Electrochemical and Spectroscopic Characterization of Oxidized Intermediate Forms of Vitamin E.

Vitamin E, a collection of lipophilic phenolic compounds based on chroman-6-ol, has a rich and fascinating oxidative chemistry involving a range of intermediate forms, some of which are proposed to be important in its biological functions. In this review, the available electrochemical and spectroscopic data on these oxidized intermediates are summarized, along with a discussion on how their lifetimes and chemical stability are either typical of similar phenolic and chroman-6-ol derived compounds, or atypical and unique to the specific oxidized isomeric form of vitamin E. The overall electrochemical oxidation mechanism for vitamin E can be summarized as involving the loss of two-electrons and one-proton, although the electron transfer and chemical steps can be controlled to progress along different pathways to prolong the lifetimes of discreet intermediates by modifying the experimental conditions (applied electrochemical potential, aqueous or non-aqueous solvent, and pH). Depending on the environment, the electrochemical reactions can involve single electron transfer (SET), proton-coupled electron transfer (PCET), as well as homogeneous disproportionation and comproportionation steps. The intermediate species produced via chemical or electrochemical oxidation include phenolates, phenol cation radicals, phenoxyl neutral radicals, dications, diamagnetic cations (phenoxeniums) and