Formation and Decay Dynamics of Vitamin E Radical in the Antioxidant Reaction of Vitamin E

Abstract

Abstract In order to understand the dynamics of antioxidant actions of vitamin E (α-, β-, γ-, and δ-tocopherols, TocH) in biological systems, kinetic study of the formation and decay reactions of vitamin E radicals (α-, β-, γ-, and δ-tocopheroxyls, Toc•) has been performed in organic solvents, using stopped-flow spectrophotometry. By mixing α-, β-, γ-, and δ-TocH with aryloxyl radical (ArO•) in ethanol, the peaks of the UV–vis absorption due to α-, β-, γ-, and δ-Toc• radical appeared rapidly at ca. 430–340 nm, showed maxima, and then decayed gradually. The second-order rate constants (kf and 2kd) for the formation and decay (that is, bimolecular disproportionation) reactions of α-Toc• were determined by comparing the observed curves with the simulation ones obtained by the numerical calculation of differential equations related to the above reactions. From the results, the wavelengths of absorption maxima (λmaxi) and molar extinction coefficients (εi) (i = 1–4) of the optical spectra were determined for α-Toc• radical. Notable solvent effects have been observed for the reaction rates (kf and 2kd) and absorption spectra (λmaxi and εi) of α-Toc• radical. The scheme of the formation and decay reactions of α-, β-, γ-, and δ-Toc• radicals has been discussed based on the results obtained.