Theoretical investigation on free radical scavenging activity of 6,7-dihydroxyflavone

The difference or heat of formation between parent molecule and its free ra dical (Delta HOF), calculated by semiempirical quantum chemical method AM1, was employed as a theoretical index to characterize the O-H bond dissociat ion energy (BDE) of antioxidant. Through a comparison between Delta HOF of model molecules, it was found that 6,7-dihydroxyflavone (DHF) possessed two Favorable structural factors, the presence of ortho hydroxyls and the exis tence of para oxygen, to weaken its O-H bond at position 6, but the passive effect produced by electron-withdrawing property of the chromonoid ring C surpassed the active effect of the para oxygen. Therefore the free radical scavenging activity of DHF was lower than that of quercetin which possessed catecholic hydroxyl in ring B and was scarcely influenced by ring C. As a result of the analysis, two more active flavonoid antioxidants were constru cted theoretically.