It was observed by ESR measurement that the oxidation of α-, β-, γ-, and δ-tocopherols (vitamin E) with a stable phenoxyl radical in benzene immediately gives corresponding tocopheroxyl radicals. The rates of reaction of α-, β-, γ-, and δ-tocopherols with the stable phenoxyl radical in ethanol solution have been determined spectrophotometrically using stopped-flow technique, as a model reaction of tocopherols with unstable free radicals (ROO·, RO·, and HO·) in biological systems. The second-order rate constants obtained are (5.12±0.36)×103 (α-Toc), (2.24±0.04)×103 (β-Toc), (2.42±0.16)×103 (γ-Toc), and (0.51±0.01)×103 (δ-Toc), M−1 s−1 in ethanol at 25.0 °C. The relative rates agree well with those obtained from studies of the reactivities of tocopherols toward poly(styrylperoxyl) and galvinoxyl radicals by O2 consumption and by ESR method, respectively. The results suggest that the relative reactivities, that is, relative antioxidant activities of tocopherols do not depend on the kinds of unstable free radicals reacted.
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.
An ENDOR study of vitamin E derivatives was carried out. The molecular structures of the cation radicals of models for vitamin E derivatives were determined by means of ENDOR. The relation between the antioxidant activity and the molecular structure of vitamin E was discussed.
ESR and ENDOR spectra of cation radicals of benzodipyran compounds were measured in CH2Cl2, and the proton hyperfine coupling constants were correctly determined for each radical. From detailed analysis of the temperature dependence of hyperfine coupling of β- and γ-methylene protons in the heterocyclic ring, the equilibrium conformation of the β-methylene group and activation energy for the conformational interconversion have been determined. The structure of the heterocyclic ring was compared with those of α-tocopherol (vitamin E) model previously obtained by X-ray analysis. The result suggests that the solution and crystal conformations of the heterocyclic ring are distinctly different.
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