Kinetic Studies of the Free Radical-Scavenging Actions of Tocopherol Metabolites (α-, γ-, and δ-Carboxyethyl-6-hydroxychroman) and Trolox in Ethanol and Micellar Solutions

The reaction rates (k s) of tocopherol metabolites (α-, γ-, and δ-CEHC) and Trolox with aroxyl radical have been measured in ethanol and micellar solution by a stopped-flow spectrophotometer, and the k s values obtained were compared with those reported for tocopherols (α-, β-, γ-, and δ-tocopherol, TocH) and tocol. The rate constants (k s) increased in the order of Tocol < δ-CEHC < δ-TocH < γ-CEHC < Trolox ∼ γ-TocH ∼ β-TocH < α-CEHC < α-TocH in ethanol. The antioxidants that have lower oxidation potentials (E p) showed higher reactivities. The k s values of α-, β-, γ-, and δ-tocopherol and tocol in micelle remained constant between pH 4 and pH 10 and decreased rapidly at pH 11∼12 by increasing pH value. On the other hand, the k s values of α-CEHC, γ-CEHC, and Trolox showed notable pH dependence. As a result of the detailed analysis of the pH dependence of the rate constants (k s), the structure–activity relationship in the free radical-scavenging action of the tocopherol metabolites and Trolox has been clarified.