Vibrational spectroscopy of a non-aromatic amino acid-based model peptide:: identification of the γ-turn motif of the peptide backbone

The first infrared hole burning spectrum of a gas-phase neutral peptide not containing an aromatic amino acid is presented. In the model peptide Z-Pro-NHMe, the amide I and II bands in the 1500 - 1800 cm(-1) region appear to be a clear diagnostic for the secondary structure of the backbone, while the analysis of a series of coupled CH bending modes in the 1000 - 1500 cm(-1) region allows to distinguish between different possible orientations of the chromophore. The geometry of the peptide is strongly constrained by the proline and only one conformation of the backbone is observed, which is identified as a hydrogen bonded gamma-turn.