On the question of salt bridges of cationized amino acids in the gas phase: glycine and arginine

The geometrical shapes of the sodiated and cesiated amino acids glycine and arginine were probed in the gas phase by using the ion mobility based ion chromatography method. The data were compared to those obtained for alkali cationized methyl esters and for all the protonated species. Molecular mechanics, semiempirical, and ab initio/density functional theory (DFT) calculations were carried out to generate model structures for comparison with experiment and to determine the relative energies of different structures. For alkali cationized glycine the experimental cross sections agreed with charge solvation structures which were found by calculation to be the most stable forms as well. Both experiment and theory indicated that sodium is solvated by both the amino and the carbonyl groups, while cesium is solvated by one or both oxygen(s) of the carboxyl group. Alkali cationized arginine was found to form a salt bridge structure. The carboxylate group is stabilized by both the charged guanidinium group and the alkali ion. High level (6-311++G7 and DZVP) ab initio/DFT calculations carried out on sodiated and rubidiated N amidinoglycine, which contains a guanidino group and which was used as a model for the larger arginine molecule, indicated that the salt bridge structures are ～10 kcal/mol more stable than the charge solvation forms for both alkali ions. The structure of protonated arginine, i.e. salt bridge or charge solvation, could not be unambiguously determined.