The Different Nature of Bonding in Cu⁺-Glycine and Cu²⁺-Glycine

A comparative study of the metal−glycine bonding for the biologically relevant Cu + and Cu 2+ pair is presented. The structure and vibrational frequencies for several coordination modes of Cu + and Cu 2+ 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 Cu 2+ -glycine. Whereas for Cu + -glycine, the ground state structure is found to have a bidentated η 2 -N,O coordination in which Cu + interacts with the nitrogen of the amino group and the carbonyl oxygen, the ground state structure of Cu 2+ -glycine is the η 2 - O, O (CO 2 - ) one, derived from the interaction of the metal cation with the CO 2 - 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 CO 2 group compared with other coordinations.