Mid- and near-infrared spectra of conformers of H-Pro-Trp-OH

We present near- and mid-infrared-UV double resonance spectra of the natural dipeptide H-Pro-Trp-OH. Two conformers are present in the supersonic expansion: a stretched conformer with fully extended backbone and a folded conformer with an OH center dot center dot center dot OCpep hydrogen bond. Both conformers are stabilized by dispersion interaction between indole ring and peptide backbone and a NHpep/N-proline contact. The vibrational and conformational assignment is supported by DFT and MP2 calculations. An adequate description of the energetic order of different conformers requires the explicit inclusion of dispersion and geometry optimization at the MP2 level. We will address the very sensitivity of the observed conformations to the structure of the end groups.