Microsolvation Structures of Protonated Glycine and l-Alanine

The IR predissociation spectra of microsolvated glycine and l-alanine, GlyH⁺(H₂O) _n and AlaH⁺(H₂O) _n, n = 1-6, are presented. The assignments of the solvation structures are aided by H₂O/D₂O substitution, IR-IR double resonance spectroscopy, and computational efforts. The analysis reveals the water-amino acid as well as the water-water interactions, and the subtle effects of the methyl side chain in l-alanine on the solvation motif are also highlighted. The bare amino acids exhibit an intramolecular hydrogen bond between the protonated amine and carboxyl terminals. In the n = 1-2 clusters, the water molecules preferentially solvate the protonated amine group, and we observed differences in the relative isomer stabilities in the two amino acids due to electron donation from the methyl weakening the intramolecular hydrogen bond. The structures in the n = 3 clusters show a further preference for solvation of the carboxyl group in l-alanine. For n = 4-6 clusters, the solvation structure of the two amino acids is remarkably similar, with one dominant isomer present in each cluster size. The first solvation shell is completed at n = 4, evidenced by a lack of free NH and OH stretches on the amino acid, as well as the first observation of H₂O-H₂O interactions in the spectra of n = 5. Finally, we note that calculations at the density functional theory (DFT) level show excellent agreement with the experiment for the smaller clusters. However, when water-water interactions compete with water-amino acid interactions in the larger clusters, DFT results show greater disagreement with experiment when compared to MP2 results.