Stability of Pyrimidine Nucleic Acid Bases with Respect to Intra- and Intermolecular Proton Transfer Reactions Induced by Excess Electrons

Chemically transformed nucleic acid bases are considered as sources of point mutations in genetic material. Our computational results and photoelectron spectra provide information about chemical transformations of pyrimidine bases induced by excess electrons. The isolated pyrimidine bases as well as their complexes with X (X= amino acid, carboxylic acid, or alcohol) have been studied with the B3LYP and MPW1K density functionals, as well as at the second order Moller-Plesset level of theory. The photoelectron spectra of some anionic complexes reveal broad features with maxima around 2 eV. These features cannot be associated with the anion of intact pyrimidine base solvated by X and indicate occurrence of chemical transformations. Our main findings are: (i) the excess electron attachment can induce a barrier-free proton transfer (BFPT) from X to the O8 atom of uracil or thymine, (ii) thymine in complexes with carboxylic acids is more resistant to BFPT than uracil, (iii) the instability of neutral rare tautomers of uracil or thymine can be significantly suppressed due to the interaction with zwitterionic amino acids.