The conformational structure of short peptide chains in the gas phase is studied by laser spectroscopy of a series of protected dipeptides, Ac-Xxx-Phe-NH2, Xxx = Gly, Ala and Val. The combination of laser desorption with supersonic expansion enables us to vaporize the peptide molecules and cool them internally; IR/UV double resonance spectroscopy in comparison to density functional theory calculations on Ac-Gly-Phe-NH2 permits to identify and characterize the conformers populated in the supersonic expansion. Two main conformations, corresponding to secondary structures of proteins, are found to compete in the present experiments. One is composed of a doubly γ-fold corresponding to the 27 ribbon structure. Topologically, this motif is very close to a β-strand backbone conformation. The second conformation observed is the β-turn, responsible for the chain reversal in proteins. It is characterized by a relatively weak hydrogen bond linking remote NH and CO groups of the molecule and leading to a 10-membered ring. The present gas phase experiment illustrates the intrinsic folding properties of the peptide chain and the robustness of the β-turn structure, even in the absence of a solvent. The β-turn population is found to vary significantly with the residues within the sequence; the Ac-Val-Phe-NH2 peptide, with its two bulky side-chains, exhibits the largest β-turn population. This suggests that the intrinsic stabilities of the 27 ribbon and the β-turn are very similar and that weakly polar interactions occurring between side-chains can be a decisive factor capable of controlling the secondary structure.
