Structure of the protected dipeptide Ac-Val-Phe-OMe in the gas phase: Towards a β-sheet model system

In this paper we report on the structure of the isolated dipeptide Ac–Val–Phe–OMe (Val=valine, Phe=phenylalanine) which is protected at the terminal positions by introducing an acetyl and a methyl group. Both resonant two-photon ionization (R2PI) and IR/R2PI spectroscopy are applied. This is the first application of IR/R2PI spectroscopy to a dipeptide. Both the region of the C–H and N–H stretching vibrations as well as the region of the C=O stretching vibrations are investigated. The chosen dipeptide exhibits only one prominent conformer in the gas phase containing a “linear” non-hydrogen-bonded structure which is an ideal candidate for a β-sheet model.