Vitamin E chemistry. Studies into initial oxidation intermediates of alpha-tocopherol: disproving the involvement of 5a-C-centered "chromanol methide" radicals

J Org Chem. 2007 Apr 27;72(9):3268-81. doi: 10.1021/jo062553j. Epub 2007 Mar 29.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Electron Spin Resonance Spectroscopy
  • Free Radicals / chemistry
  • Kinetics
  • Magnetic Resonance Spectroscopy
  • Models, Chemical
  • Oxidation-Reduction
  • Vitamin E / analogs & derivatives
  • Vitamin E / chemistry*

Substances

  • Free Radicals
  • Vitamin E