Binding energies of water to doubly hydrated cationized glutamine and structural analogues in the gas phase

The modes of metal-ion and water binding in doubly hydrated complexes of lithiated and sodiated glutamine (Gln) are probed using blackbody infrared radiative dissociation experiments and density functional theory calculations. Threshold dissociation energies, E0, for loss of a water molecule from these complexes are obtained from master-equation modeling of these data. The values of E0 are 36 +/- 1 and 38 +/- 2 kJ/mol for the lithiated and sodiated glutamine complexes, respectively, and are consistent with calculated water binding energies for the nonzwitterionic form of these complexes. Calculated water binding energies for the zwitterionic forms of these complexes are significantly higher. In contrast, calculations indicate that the zwitterionic form of Gln in these complexes is more stable than the nonzwitterionic form by 8 and 15 kJ/mol when lithiated and sodiated, respectively. Doubly hydrated lithiated and sodiated complexes of asparagine methyl ester (AsnOMe), asparagine ethyl ester (AsnOEt), and glutamine methyl ester (GlnOMe) were also studied for comparison to Gln. Although these clusters lack the acidic group of Gln and therefore have different water coordination behavior, these results further support the conclusion that Gln is nonzwitterionic in these clusters. Surprisingly, the complexes containing sodium are more stable than those containing lithium, a result that is attributed to subtle differences in how these two metal ions bind to the amino acid esters in these complexes.