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···π interaction, accompanied by a CH···π interaction. Higher-energy conformers with free OH and NH···π 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···π interaction, which is of CH···π nature for LD, while LL shows a CH···π interaction. Higher-energy conformers also display some differences in the nature of the intramolecular interactions.

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The complex formed between Cinchona alkaloids and sulphuric acid consists of a doubly protonated Cinchona alkaloid strongly bound to a bisulphate HSO4− anion. This structure explains the lack of photo-reactivity of the complex upon UV irradiation.

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