Binding properties of Cu(+/2+)-(glycyl)n glycine complexes (n = 1-3)

The structure, relative energies, and binding energies of the complexes formed by the interaction of Cu+ (d10,1S) and Cu2+ (d9,2D) cations with the (glycyl)n glycine (n = 1-3) oligomers have been theoretically determined by means of density functional methods. The most stable structures of the Cu+ systems present linear dicoordination geometries, in agreement with a recent X-ray absorption spectroscopic study of Cu(I) interacting with model dipeptides. This is attributed to an efficient reduction of metal-ligand repulsion through sd sigma hybridization in dicoordinated linear structures. In contrast, for Cu2+ systems the lowest energy structures are tricoordinated (n = 1), tetracoordinated (n = 2), and pentacoordinated (n = 3). For both copper cations, binding energy values show that the interaction energies increase when the peptide chain is elongated. Differences on the coordination properties of the ligands are discussed according to their length as well as to the electronic configuration of the metal cations, which are compared to the Cu+/2+-glycine systems.