Glycine dimers: structure, stability, and medium effects.

We report a study on different ionization states and conformations of the bimolecular (Gly)(2) system by means of quantum mechanical calculations. Optimized geometries for energy minima of the glycine dimer, as well as relative energies and free energies were computed as a function of the medium: gas phase, nonpolar polarizable solvent, and aqueous solution. The polarizable continuum model was employed to account for solvation effects. Energy calculations were done using the MP2/aug-cc-pVTZ and B3LYP/6-311+G(2df,2p) methods on B3LYP/6-31+G(d,p) optimized structures (some single-point energy calculations were also done using the B3PW91 and PBE1KCIS methods). Ionized forms of the glycine dimer (either zwitterion-zwitterion or neutral-zwitterion) are predicted to exist in all media, in contrast to amino acid monomers. In aqueous solution, dimerization is an exergonic process (-4 kcal mol(-1)). Thus, according to our results, zwitterion-zwitterion Gly dimers might be abundant in supersaturated glycine aqueous solutions, a fact that has been connected with the structure of α-glycine crystals but that remains controversial in the literature. Another noticeable result is that zwitterion-zwitterion interactions are substantially underestimated when computed using methods based on density functional theory. For comparison, some calculations for the dimer of the simplest chiral amino acid alanine were done as well and differences to the glycine dimer are discussed.