A comparison of copper(I) and silver(I) complexes of glycine, diglycine and triglycine

Density functional calculations at B3LYP/DZVP were used to obtain structural information, relative free energies of different isomers and binding energies for the following reaction in the gas phase: M+ + (glycyl)(n)glycine --> M-(glycyl)(n)glycine(+), where M = Ag or Cu and n=0-2. For the complexes with Cu+, optimizations were also performed at B3LYP/6-31++G(d,p) and single-point calculations at MP2(fc)/6-311++G(2df,2p)//B3LYP/DZVP. The calculated binding energies for the Cu+ complexes are all higher than those of the structurally similar Ag+ ions. These calculated binding energy differences become larger as the size of the ligand increases. For all the Cu+ complexes examined, the coordination number of the copper ion does not exceed two, whereas for the silver complexes tri- and tetracoordinate Ag+ structures are calculated to be at low energy minima. Significant structural and relative free energy differences occur between the lowest energy 'zwitterionic' forms of the M-(glycyl)(n)glycine(+) complexes.