Intramolecular hydrogen bonding motifs in deprotonated glycine peptides by cryogenic ion infrared spectroscopy.

The infrared spectra of deprotonated glycine peptides, (Gn-H)(-) with n = 1-4, in the 1200-3500 cm(-1) spectral region are presented. Comparisons between the experimental and calculated spectra reveal the chain length dependent hydrogen bonding motifs that define the geometries of these species. First, an interaction between the terminal carboxylate and the neighboring amide N-H is present in all the peptide structures. This interaction is strong enough to align this amide group in the same plane as the carboxylate. However, we found that the vibrational frequency shift of this hydrogen bonded N-H group is not well reproduced in the calculations. Second, in the longer (G3-H)(-) and (G4-H)(-) species, the peptide chain folds such that the terminal NH2 group also interacts with the carboxylate. Both of these folded structures display an interaction between the terminal NH2 and the neighboring N-H as well. Lastly, an amide-amide interaction is observed in the longest (G4-H)(-) structure. Analysis of the N-H peak positions reveals the interplay among the different hydrogen bonds, especially around the negatively charged carboxylate moiety.