Electron pin resonane of chromanoxy free radicals from α-, ζ2-, β-, γ-, δ-tocopherol and tocol

Abstract

Intermediate free radicals formed in the first oxidation step of tocopherols (vitamin E) are detectable at ambient temperature by electron spin resonance (ESR), even in commercial samples without previous treatment. Oxidation with diphenylpicrylhydrazyl or photolysis (400 nm) of pure or dissolved tocopherols yield high resolution ESR spectra showing 7 (α- and ζ 2-tocopherol), 5 (β- and γ-tocopherol), or 3 (δ-tocopherol and tocol) main lines and further hyperfine structure. The g-factor, g = 2.0046 ± 0.0002, is almost independent of temperature, solvent, and methyl substitution. The radicals are chromanoxyls generated by abstraction of hydrogen from the 6-hydroxyl group. They differ from known phenoxyls by nonequivalent protons at C-5 and C-7; spin density is about 20% higher at C-5. Coupling constants are a h (5-CH) = 6.0 Oe, a h (5-CCH 3) = 5.8 Oe, a h (7-CH) = 5.o Oe, ah(7-CCH 3) = 4.8 Oe. The length of the side chain only indirectly influences the ESR spectra via the state of aggregation of the tocopherols. The lifetime of α-tocopheroxyl radicals generated in the pure substance by ultraviolet irradiation at ambient temperature is about 10 min and decreases in the order α-, ζ 2-, gg-, β-, δ-tocopherol and tocol.