Effects of magnesium ion on kinetic stability and spin distribution of phenoxyl radical derived from a vitamin E analogue: mechanistic insight into antioxidative hydrogen-transfer reaction of vitamin E

The phenoxyl radical 1(.) of a vitamin E analogue, generated by the reaction of 2,2,5,7,8-pentamethylchroman-6-ol (1H) with 2,2- di(4-tert-octylphenyl)-1-picrylhydrazyl (DPPH.) or galvinoxyl (G(.)), was significantly stabilized by the presence of Mg2+. Addition of Mg2+ into a solution of 1(.) resulted in a red shift of the absorption band of 1(.) as well as a decrease in the g value of the EPR spectrum of 1(.), indicating a complex formation between 1(.) and Mg2+. The complexation between the phenoxyl radical and Mg2+ significantly retards the disproportionation reaction of 1(.) by electronic repulsion between the metal cation and a generated organic cation (1(.)), leading to stabilization of the organic radical species. No effect of Mg2+ on the rate of hydrogen atom transfer from 1H to DPPH. or to G(.) was observed, suggesting that the hydrogen-transfer reaction between 1H and DPPH. or G(.) proceeds via a one-step hydrogen atom transfer mechanism rather than electron-transfer followed by proton transfer.