only, while the antioxidants subsequently participated in chemical reactions after electron transfer, resulting in electrochemical deactivation. Ascorbic acid, cysteine and Trolox exhibited similar behavior. By contrast, reactions involving antioxidants with an m -dihydroxybenzene moiety showed time-dependent electrochemical changes for both DPPH• and the antioxidants, indicating irreversible addition reactions that produced products with redox activities distinct from those of the original reactants. Overall, the representative antioxidants examined in this study exhibited characteristic reaction patterns that were closely related to their fundamental structural features.