Hydrogen Atom Addition to Cytosine, 1-Methylcytosine, and Cytosine−Water Complexes. A Computational Study of a Mechanistic Dichotomy

Abstract

Combined ab initio and density functional theory calculations at the B3-MP2/6-311++G(3df,2p) level of theory are used to investigate the structures and energetics of radicals produced by hydrogen atom addition to cytosine tautomers, 1-methylcytosine, and cytosine−water complexes. H-atom adducts to the N-3 positions are the most stable radical isomers derived from cytosine tautomer (1), 1-methylcytosine, and cytosine−water complexes in the gas phase. Solvent effects on radical stabilities are addressed by calculations that use the polarizable continuum model. Solvation by bulk water favors C-5 and C-6 adducts which have free energies in water that are comparable to those of the N-3 adducts. H-atom additions to the C-5 positions have the lowest activation energies for all cytosine derivatives under study and are predicted to be kinetically predominant. H-atom additions to the N-3 and C-6 positions are solvent dependent. In the absence of solvation, N-3 is more reactive than C-6 in cytosine and 1-methylcytosine. Water complexation increases the activation energy for H-atom addition to N-3 and results in a reactivity reversal for the N-3 and C-6 positions.