Density functional theory (DFT) calculations have been used to explore the molecular structure, electronic structure and O—H bond dissociation enthalpy of quercetin, one of flavonoid compounds. The relationships between the properties and the active site of quercetin are analyzed. Possible reaction mechanisms between •OH, •OOH and with quercetin involved in the antioxidation activity of quercetin have been discussed. The DFT calculations under the level of B3LYP/6-31＋G(d) show that the 4 -OH of quercetin is the most active site on the basis of stability of dehydrogenated quercetin radicals, O—H bond dissociation enthalpy as well as activation energies of hydrogen abstraction from quercetin. This relatively high activity can be ascribed to weak hydrogen-bond interactions between two nearby hydroxides in quercetin molecule. The theoretical investigations on the antioxidation mechanism of quercetin are useful for the design and synthesis of novel antioxidants with enhanced activity.

Key words: density functional theory (DFT) calculations, flavonoid, antioxidant activity, reaction mechanism, dissociation enthalpy