Structure and collision-induced dissociation of the protonated cyclo His-Phe dipeptide: mechanistic studies and stereochemical effects

The role of stereochemical factors on the structure and the fragmentation paths of the protonated cyclic dipeptide cyclo histidine–phenylalanine is studied under ion traps conditions by combining tandem mass spectrometry, laser spectroscopy, quantum chemical calculations and chemical dynamics simulations. Vibrational spectroscopy obtained by Infrared Multiple Photon Dissociation (IRMPD) reveals a small difference between the two diastereomers, c- LLH + and c- LDH + , arising mainly from ancillary CH... π interactions. In contrast, there is a strong influence of the residues chirality on the collision-induced dissociation (CID) processes. Chemical dynamics simulations rationalize this effect and evidence that proton mobility takes place, allowing isomerization to intermediate cyclic structures that are different for c- LLH + and c- LDH + , resulting in different barriers to proton mobility. This effect is related to the protonation of the imidazole ring. It contrasts with the minute stereochemical effects observed for other cyclic dipeptides in which the proton is borne by an amide CO.