Radical scavenging properties of genistein

The reactivity of genistein toward reactive radical species has been investigated by means of pulse radiolysis. The values of rate constants, respectively 2.3 . 10(10) M(-1)s(-1) and 1.3 . 10(10) M(-1)s(-1) for the reaction with hydroxyl radical at pH 8.3 and 3.0, are close to diffusion limit indicating that genistein is a potent hydroxyl radical scavenger. The reactivity of genistein towards one-electron oxidants has also been investigated. The rate constants k = 4.6 . 10(9) M(-1)s(-1) (pH 8.3) and 6.7 . 10(8) M(-1)s(-1) (pH 7.6) have been determined for the reaction of genistein with N-.(3) and Br-2(.-) radicals, respectively. For both oxidants the rate constants at pH 3 does not exceed 10(8) M(-1)s(-1). The differences in reactivity of genistein towards the oxidants at different acidity of the solution have been assumed to arise from the acid-base equilibria of genistein. The dissociation constants for genistein (pK(a): 7.2, 10.0, and 13.1) have been evaluated spectroscopically. The influence of acid-base equilibria on bond dissociation energy and ionization potential for genistein has also been investigated by means of DFT calculations. It has been concluded on the basis of these calculations that monoanionic form of genistein existing at physiological pH is more powerful radical scavenger than the neutral molecule. (C) 2003 Elsevier Inc.