MECHANISM OF ANTIOXIDANT REACTION OF VITAMIN-E - CHARGE-TRANSFER AND TUNNELING EFFECT IN PROTON-TRANSFER REACTION

In order to shed light on the mechanism of proton-transfer reactions, a kinetic and ab initio study of the antioxidant action (intermolecular proton transfer) of vitamin E derivatives has been carried out. The second-order rate constants (k(s)'s) for the reaction of tocopherols (TocH's) with variously substituted phenoxyl radicals (PhO.'s) in ethanol were measured with a stopped-flow spectrophotometer. The half-wave reduction potentials (E1/2's) of PhO.'s were obtained by using a cyclic voltammetry technique. The result indicates that k(s) increases as the total electron-donating capacity of the alkyl substituents at the aromatic ring of TocH or the electron-withdrawing capacity of the substituent of PhO. increases. k(s) for the reaction of deuterated tocopherol derivatives (TocD's) with a PhO. in deuterated ethanol (C2H5OD, ethanol-d1) was also measured. A substantial deuterium kinetic isotope effect on k(s) is observed. In the reactions of each PhO. with various TocH's, a plot of log k(s) vs peak oxidation potential (E(p)) of TocH is found to be linear. The slope of its plot for TocD's is clsoe to that for TocH's. In the reactions of each TocH with various PhO.'s, a plot of log k(s) vs E1/2 of PhO. is found to be linear. The geometries of TocH's were optimized with the semiempirical modified neglect of diatomic overlap (MNDO) method. The Koopmans' theorem first ionization energies (IP) for those geometries were calculated with the ab initio method. In the reactions of a PhO. with various TocH's, plots of log k(s) vs IP, the activation energy (E(act)) vs IP, and E(p) vs IP are also found to be linear. From these results, it is considered that both the charge transfer and the proton tunneling play important roles in the antioxidant reaction of TocH. The transition state has the property of the charge-transfer species. The proton tunneling takes place below the transition state. Tunneling allows the proton to cut a corner on the potential energy surface. Our explanation will be widely applicable to many proton-transfer reactions.