The effect of solvent polarity on the antioxidant potential of echinatin, a retrochalcone, towards various ROS: a DFT thermodynamic study

Abstract

The antioxidant properties of echinatin (Ech), isolated from liquorice, have recently been reported. It is well known that the free radical species can be deactivated by phenolic antioxidants via different mechanistic pathways. In this work, the scavenging of eighteen different reactive oxygen species (ROS) has been considered, focussing on three main working mechanisms, namely hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET). The investigations have been performed in different dielectric media, viz. gas phase, benzene, methanol and aqueous solution, using density functional theory (DFT) calculations at the M05-2X/6-311++G** level. Various molecular descriptors have been elucidated for Ech as well as the ROS and compared with the reference antioxidant molecule, trolox. In addition, the redox potentials and the equilibrium constants have been calculated to discuss the feasibility of the overall scavenging process. The results demonstrate that the 4-OH group is the first site for H-atom donation, followed by 4’-OH. Further, it has been found that HAT would be the most favourable mechanism in the gas phase. The SPLET mechanism is thermodynamically favoured in polar media like water and methanol, while in the case of non-polar solvents like benzene, the two mechanisms are observed to be competitive.