Kinetic Studies of Antioxidant Activity of New Tocopherol Model Compounds in Solution

Article abstract of DOI:10.1246/bcsj.60.2163

The second-order rate constants k_s for the reaction of 10 kinds of tocopherol (vitamin E) model compounds with stable phenoxyl radical in ethanol have been measured at 25.0 deg C, using a stopped-flow spectrophotometer, as a model reaction of tocopherols with unstable free radicals ROO., RO., and HO.) in biological systems.The absolute k_s values of α-, β-, γ-, and δ-tocopherol models are similar to or slightly smaller than those of the corresponding tocopherols having a long-phytyl-chain.The relative k_s values (α:β:γ:δ = 100:53:50:24), that is, relative antioxidant activities, of α-, β-, γ-, and δ-tocopherol models are in good agreement with those (100:44:47:20) of α-, β-, γ-, and δ-tocopherols.The antioxidant activities of tocopherol models having two alkyl substituents, such as methyl, ethyl, isorpopyl, and t-butyl groups, at ortho positions of OH group are similar to each other, suggesting that the effect of steric hindrance on the reaction rate is small. 5,7-Dimethyltocol model has quite similar rate constants with those of β- and γ-tocopherol models, whereas γ-tocopherol model is only ca. 24percent as reactive as α-tocopherol model and tocol model is only ca. 10percent as reactive as α-tocopherol model.The result indicates that the rate constants increase as the total electron donating capacity of the alkyl substituents at aromatic ring increases.For the tocopherol models log k_s was found to correlate with Σ?⁺ substituent constants with a ρ⁺ value of -1.0.