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(+)(H(2)O) and Maiba.Na(+)(H(2)O) 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(+)(H(2)O) 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(+)(H(2)O). These results indicate that the threshold dissociation energy for loss of a water molecule from these salt-bridge complexes is approximately 1 kcal/mol less than that for loss of water from the charge-solvated complexes.
Department of Chemistry, University of California, Berkeley, California 94720-1460, USA.
Andrew S
AS
Lemoff
Matthew F
MF
Bush
Evan R
ER
Williams
D002087:Butyrates / Q000737:chemistry; D005740:Gases; D058610:Isobutyrates; D007700:Kinetics; D008958:Models, Molecular; D011522:Protons; D012964:Sodium / Q000737:chemistry; D013816:Thermodynamics; D014633:Valine / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
D002087:Butyrates; D005740:Gases; D058610:Isobutyrates; D011522:Protons; D014867:Water; C020380:isobutyric acid; D012964:Sodium; D014633:Valine