alpha-Tocopherol oxidation mediated by superoxide anion (O2-). I. Reactions in aprotic and protic conditions

The reaction of alpha-tocopherol (alpha-T) with superoxide anion (O2-) in both dry acetonitrile and in aqueous acetonitrile solution is described. The O2- was generated by the electrochemical reduction of molecular oxygen in acetonitrile, using tetrabutylammonium bromide as an electrolyte. alpha-T was reacted with O2- either in dry acetonitrile or in a 10% aqueous acetonitrile solution. In dry acetonitrile, alpha-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 alpha-T by treatment with LiAlH4 or ascorbic acid. However, the secondary intermediate or the stable oxidation products could not be reduced to alpha-T. In the 10% aqueous acetonitrile, alpha-T was oxidized to alpha-tocopheryl quinone, alpha-tocopherol dimer and alpha-tocopherol dihydroxy dimer, and an unknown compound. In the aqueous medium, no intermediates were formed by the action of O2-. The results of this study indicate that the reaction of alpha-T with O2- under aprotic conditions is different from that observed under protic conditions.