Is Vitamin A an Antioxidant or a Pro-oxidant?

Antioxidant efficiency of all-trans-retinol has been studied on the basis of characteristic thermochemical properties using density functional theory. The influence of the solvent polarity has also been evaluated. It is found that retinol may act in parallel as an effective antioxidant via H atom donating as well as a pro-oxidant in yielding reactive hydroxyl radical. In fact, the lowest values of bond dissociation enthalpy were found at the C18-H and C18-OH positions. Retinol was also determined to be a good electron donor but bad acceptor in the single electron transfer (ET) reaction with hydroperoxyl (HOO^•) radical. In addition, potential energy surfaces of H atom transfer (HAT) and radical adduct formation (RAF) reactions between retinol and HOO^• radical were also investigated in the gas phase and in the solvent. The results demonstrated that the RAF mechanism was generally more predominant than ET and HAT ones. The most favored radical addition position was found at the C2═C3 double bond in the cyclohexenyl ring. Moreover, the radical scavenging reactivity via RAF reactions was strongly exergonic and thermodynamically feasible while the ET one was endergonic. Natural bond orbital analysis showed that the lone pairs of electrons on the oxygen atom of the HOO^• radical were donated to the unoccupied antibonding orbital of transferred H atom in HAT reactions. In contrast, in the case of RAF reactions, strong interactions between 2p orbitals on oxygen atoms of the radical and the π orbital of the double bond on the retinol molecule were recognized. The results obtained in this work were in agreement with previous experimental observations.