5-Aminouracil as Effective Inhibitor of Peroxyl Radicals. Experimental and Theoretical Studies

The reaction of 5-aminouracil with peroxyl radicals generated by the thermal decomposition of 2,2'-azo-bis(2-methylpropionitrile) (AIBN) and 2,2'-azo-bis(2-amidinopropane) dihydrochloride was studied at 50 degrees C in ethanol and water (pH 7.0) solution respectively. The oxidation product of 5-aminouracil formed by peroxyl radicals was dihydro-5,5,6-trihydroxypyrimidine-2,4-dione. The relative rate constant of 5-aminouracil vs. quercetin and 2,6-di-tert-butyl-4-methylphenol by peroxyl radicals generated from AIBN was measured in ethanol and found to be 0.19 (50 degrees C) and 3.6 (70 degrees C) respectively. Theoretical data of the redox potential and the bound dissociation energy oppose against single electron/proton transfer mechanism and provide support for a hydrogen atom abstraction mechanism. Transition structures and activation barriers of the hydrogen abstraction from 5-aminouracil, 5-hydroxy-6-methyluracil and 2,6-di-tert-butyl-4-methylphenol by methyl peroxyl radical were determined with the BB1K/6-31+G(d,p) level of theory. The relative theoretical reactivity was found to be in a good agreement with the experimental results and also supported the hydrogen abstraction mechanism.