Computational Study of the Antioxidant Properties of Vitamin E and Its Ability to Scavenge Alkyl Peroxide Radicals

In this study, the antioxidant activity of alpha-tocopherol, α-TOCO, one of the eight forms of vitamin E with CCl3O2 radical using density functional theory, DFT, and conductor-like polarizable continuum model, CPCM, at the B3LYP/6-31G** level of theory and three different phases of gas, oil and water have been studied. Interaction energy calculations show that there is a gravitational force between the radical and α-TOCO and their values in the oil and water phases are more than the gas phase. Mechanisms study of electron transfer, hydrogen atom abstraction, and radical addition show that the radical addition process is more thermodynamically desirable than other reactions and that the oil and aqueous phases are both suitable for this type of process. The values of the calculated reactivity quantities show that α-TOCO tends to oxidize against radicals, that this tendency is more in the aqueous phase than the others. Also, by radical adsorption on α-TOCO in each phase, the antioxidant property of α-TOCO decreases and its electroaccepting increases. The results of TD-DFT calculations show that the maximum absorption wavelength, λmax, and the corresponding oscillator strength, fmax, for α-TOCO in the oil phase are greater than those in the gas and aqueous phases. λmax and fmax change significantly by radical adsorption on α-TOCO, and in the oil and water phases, λmax of the complex is approximately equal to the wavelength of red light. The results of calculations of atoms in molecules, AIM, show that the interactions between α-TOCO and radical in all three phases are of the type of hydrogen bond and van der Waals, with the difference that the number of bonds and the strength of the bonds are reduced from the oil to water phase and from water to gas phase, respectively. This result corresponds to the calculated values for the interaction energies.