An investigation of inter-ligand coordination and flexibility: IRMPD spectroscopic and theoretical evaluation of calcium and nickel histidine dimers

Metallated gas-phase structures consisting of an intact and deprotonated histidine (His) ligand, M(His-H)(His)⁺, where M = Ca and Ni, were examined using infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light from a free-electron laser (FEL). In parallel, ab initio quantum-chemical calculations identified several low-energy isomers for each complex. Experimental action spectra were compared to linear absorption spectra calculated at the B3LYP level of theory, using the 6-311+G(d,p) basis set. Single-point energies were calculated at B3LYP, B3LYP-GD3BJ, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set. For Ca(His-H)(His)⁺, the dominant structure has the metal center coordinating with the π nitrogen of the imidazole ring (N_π) and both oxygen atoms of the carboxylate group of the deprotonated His ligand while coordinating with the backbone amine (N_α), N_π, and the carbonyl oxygen of the carboxylic acid of the intact His ligand. The Ni(His-H)(His)⁺ species coordinates the metal ion through N_α, N_π, and the carbonyl oxygen for both the deprotonated and intact His ligands, but also shows evidence for a minor secondary structure where the deprotonated His coordinates the metal at N_α, N_π, and the deprotonated carbonyl oxygen and the intact His ligand is zwitterionic, coordinating the metal with both carboxylate oxygens. Different levels of theory predict different ground structures, highlighting the need for utilizing multiple levels of theory to help identify the gas-phase structure actually observed experimentally.