Computational Study on the Conformational Preferences of Neutral, Protonated and Deprotonated Glycine Dimers

A conformational analysis has been carried out for monoprotonated, unprotonated and deprotonated glycine dimers in the gas phase and an aqueous solution. MP2/6-311++(d,p), B3LYP/6-311++(d,p) and M06/6-311++(d,p) optimizations were performed for more than 200 initial conformations comprising nonionic (COOH–CH2–NH2) (N) and zwitterionic (COO−–CH2–NH3+) (Z) structures for neutral monomers. All the methods indicate that Z monomers are preferred over N ones for the neutral and deprotonated dimers in aqueous solutions, whereas the reverse trend is observed in the gas phase (including also protonated dimers). NC and ZC structures coexist in aqueous solutions for the protonated glycine dimer. The preferred geometries are significantly different depending on the media and total dimer charge. Moreover, several minima display close energies in each series (media and total dimer charge). New conformers, not previously reported, are found to be significantly populated in those conformational mixtures. Dimers containing Z monomers are associated with larger absolute solvation energies and are more prone than N-containing ones to experience protonation and deprotonation in the gas phase, whereas the reverse trend is observed in the aqueous solution. The Quantum Theory of Atoms in Molecules (QTAIM) analysis reveals that uncharged dimers display trifling electron density transfer between monomers, whereas it is significant in anionic and cationic dimers.