Quantum-chemical study on the antioxidation mechanisms of trans-resveratrol reactions with free radicals in the gas phase, water and ethanol environment

To explain the structure-antioxidant activity relationships of trans-resveratrol, its anions and phenoxy radical in the gas phase, water and ethanol media, the quantum-chemical calculations based on CPCM solvation model have been performed for the first time. For fully optimised geometries, the antioxidative parameters have been calculated at the B3LYP/6-311G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The calculated values of the Gibbs free enthalpies revealed that the reactions of trans-resveratrol with water molecule are thermodynamically unfavourable, while the reaction of this compound with hydroxide anion is thermodynamically preferable in the media studied. The calculations performed have shown that in polar media, the 4′-OH group is a stronger proton donor than the 3-OH group and hence, the 4′-OH group contributes most to the antioxidant activity of trans-resveratrol. In addition, it has been established that the ability of 4′-O-monoanion to donate proton is higher than that of 3,4′-O-dianion and 3,5-O-dianion of trans-resveratrol in the media investigated. The results obtained demonstrate that the hydrogen atom transfer mechanism for scavenging of free radicals is more preferable than the single-electron transfer or the sequential proton loss electron transfer mechanism in all types of environment. Trans-resveratrol is found to be a more active donor of hydrogen atoms in the gas phase than in polar environments and in polar media it is more susceptible to electron and proton donation than in the gas phase. Polar media are shown to stabilise trans-resveratrol and its 4′-O-radical, cation radical and all anionic forms stronger than the gas phase. Furthermore, our study shows that the antioxidant capacity of trans-resveratrol is related to the planar and semi-quinone structure of its 4′-O-radical and all anionic forms in the environments investigated.