Identification of histidine-Ni (II) metal complex by Raman spectroscopy

Metal ions such as nickel have a strong binding affinity to amino acids and form metal complexes of different geometry. This complex formation is influenced by many factors among others the kind of reactants and their relative concentrations, pH, and its effect on the net charge of the amino acid molecule. In this work, the formation of the histidine-nickel (II) complex in different conditions was examined by Raman spectroscopy. In addition to the experiments, density functional theory (DFT) calculations on histidine-nickel complexes were performed in order to elucidate the complex formation mechanism and optimal geometry of the structures as well as to investigate their vibrational properties. The Raman measurements showed double peaks at 1272 and 1297 cm(-1), and triple peaks at 1322, 1336, and 1355 cm(-1) that belong to the metal complex. The geometry optimizations and total energy calculations of His-Ni (II) complex revealed that the octahedral geometry and the triplet spin state of Ni ion is the energetically favorable structure. This metal complex is formed through the nitrogen atom of the imidazole side chain, the nitrogen atom of the terminal amino group, and the oxygen atom of the carboxyl group. Experiments with pH revealed that the alkaline pH favors while the change of the concentration of the metal ions does not affect the the His-Ni (II) complex formation.