The gas-phase model tripeptides N-acetyl-Phe-Pro-NH₂ and N-acetyl-Pro-Phe-NH₂ have been studied experimentally and theoretically in order to investigate the local conformational preferences of the peptide backbone and their competition with secondary structures under solvent-free conditions. The combination of UV and IR spectroscopies shows that, under supersonic beam conditions, only a reduced number of conformations are formed, indicating efficient conformational relaxation processes in these species. IR spectroscopy in the NH stretch spectral range combined with density functional theory calculations proves to be a very efficient tool to assign the structure of these species in terms of intramolecular H-bonding. Classical secondary structures of biology, like repeated γ-turns are observed as major conformations. Only one minor conformation of N-Ac-Phe-Pro-NH₂ was assigned to a β-turn structure. According to the nature of the main conformers, the backbone conformational trends on the phenylalanine (Phe) residue is shown to be very dependent upon the neighbouring residues: Phe adopts a β conformation when alone (in N-acetyl-Phe-NH₂) or when followed by a proline residue (in N-acetyl-Phe-Pro-NH₂) but favours a γ_L conformation when preceded by proline (in N-acetyl-Pro-Phe-NH₂). These subtle preferences, resulting from a competition between weakly polar or dispersive interactions, constitute a very stringent test of the theoretical tools for protein modelling and simulation.