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

Abstract

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.