Oxidation of Lipids. IV. Formation and Reaction of Chromanoxyl Radicals as Studied by Electron Spin Resonance

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In order to study the inhibition of oxidation by vitamin E, the formation and reaction of chromanoxyl radicals and hindered phenoxyl radicals were examined by ESR. t-Butoxyl radical abstracted the phenolic hydrogens of α-, β-, γ-, and δ-tocopherols, 2,4,6-tri-t-butylphenol (TBP), and 2,6-di-t-butyl-4-methylphenol (BMP) to give corresponding phenoxyl radicals which were stable enough to be observed by ESR. 2,2-Diphenyl-1-picrylhydrazyl (DPPH) also induced the formation of chromanoxyl and phenoxyl radicals. The ESR spectra of chromanoxyl radicals were recorded and their hyperfine coupling constants were determined. The phenoxyl radicals from TBP and BMP disappeared quite rapidly when excess t-butyl hydroperoxide was added, whereas α-chromanoxyl radical from α-tocopherol remained considerably. α-Chromanoxyl radical and phenoxyl radicals reacted with p-t-butylbenzenethiol to regenerate α-tocopherol and corresponding phenols respectively. The implications of these results are discussed.