IRMPD Spectroscopic and Theoretical Structural Investigations of Zinc and Cadmium Dications Bound to Histidine Dimers

Metallated gas-phase structures consisting of a deprotonated and an intact histidine (His) ligand, yielding M(His-H)(His)(+), where M = Zn and Cd, were examined with infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light from a free-electron laser (FEL). In parallel, quantum chemical calculations identified several low-energy isomers for each complex. Experimental action spectra were compared to linear spectra calculated at the B3LYP level of theory using the 6-311+G(d,p) and def2-TZVP basis sets for the zinc and cadmium complexes, respectively. For both Zn and Cd species, the definitive assignment is complicated by conflicting relative energetics, which were calculated at B3LYP, B3LYP-GD3BJ, B3P86, and MP2(full) levels. Spectral comparison for both species indicates that the dominant conformation, [N-alpha, N-pi, CO-][CO2-](N pi H+), has the deprotonated His chelating the metal at the amine nitrogen, Jr nitrogen of the imidazole ring, and the deprotonated carbonyl oxygen and that the intact His ligand adopts a salt-bridge bidentate binding motif, coordinating the metal with both carboxylate oxygens. There is also evidence for a conformation where the deprotonated His coordination is maintained, but the intact His ligand adopts a more canonical structure, coordinating with the metal atom at the amine nitrogen and pi nitrogen, [N-alpha, N-pi, CO-][N-alpha, N-pi]gtgg. For both metallated species, B3LYP, B3P86, and B3LYP-GD3BJ levels of theory appear to describe the relative stability of the dominant zwitterionic species more accurately than the MP2(full) level.