Binding Energies of Water to Sodiated Valine and Structural Isomers in the Gas Phase:  The Effect of Proton Affinity on Zwitterion Stability

The structures of valine (Val) and methylaminoisobutyric acid (Maiba) bound to a sodium ion, both with and without a water molecule, are investigated using both theory and experiment. Calculations indicate that, without water, sodiated Val forms a charge-solvated structure in which the sodium ion coordinates to the nitrogen and the carbonyl oxygen (NO-coordination), whereas Maiba forms a salt-bridge structure in which the sodium ion coordinates to both carboxylate oxygens (OO-coordination). The addition of a single water molecule does not significantly affect the relative energies or structures of the charge-solvated and salt-bridge forms of either cluster, although in Maiba the mode of sodium ion binding is changed slightly by the water molecule. The preference of Maiba to adopt a zwitterionic form in these complexes is consistent with its higher proton affinity. Experimentally, the rates of water evaporation from clusters of Val·Na+(H2O) and Maiba·Na+(H2O) are measured using blackbody infrared radiative dissociation (BIRD). The dissociation rates from the Val and Maiba complexes are compared to water evaporation rates from model complexes of known structure over a wide range of temperatures. Master equation modeling of the BIRD kinetic data yields a threshold dissociation energy for the loss of water from sodiated valine of 15.9 ± 0.2 kcal/mol and an energy of 15.1 ± 0.3 kcal/mol for the loss of water from sodiated Maiba. The threshold dissociation energy of water for Val·Na+(H2O) is the same as that for the charge-solvated model isomers, while the salt-bridge model complex has the same water threshold dissociation energy as Maiba·Na+(H2O). These results indicate that the threshold dissociation energy for loss of a water molecule from these salt-bridge complexes is ∼1 kcal/mol less than that for loss of water from the charge-solvated complexes.