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 mech anics, semiempirical, and ab initio/density functional theory (DFT) calcula tions were carried out to generate model structures for comparison with exp eriment and to determine the relative energies of different structures. For all;ali cationized glycine the experimental cross sections agreed with cha rge solvation structures which were found by calculation to be the most sta ble forms as well. Both experiment and theory indicated that sodium is solv ated 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 wa s found to form a salt bridge structure. The carboxylate group is stabilize d by both the charged guanidinium group and the alkali ion. High level (6-3 11++G** and DZVP) ab initio/DFT calculations carried out on sodiated and ru bidiated N amidinoglycine, which contains a guanidino group and which was u sed as a model for the larger arginine molecule, indicated that the salt br idge structures are similar to 10 kcal/mol more stable than the charge solv ation forms for both alkali ions. The structure of protonated arginine, i.e . salt bridge or charge solvation, could not be unambiguously determined. ( C) 1999 Elsevier Science B.V.