1. Department of Physics, Banaras Hindu University, Varanasi, 221 005, India
Abstract:
The reaction mechanisms involved in the scavenging of hydroxyl (OH·), methoxy (OCH3·), and nitrogen dioxide (NO2·) radicals by ellagic acid and its monomethyl and dimethyl derivatives were investigated using the transition state theory and density functional theory. The calculated Gibbs barrier energies associated with the abstraction of hydrogen from the hydroxyl groups of ellagic acid and its monomethyl and dimethyl derivatives by an OH· radical in aqueous media were all found to be negative. When NO2· was the radical involved in hydrogen abstraction, the Gibbs barrier energies were much larger than those calculated when the OH· radical was involved. When OCH3· was the hydrogen-abstracting radical, the Gibbs barrier energies lay between those obtained with OH· and NO2· radicals. Therefore, the scavenging efficiencies of ellagic acid and its monomethyl and dimethyl derivatives towards the three radicals decrease in the order OH· >> OCH3· > NO2·. Our calculated rate constants are broadly in agreement with those obtained experimentally for hydrogen abstraction reactions of ellagic acid with OH· and NO2· radicals.
Figure
Reactant complex (RC), transition state (TS), and product complex (PC) for hydrogen abstraction from ellagic acid by an OH· radical