Does the Residues Chirality Modify the Conformation of a Cyclo-Dipeptide? Vibrational Spectroscopy of Protonated Cyclo-diphenylalanine in the Gas Phase

The structure of a protonated diketopiperazine dipeptide, cyclo-diphenylalanine, is studied by means of infrared multiple photon dissociation spectroscopy combined with quantum chemical calculations. Protonation exclusively occurs on the oxygen site and, in the most stable conformer, results to an intramolecular OH center dot center dot center dot pi interaction, accompanied by a CH-pi interaction. Higher energy conformers with free OH and NH center dot center dot center dot pi interactions are observed as well, due to kinetic trapping. Optimization of the intramolecular interactions involving the aromatic ring dictates the geometry of the benzyl substituents. Changing the chirality of one of the residues has consequences on the CH center dot center dot center dot pi interaction, which is of C alpha H center dot center dot center dot pi nature for LD, while LL shows a C beta H center dot center dot center dot pi interaction. Higher-energy conformers also display some differences in the nature of the intramolecular interactions.