Mechanisms of the hydroxyl and superoxide anion radical scavenging activity and protective effect on lipid peroxidation of thymoquinone: a DFT study

The mechanism of thymoquinone (2-isopropyl-5-methyl-1,4-benzoquinone) for scavenging peroxide radicals was explicated in gas phase and in liquid phase (benzene, water) using density functional theory approach. The calculated bond dissociation enthalpy, ionization potential, and proton dissociation enthalpy revealed that the H-atoms at positions 2-isopropyl and 5-methyl groups can be readily abstracted. In fact, the 2-isopropyl C-H bond dissociation enthalpy is lower than the 5-methyl C-H dissociation enthalpy. Analysis of spin density distribution indicates that H-atom abstraction in 2-isopropyl site generates more stable radical. The plots of highest occupied molecular orbital of thymoquinone demonstrate that the whole molecule can be easily attacked by electrophilic agents like radicals. In this paper, we studied the interaction of thymoquinone with hydroxyl, hydroperoxyl, and superoxide anion radicals, using potential energy surface scans and natural bond orbital analysis. Calculated potential energy barriers for hydroxyl radical attack (similar to 2.5 kJ/mol) revealed an instant scavenging by thymoquinone. Modeling of the reaction between thymoquinone and lipid peroxyl radical shows a barrier energy around 26.23 kJ/mol. .