Structural and electronic characterization of the complexes obtained by the interaction between bare and hydrated first-row transition-metal ions (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) and glycine

The complexes formed by the simplest amino acid, glycine, with different bare and hydrated metal ions (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) were studied in the gas phase and in solvent in order to give better insight into the field of the metal ion-biological ligand interactions. The effects of the size and charge of each cation on the organization of the surrounding water molecules were analyzed. Results in the gas phase showed that the zwitterion of glycine is the form present in the most stable complexes of all ions and that it usually gives rise to an eta O-2,O coordination type. After the addition of solvation sphere, a resulting octahedral arrangement was found around Ni2+, Co2+, and Fe2+ ions in their high-spin states, whereas the bipyramidal-trigonal (Mn2+ and Zn2+) or square-pyramidal (Cu2+) geometries were observed for the other metal species, according to glycine behaves as bi- or monodentate ligand. Despite the fact that the zwitterionic structure is in the ground conformation in solution, its complexes in water are less stable than those obtained from the canonical form. Binding energy values decrease in the order Cu2+ > Ni2+ > Zn2+ approximate to Co2+ > Fe2+ > Mn2+ and Cu2+ > Ni2+ > Mn2+ approximate to Zn2+ > Fe2+ > Co2+ for M2+-Gly and Gly-M2+(H2O)(n) complexes, respectively. The nature of the metal ion-ligand bonds was examined by using natural bond order and charge decomposition analyses.