Interaction of alkali metal cations (Li+-Cs+) with glycine in the gas phase: A theoretical study
Résumé
The complexes formed by alkali metal cations (Cat(+)) and glycine (Gly) were studied by means of ab initio quantum chemical methods. Seven types of Gly-Cat(+) interaction were considered in each case. It was found that in the most stable forms of Gly-Li+ and Gly-Na+ the metal ion is chelated between the carbonyl oxygen and nitrogen ends of glycine. For Gly-K+ an isomer involving complexation with both oxygens of the carboxylic function is found to be degenerate with the above chelate, and becomes slightly more stable for Gly-Rb+ and Gly-Cs+. In all cases, interaction of the ion with the carboxylate group of zwitterionic glycine is also low in energy. Computed binding energies (Delta H-298, kcal mol(-1)) are 54.5 (Gly-Li+), 36.3 (Gly-Na+), 26.5 (Gly-K+), 24.1 (Gly-Rb+) and 21.4 (Gly-Cs+). The values for Gly-Na+ and Gly-K+ are in good agreement with recent experimental determinations. For Gly-Li+, a revised experimental value of 54.0 kcal mol(-1) is obtained, based on the computed complexation enthalpy and entropy of Li+ with N,N-dimethylformamide (51.7 kcal mol(-1) and 23.8 cal mol(-1) K-1, respectively). Three isomers among the most stable of the lithiated dimer Gly-Li+-Gly have been determined and found to involve local Gly-Li+ interactions analogous to those in the monomer However, the relative energies of the various isomers show nonnegligible differences between the monomer and the dimer, implying that the kinetic method must be used with care for the determination of cation affinities of larger molecules. Finally, the fluxionality of the Gly-Na+ complex has been considered by locating the transition states for interconversion of the lowest energy isomers. In particular it is found that the lowest isomer can be transformed into the one involving zwitterionic Gly with a rate-determining barrier of 20.4 kcal mol(-1).