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

Abstract

Listen

Translate article

Abstract The second-order rate constants ks for the reaction of 10 kinds of tocopherol (vitamin E) model compounds with stable phenoxyl radical in ethanol have been measured at 25.0°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 ks 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 ks 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, isopropyl, 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 δ-tocopherol model is only ca. 24% as reactive as α-tocopherol model and tocol model is only ca. 10% 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 logks was found to correlate with Σσ+ substituent constants with a ρ+ value of −1.0.