Barrier-free intermolecular proton transfer induced by excess electron attachment to the complex of alanine with uracil.

The photoelectron spectrum of the uracil-alanine anionic complex (UA)(-) has been recorded with 2.540 eV photons. This spectrum reveals a broad feature with a maximum between 1.6 and 2.1 eV. The vertical electron detachment energy is too large to be attributed to an (UA)(-) anionic complex in which an intact uracil anion is solvated by alanine, or vice versa. The neutral and anionic complexes of uracil and alanine were studied at the B3LYP and second-order Møller-Plesset level of theory with 6-31++G(*) (*) basis sets. The neutral complexes form cyclic hydrogen bonds and the three most stable neutral complexes are bound by 0.72, 0.61, and 0.57 eV. The electron hole in complexes of uracil with alanine is localized on uracil, but the formation of a complex with alanine strongly modulates the vertical ionization energy of uracil. The theoretical results indicate that the excess electron in (UA)(-) occupies a pi(*) orbital localized on uracil. The excess electron attachment to the complex can induce a barrier-free proton transfer (BFPT) from the carboxylic group of alanine to the O8 atom of uracil. As a result, the four most stable structures of the uracil-alanine anionic complex can be characterized as a neutral radical of hydrogenated uracil solvated by a deprotonated alanine. Our current results for the anionic complex of uracil with alanine are similar to our previous results for the anion of uracil with glycine, and together they indicate that the BFPT process is not very sensitive to the nature of the amino acid's hydrophobic residual group. The BFPT to the O8 atom of uracil may be relevant to the damage suffered by nucleic acid bases due to exposure to low energy electrons.