MECHANISM OF PROOXIDANT REACTION OF VITAMIN-E - KINETIC, SPECTROSCOPIC, AND ABINITIO STUDY OF PROTON-TRANSFER REACTION

To shed light on the mechanism of proton-transfer reactions, a kinetic, spectroscopic, and ab initio study of the prooxidant action of vitamin E derivatives has been carried out. The second-order rate constants (k(-s)'s) for the reaction of six tocopheroxyl radicals (Toc.'s) with five alkyl hydroperoxides (ROOHs) in benzene were determined spectrophotometrically. The first adiabatic ionization potentials (I(a)'s) of ROOHs were obtained by means of photoelectron spectroscopy. The result indicates that k(-s) increases as the electron-donating capacity of the alkyl substituents of ROOH increases and I(a) decreases. The methylation at the aromatic ring of Toc. reduces the k(-s) for a given ROOH. k(-s) for the reaction of deuterated alkyl hydroperoxides (ROODs) with a Toc. in a mixed solution of benzene and ethanol-d1 was also measured. A deuterium kinetic isotope effect on k(-s) is observed. For a given Toc., plots of log k(-s) vs I(a) for various ROOHs and log k(-s) vs Tafts sigma* constant of alkyl substituents of ROOH are found to be linear. The slope of the plot of log k(-s) vs sigma* for ROOD is similar to that for ROOH. The geometries of ROOHs were optimized, and the Koopmans' theorem first ionization potentials (I(K)'s) for those geometries were calculated with the ab initio method. A plot of log k(-s)the reactions of a Toc. with various ROOHs vs I(K) of the ROOH is also found to be linear. From these results, it is considered that both charge transfer and proton tunneling play important roles in the prooxidant reaction of TocH. The transition state in the prooxidant reaction has properties 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 transfer reactions.