A comparative study of the metal-glycine bonding for the biologically relevant Cu+ and Cu2+ pair is presented. The structure and vibrational frequencies for several coordination modes of Cu+ and Cu2+ to glycine have been determined using the hybrid three-parameter B3LYP density functional approach. Single-point calculations have also been carried out at the modified coupled pair functional (MCPF) and single- and double-(triple) excitation coupled cluster (CCSD(T)) levels of theory and using larger basis sets. Calculations have shown that the metal-glycine bonding and the energy ordering of the different conformers are very different in Cu+-glycine than in Cu2+-glycine. 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 this case, the results seem to indicate that glycine acquires an important radical 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.