L46. Chemical Kinetic Factors Involved in the Antioxidant Activity of Methoxy Phenols

K. Indira Priyadarsini and S. N. Guha

Chemistry Division, Bhabha Atomic Research Centre

Trombay, Mumbai-400085, India

Several naturally occuring para-substituted o-methoxy phenols like, curcumin, dehydrozingerone, bromopentenone, eugenol, isoeugenol and ferullic acid show antioxidant activity mainly by scavenging reactive oxygen free radicals produced during the peroxidative damage of membrane lipids. With a view to understand their efficiency as antioxidants, rate constants for the reactions of these phenols with several oxidising radicals like hydroxyl (OH), azide (N₃ ), nitrogen dioxide (NO₂), and haloperoxyl (CCl₃O₂ ) raducaks were determined in aqueous solutions using pulse radiolysis technique. Depending on the pH of the solution and the nature of substitution on the phenol, the bimolecular rate constants varied from 10¹⁰ to 10⁸ dm³mol^-1s^-1. Hydroxyl radicals reacted with phenols by both addition and oxidation, where as all other radicals reacted mainly be electron transfer producing the corresponding phenoxyl radicals. The phenoxyl radicals having a strong absorption from 300-500 nm region decay by second order radical-radical reactions with 2k values of the order of 10⁹ to 10⁸ dm³mol^-1s^-1. The one-electron reduction potentials were determined for all the phenols except curcumin, by estimating the equilibrium constants for reversible electron transfer between the phenol and phenothiazines as standards.