Determining gas-phase chelation of zinc, cadmium, and copper cations with HisHis dipeptide using action spectroscopy and theoretical calculations

Using light generated by an infrared free electron laser, action spectroscopy was performed on doubly charged complexes of the metalated dipeptide histidyl-histidine (HisHis). Metal cations used were zinc, cadmium, and copper. Molecular dynamics and quantum-chemical calculations were used to screen a large number of conformers, whose theoretical infrared spectra were compared to the recorded action spectra of these metalated complexes. The zinc and cadmium spectra display dominant features associated with an iminol binding motif of the HisHis ligand, where the metal ion coordinates with both pros (π) nitrogens of the imidazole sidechains, the terminal carbonyl oxygen, and the backbone nitrogen for which the hydrogen ordinarily bound here has migrated to a carbonyl. The copper complex was difficult to assign to a single species, because a few predicted bands are absent from the experimental spectrum. The theoretical single point energies were also calculated for all structures examined, and DFT methods were found to describe the ion conformer populations in the case of the zinc and cadmium chelates better than the MP2 prediction.