A theoretical study of the different radical-scavenging activities of catechin, quercetin, and a rationally designed planar catechin.

To improve the radical-scavenging activity of catechin, a planar catechin analogue was designed and synthesized by Fukuhara [J. Am. Chem. Soc. 124 (2002) 5952]. Although the planar catechin is less active than quercetin, it is much more active than catechin in its ability to scavenge galvinoxyl radical, suggesting that the rational design was successful. However, an interesting question remains: what is the basis for the enhanced radical-scavenging activity of the planar catechin? By DFT calculations, we determined that the galvinoxyl radical is scavenged through an electron-transfer mechanism rather than a hydrogen-atom-transfer mechanism. Moreover, the antioxidant anion, derived from proton dissociation, plays a key role in the radical-scavenging process. Hence, the different radical-scavenging activities of the three antioxidants may result from the different ionization potentials of their anions.