Jet-cooled laser spectroscopy and solid-state vibrational circular dichroism of the cyclo-(Tyr-Phe) diketopiperazine dipeptide

• The gas-phase structure of a cyclic dipeptide is determined by laser spectroscopy. • Cyclo Tyr-Phe-exists in two geometries, with one ring folded and the other extended. • Minor structural differences happen depending on the chirality of the residues. • The solid-state structure is studied by vibrational circular dichroism. • Dimers involving a double hydrogen bond account for the solid-state results. The structures of the diastereomer diketopiperazine dipeptides cyclo(LTyr-LPhe) and cyclo-(DTyr-LPhe) are studied in the gas phase using resonance-enhanced two-photon ionization, IR/UV double resonance spectroscopy and quantum chemical calculation. Both diastereomers exhibit two conformations, with the tyrosine ring folded over the DKP ring and the phenylalanine ring extended outwards, or vice versa . The two diastereomers differ only slightly by the nature of weak CH···π and NH···π interactions. The crystal structure of cyclo(LTyr-LPhe) is determined by X-ray crystallography and is composed of monomers with folded tyrosine ring. The vibrational circular dichroism spectrum is interpreted by the existence of dimers in the solid state. Quantum chemical calculation shed light on the structural modifications between the gas phase and the solid state.