SALT BRIDGE STRUCTURES IN THE ABSENCE OF SOLVENT - THE CASE FOR THE OLIGOGLYCINES

Protonated and sodiated oligoglycines, Gly(n) (n = 1-6), were generate d in the gas phase using matrix-assisted laser desorption ionization a nd their structure probed by measuring collision cross sections in hel ium. It was found that the sodiated oligoglycines have larger cross se ctions than the protonated forms and that the difference between the c ross sections of the two forms increases with increasing oligoglycine size (n = 2-5) reaching a value of >11% for pentaglycine. This observa tion indicated that the protonated forms are more compact and spherica l than the sodiated species. Theoretical studies including ab initio M P2, density functional theory, and molecular mechanics calculations in dicated that protonated oligoglycines assume almost spherical shapes. The same was true for sodiated forms if it was assumed that the sodium ion was bound to a zwitterion oligoglycine structure via a salt bridg e system. However, structures obtained when the sodium ion was solvate d by nonzwitterionic oligoglycines were fairly extended with strongly oblate shapes. Cross sections calculated for these latter structures a greed well with the experimental data of the sodiated species, while c ross sections calculated for the spherical shapes agreed well with the experimental data of the protonated forms. Relative energies obtained from calculations (B3LYP/6-311++G*) on Gly(n)Na(+) (n = 1-4) indicat ed that the salt bridge forms are less stable than the charge solvatio n forms by 3, 14, 8, and 8 kcal/mol for n 1, 2, 3, and 4, respectively . Both theory and experiment indicated that sodiated oligoglycines do not form salt bridge structures in the gas phase.