Mid-infrared spectroscopy of protected peptides in the gas phase: A probe of the backbone conformation

Infrared/UV hole-burning spectroscopy is performed on individual conformers of the protected dipeptide Z-Aib-Pro-NHMe. The extended IR range probed in this study allows one to elucidate both the H-bonding motif (5-7 mu m) as well as the backbone structure (7-10 mu m). Comparison with DFT calculations shows that the backbone is locally constrained to an alpha-conformation by Aib and to a gamma-turn by Pro. The gamma-turn motif observed here is intriguing since the condensed phase structure is known to be a beta-turn. This is the first actual observation of such a discrepancy, and it emphasizes the subtle balance between intraand intermolecular forces, which is responsible for the relative stability of the different secondary structure motifs.