Abstract: Isoflavones are a large class of polyphenols with biological activity. They are currently the subject of extensive
research for their significant antioxidant activity. This investigation determined and compared the in vitro antioxidant activity
of genistein, daidzein, pomiferin and osajin isoflavones as representative isoflavones with reported data in the literature and
further explored their structure-activity relationships for 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging capacity and
oxygen radical absorbance capacity (ORAC). Molecular simulation of the four molecules was carried out. As a result, stable
dominant conformation was obtained and studied by quantum chemistry calculation. It was found that the four isoflavones
indicated an association between free radical scavenging ability and electron transfer at the phenolic hydroxyl position as
well as dehydrogenation capacity. Frontier molecular orbital could intuitively show the active sites of the molecules and
based on energy gap and the enthalpy of formation of free radicals the antioxidant activity of the four isoflavones could be
accurately ranked. Based on the enthalpy of formation of polyhydroxyl free radicals, 4’-OH was the main hydroxyl active
site of genistein and daidzein, and 3’-OH was the main hydroxyl active site of pomiferin and osajin. Therefore, 4’-OH and
3’-OH can be considered as the representative active sites of the four isoflavonemolecules for scavenging free radicals.

Key words: isoflavone, quantum chemistry calculation, free radical scavenging, frontier molecular orbital, heat of formation of free radical