Theoretical insight into the antioxidative activity of isoflavonoid: The effect of the C2=C3 double bond

Isoflavonoids are one of the most important groups of naturally occurring antioxidants. Their structural features are important for evaluating their antioxidative activity. In this work, density functional theory (DFT) methods were applied to investigate the influence of the C2=C3 double bond on the antioxidative activity of isoflavonoids based on three currently accepted radical scavenging mechanisms from the viewpoint of thermodynamics. The C2=C3 double bond can make the compounds more flat, which would extend the conjugated system in the molecule and make the isoflavonoids higher antioxidant activity. The C2=C3 double bond would not alter the strongest antioxidative hydroxyl group of the isoflavonoids. In the gas, benzene and CHCl₃ phases, the C2=C3 double bond will enhance the antioxidative activity of isoflavonoids by lowering the bond dissociation enthalpies of the hydroxyl groups in the B ring that are the strongest antioxidative sites for the hydrogen atom transfer (HAT) mechanism. In polar phases, a similar result is obtained by weakening the proton affinity of 7-OH that is the strongest antioxidative hydroxyl group in the sequential proton loss electron transfer (SPLET), mechanism. Thus, the C2=C3 double bond will enhance the antioxidative activity of isoflavonoids irrespective of the studied phases.