Antioxidant activity of mildbone and mildbenone secondary metabolites of Erythrina mildbraedii Harms: A theoretical approach

Flavanone mildbone and its isomeric chalcone mildbenone isolated from an African Erythrina species herms, Erythrina mildbraedii showed significant antioxidant activity to scavenge 2,2-diphenyl-1-picrylhydrazyl free radical (DPPH). To shed light and explain the high antioxidant activity of mildbone compared with mildbenone, density functional theory (DFT) calculations have been carried out at the B3P86/6-311++G(d,p) and MPWB1K/6-311++G(d,p) levels of theory. The solvent effects were taken into account implicitly using polarizable continuum model (PCM). The results showed that the high antioxidant activity of mildbone is mainly related to the second bond dissociation enthalpy BDE_d of a second hydrogen atom transfer from i-OH phenoxyl radical to the free radical. Thermodynamic and kinetic results showed that DPPH and peroxyl OOCH₃ free radicals scavenging by mildbone and mildbone mainly proceed through a proton-coupled electron transfer (PC-ET) mechanism.