Thermodynamic and kinetic studies on antioxidant capacity of amentoflavone: a DFT (density functional theory) computational approach

Density functional theory (DFT) at the theoretical M06-2X/6-311G(d,p) level was used to assess thermodynamics and kinetics in the antioxidative action of amentoflavone (AF). The antioxidative HAT pathway (H-atom transfer) is assigned to this compound in gas, but the SPL-ET (sequential proton loss-electron transfer) is the main route in polar solvents methanol and water. In all four mediums gas, benzene, methanol, and water, 4‴-OH is the most active site in free radical quenching with the lowest BDE (bond dissociation enthalpy) values of 81.8-84.8 kcal/mol, as well as it exerted the PA (proton affinity) values of 29.8-33.0 kcal/mol in methanol and water. Regarding kinetics, when interacted with •OOH and •NO2 in gas and methanol, 4‴-OH group is also responsible for the lowest ΔG# values (Gibbs free energy of activation), and the highest rate constant K values. Acidic assessment also indicated that 4‴-OH is associated with the strongest acidity (the lowest pKa). Two favorable oriented 4‴-OH and 7-OH groups further exhibited antioxidative activity since they prevented metal ions Zn2+ and Fe2+ from participating in free radical producing processes, in which the most stable complex [FeAF(H2O)4] generated the lowest IE value of -206.2 kcal/mol, and Egap value of 3.491 kcal/mol, but the highest MIA values of 184.6 kcal/mol in methanol.