Salt bridge structures in the absence of solvent? The case for the oligoglycines

Protonated and sodiated oligoglycines, Gly(n) (n = 1-6), were generated in the gas phase using matrix-assisted laser desorption ionization and their structure probed by measuring collision cross sections in helium. It was found that the sodiated oligoglycines have larger cross sections than the protonated forms and that the difference between the cross sections of the two forms increases with increasing oligoglycine size (n = 2-5) reaching a value of >11% for pentaglycine. This observation indicated that the protonated forms are more compact and spherical than the sodiated species. Theoretical studies including ab initio MP2, density functional theory, and molecular mechanics calculations indicated 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 bridge system. However, structures obtained when the sodium ion was solvated by nonzwitterionic oligoglycines were fairly extended with strongly oblate shapes. Cross sections calculated for these latter structures agreed well with the experimental data of the sodiated species, while cross 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) indicated that the salt bridge forms are less stable than the charge solvation 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.