Hydration of valine-cation complexes in the gas phase: On the number of water molecules necessary to form a zwitterion

The stepwise hydration of valine-alkali metal ion complexes, Val·M⁺(H₂O)_n, n = 2-6, M = Li, Na, and K, is investigated using both theory and experiment. Experimentally, the rate of water loss from the valine clusters is measured using blackbody infrared radiative dissociation. The kinetics for the loss of one water molecule from these clusters are compared to those from model clusters of known zwitterionic vs nonzwitterionic structure. Both theory and experiment indicate that the structure of Val·Li⁺(H₂O)₂ is very similar to that of the singly and nonhydrated complexes investigated previously; the lithium is coordinated between the nitrogen and carbonyl oxygen of nonzwitterionic valine, and the water molecules interact solely with the metal ion. The third water molecule changes the structure of the Val·Li⁺ cluster significantly. The metal ion coordinates to the C-terminal end of zwitterionic valine and to two of the water molecules. The third water molecule hydrogen bonds to the protonated N terminus of valine. Thus, the third water molecule is the first one that interacts directly with the valine, and this stabilizes the zwitterionic form of valine over the nonzwitterionic form. The dissociation of the sixth water molecule from the valine cluster is slower than that of the fifth, indicating that the cluster with six waters is especially stable relative to the cluster with five water molecules. This provides further support for zwitterionic valine in the presence of only a limited number of water molecules. For M = Na, two water molecules changes the metal binding position from NO coordination to the C terminus of valine. The experiment is unable to distinguish the zwitterionic vs nonzwitterionic character of valine in this complex, but theory indicates the nonzwitterion form. As is the case with lithiated clusters, Val·Na⁺-(H₂O)₆ is more stable than Val·Na⁺(H₂O)₅. Computational results for M = K predict that the most stable conformation of Val·K⁺(H₂O)₂ resembles Val·Na⁺(H₂O)₂, whereas the kinetic data for the sodiated and potassiated clusters, although inconclusive, suggest the zwitterion form. The stepwise hydration studies presented here indicate that very few water molecules are necessary to cause valine to adopt its solution-phase zwitterionic structure.