Antioxidant Properties of Kanakugiol Revealed Through the Hydrogen Atom Transfer, Electron Transfer and M2+ (M2+=Cu(II) or Co(II) Ion) Coordination Ability Mechanisms. A DFT Study In Vacuo and in Solution

Abstract

DFT studies on the conformational and antioxidant properties of kanakugiol were performed to investigate factors that contribute to its conformational preferences and to elucidate its antioxidant properties and mechanisms. The antioxidant activity has been considered through the ability of kanakugiol to scavenge free radical species and through its ability to chelate metal ions. The antiradical activity was performed using the B3LYP and the B3P86 and by using both the 6-31+G(d,p) and the 6-311+G(d,p) basis sets. The gas-phase interaction of kanakugiol with Cu (II) and Co (II) ions were studied by means of G96LYP/6-311+G(3df,2p)//G96LYP/6-31G(d) calculations. The results show that the preferred neutral conformers are stabilised by the presence of intramolecular hydrogen bonds (IHBs) and the orientation of both the OCH? groups and the prop-2-en-1-one chain. The neutral and cationic radical species are stabilised by both IHBs and the spin density delocalisation of the unpaired electron. A comparison of the OH bond dissociation enthalpy and the ionisation potential values across media suggests that each parameter has the lowest value in acetonitrile and highest value in vacuo, which indicates that kanakugiol may have an optimal tendency to scavenge radical species in polar aprotic media. Among the metal ion complexes, the preferred geometry is dependent on the media; in vacuo the preferred complex is one in which the metal ion is dentated to the ? system of the aromatic ring while in water solution, the preferred complex is one in which the metal ion is dentated between O atoms. An estimation of metal ion affinity in aqueous solution shows a remarkable decrease with respect to the results in vacuo. The charge on metal ions decreases on interaction with kanakugiol, suggesting that both metal ions are reduced while kanakugiol is oxidised. An analysis of the spin density distribution and the electron delocalisation index elucidates the difference in the type of bonding between the Co(II) and Cu(II) complexes.