A quantum-chemical study on the antioxidant properties of natural phenolic compounds

The influence of the structure of flavonolignans (silybin, silychristin), flavonoids (quercetin and taxifolin) on the antioxidant properties of silymarin - a standardized herbal extract obtained from the milk thistle seeds (lat. Silybum marianum), which contains a mixture of these compounds has been studied. Molecular structure was optimized at HF/6-31G(d,p) level of theory by means of the GAMESS program package. The O–H bond dissociation enthalpies for all OH groups and ionization potentials of molecules were calculated using density functional theory approach with B3P86 functional and 6-31G(d,p) basis set to analyze the redox properties. The solvation model IEF PCM was used to account for the solvent effects. It has been shown that the OH groups of the side phenolic ring give the maximum contribution to the hydrogen atom transfer mechanism (HAT). СН2ОН groups and OH groups of flavonoids moieties of silybin and silycristin weakly participate in the HAT (high dissociation enthalpies). The presence of 2,3-double bond in the flavonoid moiety affects the redistribution of charge and increases the contribution of the 3-OH group to a HAT mechanism for the quercetin molecule compared to taxifolin. The contribution of the electron transfer mechanism to the antioxidant activity is reduced in a series of quercetin > silychristin > taxifolin > silybin.