Interaction of Proline with Cu+ and Cu2+ Ions in the Gas Phase

The structures and metal affinities of the binding configurations of Cu+ and Cu2+ to proline have been investigated using the hybrid three-parameter Density Functional Theory(DFT/B3LYP). We found that the metal-proline bonding and the energy ordering of several conformers were very different in Cu+-proline and Cu2+-proline. For Cu+-proline, the ground state structure was found to have a bidentated coordination in which Cu+ was coordinated to the carbonyl oxygen and imino group nitrogen of neutral proline. On the contrary, the ground state structure of Cu2+-proline involves chelation between the two oxygens of the carboxylate group in a zwitterionic proline. The metal ion affinity of proline of the most stable Cu+-proline complex was calculated as 76.0 kcal/mol at 6-311++ G(d, p) level, whereas the Cu2+ ion affinity of proline was calculated as 258.5 kcal/mol.