Barrier-free proton transfer in anionic complex of thymine with glycine

We report the photoelectron spectrum of the thymine-glycine anionic complex (TG(-)) recorded with low energy photons (2.540 eV). The spectrum reveals a broad feature with a maximum between 1.6-1.9 eV. The measured electron vertical detachment energy is too large to be attributed to a complex in which an anion of intact thymine is solvated by glycine, or vice versa. The experimental data are paralleled by electronic structure calculations carried out at the density functional theory level with 6-31++G** basis sets and the B3LYP and MPW1K exchange-correlation functionals. The critical structures are further examined at the second order Moller-Plesset level of theory. The results of calculations indicate that the excess electron attachment to the complex induces an intermolecular barrier-free proton transfer from the carboxylic group of glycine to the O8 atom of thymine. As a result, the four most stable structures of the thymine-glycine anionic complex can be characterized as a neutral radical of hydrogenated thymine solvated by an anion of deprotonated glycine. The calculated vertical electron detachment energies for the four most stable anionic complexes lie in a range 1.6-1.9 eV, in excellent agreement with the maximum of the photoelectron peak.