Gas phase intramolecular proton transfer in cationized glycine and chlorine substituted derivatives (M-gly, M = Na+, Mg2+, Cu+, Ni+, and Cu2+): Existence of zwitterionic structures?

The intramolecular proton transfer in cationized glycine and chlorine subst ituted derivatives with M=Na+, Mg2+, Ni+, Cu+, and Cu2+ has been studied wi th the three parameter B3LYP density functional method. The coordination of metal cations to the oxygens of the carboxylic group of glycine stabilizes the zwitterionic structure. For all monocations the intramolecular proton transfer occurs readily with small energy barriers (1-2 kcal mol(-1)). For the dication Mg2+ and Cu2+ systems, the zwitter-ionic structure becomes ver y stable. However, whereas for Mg2+, the proton transfer process takes plac e spontaneously, for Cu2+ the reaction occurs with an Important energy barr ier. The substitution of the hydrogens of the amino group by chlorine atoms decreases the basicity of nitrogen, which destabilizes the zwitterionic st ructure. For monosubstituted glycine complexed with Na+, the zwitterionic s tructure still exists as a minimum, but for disubstituted glycine no minimu m appears for this structure. In contrast, for Mg2+ complexed to mono- and disubstituted glycine, the zwitterionic structure remains the only minimum, since the enhanced electrostatic interaction with the dication overcomes t he destabilizing effect of the chlorine atoms.