The different nature of bonding in Cu+-glycine and Cu2+-glycine

A comparative study of the metal-glycine bonding for the biologically relev ant Cu+ and Cu2+ pair is presented. The structure and vibrational frequenci es for several coordination modes of Cu+ and Cu2+ to glycine have been dete rmined using the hybrid three-parameter B3LYP density functional approach. Single-point calculations have also been carried out at the modified couple d pair functional (MCPF) and single- and double- (triple) excitation couple d cluster (CCSD(T)) levels of theory and using larger basis sets. Calculati ons have shown that the metal-glycine bonding and the energy ordering of th e different conformers are very different in Cu+-glycine than in Cu2+-glyci ne. Whereas for Cu+-glycine, the ground state structure is found to have a bidentated eta(2)-N,O coordination in which Cu+ interacts with the nitrogen of the amino group and the carbonyl oxygen, the ground state structure of Cu2+-glycine is the eta(2)-O,O (CO2-) one, derived from the interaction of the metal cation with the CO2- terminus of the zwitterionic glycine. In thi s case, the results seem to indicate that glycine acquires an important rad ical character that changes the relative metal affinities of the different basic sites, which favors the interaction of the metal cation with the CO2 group compared with other coordinations.