Complexation of glycine by atomic metal cations in the gas phase

The interaction of glycine with 15 metal cations (M⁺ or M²⁺) in the gas phase has been studied by quantum chemical calculations. Three types of complexation have been considered: (i) chelation between nitrogen and the carbonyl oxygen, (ii) attachment to the carboxyl group of neutral glycine and (iii) attachment to the carboxylate group of zwitterionic glycine. It is found that the relative energies of these structures and, therefore, the nature of the lowest energy isomer, depend dramatically upon the metal ion. In several cases, metal ion attachment to glycine results in a switch from the neutral form (the most stable form of gaseous glycine) to the zwitterion (the most stable form of glycine in solution). This occurs with doubly-charged cations and, in some cases, with monocations. Several metal properties are invoked to explain these results: metal charge, size, electron affinity and polarizability. The role of metal-ion polarizability is illustrated by the computed geometries of M(CH₃OH)₂ⁿ⁺ complexes.