Structural elucidation of biological and toxicological complexes: investigation of monomeric and dimeric complexes of histidine with multiply charged transition metal (Zn and Cd) cations using IR action spectroscopy

The gas-phase structures of singly and doubly charged complexes involving transition metal cations, Zn and Cd, bound to the amino acid histidine (His) as well as deprotonated His (His-H) are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser. IRPMD spectra are measured for CdCl+(His), [Zn(His-H)]+, [Cd(His-H)]+, Zn2+(His)2, and Cd2+(His)2 in the 550-1800 cm-1 range. These studies are complemented by quantum mechanical calculations of the predicted linear absorption spectra at the B3LYP/6-311+G(d,p) and B3LYP/Def2TZVP levels. The monomeric spectra are similar to one another and indicate that histidine coordinates to the metal in a charge-solvated (CS) tridentate form in the CdCl+(His) complex and has a similar tridentate configuration with a deprotonated carboxylic acid terminus in the [M(His-H)]+ complexes. The preference for these particular complexes is also found in the relative energetics calculated at the B3LYP, B3P86, and MP2(full) levels. The spectra of the dimer complexes have obvious CS characteristics, suggesting that at least one of the His ligands is charge solvated; however, there are also signatures for a salt-bridge (SB) formation in the second His ligand. The definitive assignment of a SB ligand is complicated by the presence of the CS ligand and conflicting relative energetics from the different levels of theory