Glycine in aqueous solution: solvation shells, interfacial water, and vibrational spectroscopy from ab initio molecular dynamics.

An aqueous glycine solution is studied with ab initio molecular dynamics to investigate the structural aspects of the different solvation shells within the zwitterion and their impact on the infrared spectrum. The individual contributions to the total IR spectrum from glycine and solvation water are decomposed systematically using the standard schemes in terms of maximally localized Wannier orbitals to define approximate molecular dipole moments in solution. The IR spectra of the aqueous solution and of the solvated zwitterionic glycine molecule itself are compared to those stemming from neutral glycine in the gas phase and a virtual "isolated" zwitterionic glycine molecule vertically transferred from solution into vacuum. Furthermore, electronic polarization effects due to solute-solvent coupling are discussed in detail for the solute and for the interfacial solvent molecules based on dipole moment distribution functions.