Conformations and Vibrational Spectra of a Model Tripeptide: Change of Secondary Structure upon Micro-Solvation

Abstract

Mid-infrared (IR) hole burning spectra of the model tripeptide Z-Aib-Pro-NHMe (Z = benzyloxycarbonyl) in gas phase and its micro-clusters with one and two methanol molecules are presented. To establish a relation between experimental spectra and the underlying conformations, calculations at the DFT [B3LYP/6-311++G(d,p)] level of theory are performed. In particular, the intra-peptide and the peptide-methanol hydrogen bonds can be identified from spectral shifts in the amide I, II, and III regions. While the unsolvated tripeptide as well as its one-methanol cluster prefer a γ-turn structure, a β-turn structure is found for the two-methanol cluster, in agreement with previous condensed phase studies. Comparison of measured and simulated spectra reveal that the favorable methanol binding sites are at the head and tail part of the tripeptide. The interconversions between γ-turn and β-turn structures may occur within one deep and wide potential basin with potential barriers below 10 kJ/mol.