Probing the Reactivity of Hydroxyl Radicals toward Isolated Thymine Using Theoretical Calculations

Abstract

B3LYP and MP2 methods are used to study the hydroxylation and dehydrogenation of thymine at different sites in the presence of hydroxyl radicals (⋅OH). With a 6-311++G(d,p) level of description, the consistent results thermodynamically show that the relative free energy increases in the order of CM5 < N1 < C6 < N3 on the hydrogen abstraction. However, toward the hydroxylation, though the two methods give a similar trend that free energy barrier increases in the order of C6 < C5 < C4 < C2, they describe the reaction at C6 site in a different way: B3LYP calculation gives a barrierless pathway, whereas MP2 calculation gives a reactant complex with an energy of 19.5 kJ/mol and a pathway with a barrier energy of 64.9 kJ/mol. The calculated results are consistent with experimental data and the preference for the C5 site is due to the facility of ⋅OH in approaching the substrate. © 2013 Wiley Periodicals, Inc.