Intramolecular interactions in protonated peptides: H + PheGlyGly and H + GlyGlyPhe

The conformations of protonated PheGlyGly and GlyGlyPhe tripeptides, generated at temperatures approximately 300-350 K through a photochemical mechanism, were investigated in the gas phase using R-IRMPD spectroscopy in the OH and NH stretch region in combination with quantum chemistry calculations. The results aid characterisation of their conformational landscapes and specifically, help identify the intramolecular interactions that control the peptide conformations. The dominant intramolecular interaction in protonated PheGlyGly operates between the N-and C-termini but in protonated GlyGlyPhe there is a strong cation-pi interaction. The observation of a cation-pi interaction in the warm ions highlights its strong, stable nature.