Vitamin E chemistry. Studies into initial oxidation intermediates of alpha-tocopherol: disproving the involvement of 5a-C-centered "chromanol methide" radicals.
Contrary to concepts handed down in the literature from the early days of vitamin E research, one-electron oxidation of vitamin E does not involve 5a-C-centered radicals. A combined approach of analytical techniques, in particular electron paramagnetic resonance spectroscopy (EPR), organic synthesis of special derivatives, isotopic labeling, kinetic studies, and computational chemistry was used to re-evaluate the one-electron and two-electron oxidation chemistry of alpha-tocopherol (alpha-toc). EPR in combination with 5a-13C-labeled compounds provided no indication of the involvement of 5a-C-centered radicals. Oxidation of special tocopherol derivatives were used to disprove the occurrence of 5a-C-centered one-electron intermediates. Additionally it was shown that those vitamin E reactions that were commonly evoked to plead for the involvement of C-centered tocopheryl radicals actually proceeded via heterolytic, i.e., non-radical, intermediates. The results will help to clear widely spread misunderstandings about the chemistry of vitamin E and will have mechanistic implications for the synthesis of tocopherol-based supramolecular structures and 5a-substituted alpha-tocopherol derivatives.
Rosenau
Thomas
T
Department of Chemistry, University of Natural Resources and Applied Life Sciences, Muthgasse 18, A-1190 Vienna, Austria. thomas.rosenau@boku.ac.at
Kloser
Elisabeth
E
Gille
Lars
L
Mazzini
Francesco
F
Netscher
Thomas
T
D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D010084:Oxidation-Reduction; D014810:Vitamin E / Q000031:analogs & derivatives / Q000737:chemistry*
D005609:Free Radicals; D014810:Vitamin E