Energetics of Uracil Cation Radical and Anion Radical Ion−Molecule Reactions in the Gas Phase

Abstract

The uracil cation radical was calculated to exist predominantly as the 1,3-dioxo tautomer 1•+, similar to the most stable tautomer of neutral uracil (1). The enol forms of 1•+ were found to be 10−173 kJ mol-1 less stable than 1•+ and should not be significantly populated at 298 K thermal equilibrium. Cation radical 1•+ is a moderately strong gas-phase acid of topical acidities ΔHacid = 829, 921, 916, and 879 kJ mol-1 for the H-1, H-3, H-5, and H-6 protons, respectively. Ion 1•+ is capable of exothermic protonation of adenine, guanine, and cytosine, and of the arginine, lysine, histidine, and tryptophan amino acid residues in proteins. The hydrogen atom affinities of 1•+ were −ΔHrxn = 432, 371, and 360 kJ mol-1 for H-atom additions to O-4, O-2, and C-5, respectively. 1•+ was calculated to exothermically abstract the thiol hydrogen atom from CH3SH, the hydroxyl hydrogen from phenol, and an α-hydrogen atom from glycine N-methylamide. Uracil radicals formed by deprotonation of 1•+ were calculated to have large hydrogen atom affinities that should allow for exothermic abstraction of H-atoms from thiol groups, phenolic hydroxyls, and amino acid backbone α-methylene and methine groups. Protonation by a uracil cation radical followed by hydrogen atom abstraction can propagate radiation damage from the initial ionization site. In contrast to the highly reactive uracil cation radicals and radicals, the weakly bound uracil anion radical (1•-) was predicted to be much less reactive in the gas phase. Ion−molecule reactions of 1•- by proton and hydrogen atom abstractions from thiols, phenol, and α-positions of amino acids were calculated to be endothermic and thus very slow in the gas phase. 1•- can selectively deprotonate carboxylic groups as calculated for the reaction with glycine.