Antioxidant activity of methyl caffeate: Evidence of proton-coupled electron transfer reaction mechanism

Methyl caffeate (MC), a naturally occurring methyl ester of caffeic acid (CA), exhibits potent antioxidant activity and a broad spectrum of biological effects. This study investigates the antioxidant mechanism of MC through its reaction with the stable radical DPPH•, employing both experimental and computational approaches. Kinetic measurements were conducted in a predominantly nonpolar medium (1,4-dioxane with phosphate buffer), revealing concerted proton and electron transfer. This experimental evidence was supported by values of kinetic isotope effects (KIEs) and thermodynamic activation parameters. Analysis of the intrinsic bond orbitals (IBOs) along the calculated intrinsic reaction coordinate (IRC) trajectories supported the proposed proton-coupled electron transfer (PCET) reaction mechanism. Additionally, the Fe(II) complexation ability of MC was evaluated spectrophotometrically, demonstrating stable complex formation at pH 7.0, suggesting potential for mitigating hydroxyl radical generation in physiological conditions. These findings offer new insights into the antioxidant behaviour of MC and its potential applications in pharmaceutical and nutraceutical formulations.