Solvent-Polarity Dependence of Antioxidant Kinetics of Vitamin E

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We have studied antioxidant reactions of vitamin E (α-tocopherol) with an aroxyl radical in several organic solvents (ethanol, diethyl ether, benzene, hexane, and heptane) by using a stopped-flow spectrophotometer. The reaction mechanism is electron-transfer followed by proton-transfer from vitamin E to the radical, and the rate constant (ks) decreases with increasing solvent-polarity. To clarify the reason for the solvent-polarity dependence, we calculated atomic charges of hydrogen atoms in the solvent molecules. A linear relationship between the ks values and the atomic charges was obtained except for the point of ethanol. The reason for the deviation in ethanol is that ethanol solvent has strong intermolecular hydrogen-bonding and is not easy to be reorganized. It was found that the reorganization energy of Marcus theory has an important influence on the ks value. As the solvent-polarity and the reorganization energy increase, the curvature of the potential curve increases and therefore the ks value decreases.
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