α-tocopherol oxidation mediated by superoxide anion (O(Formula presented.)) I. Reactions in aprotic and protic conditions

The reaction of α-tocopherol (α-T) with superoxide anion (O(Formula presented.)) in both dry acetonitrile and in aqueous acetonitrile solution is described. The O(Formula presented.) was generated by the electrochemical reduction of molecular oxygen in acetonitrile, using tetrabutylammonium bromide as an electrolyte. α-T was reacted with O(Formula presented.) either in dry acetonitrile or in a 10% aqueous acetonitrile solution. In dry acetonitrile, α-T was oxidized to a very unstable primary intermediate, which was further oxidized to a secondary, more stable intermediate. The formation of the secondary intermediate depended upon the presence of molecular oxygen. This intermediate readily converted into two compounds in equimolar amounts (designated A and B). The primary, very unstable intermediate was readily reduced again to α-T by treatment with LiAlH₄ or ascorbic acid. However, the secondary intermediate or the stable oxidation products could not be reduced to α-T. In the 10% aqueous acetonitrile, α-T was oxidized to α-tocopheryl quinone, α-tocopherol dimer and α-tocopherol dihydroxy dimer, and an unknown compound. In the aqueous medium, no intermediates were formed by the action of O(Formula presented.). The results of this study indicate that the reaction of α-T with O(Formula presented.) under aprotic conditions is different from that observed under protic conditions.