A DFT study on the structural, electronic properties and radical scavenging mechanisms of calycosin, glycitein, pratensein and prunetin

Abstract

Antioxidative properties of naturally occurring isoflavonoids calycosin, glycitein, pratensein, prunetin and their radicals were investigated through density functional theory (DFT) method employing B3LYP/6-311G(d,p) and B3LYP/6-311+G(d,p) methods.The reaction enthalpies related to the individual steps of three phenolic antioxidant action mechanisms–hydrogen atom transfer (HAT), single-electron transfer–proton transfer (SET–PT) and sequential proton loss electron transfer (SPLET) have been computed. The bond dissociation enthalpy (BDE), ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA), electron transfer enthalpy (ETE) were determined in gas phase and in two solvents. The relative energies of each OH groups, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and spin density distribution for radicals were computed and discussed. From the molecular structure and thermodynamic point of view, it was found that B-ring of isoflavonoids to be the active center and the hydrogen atom transfer (HAT) appeared as a major mechanism which contributes much in antioxidant action in gas phase. The SPLET mechanism will be the thermodynamically preferred pathway in polar solvents. On the basis of the computed IP, BDE and PA values most active system was found to be pratensein followed by calycosin, glycitein and prunetin. The analysis of structure–activity relationships for four isoflavonoids clearly indicates that hydroxyl group at position 7 to be the most acidic site.