  Ion hydration structure and free energy establish criteria for understanding
selective ion binding in potassium K+ ion channels, and may be significant to
understanding blocking mechanisms as well. Recently, we investigated the
hydration properties of Ba2+, the most potent blocker of K+ channels among the
simple metal ions. Here, we use a similar method of combining ab initio
molecular dynamics simulations, statistical mechanical theory, and electronic
structure calculations to probe the fundamental hydration properties of Sr2+,
which does not block bacterial K+ channels. The radial distribution of water
around Sr2+ suggests a stable 8-fold geometry in the local hydration
environment, similar to Ba2+. While the predicted hydration free energy of
-331.8 kcal/mol is comparable with the experimental result of -334 kcal/mol,
the value is significantly more favorable than the -305 kcal/mol hydration free
energy of Ba2+. When placed in the innermost K+ channel blocking site, the
solvation free energies and lowest energy structures of both Sr2+ and Ba2+ are
nearly unchanged compared with their respective hydration properties. That
result suggests that differences in blocking behavior arise due to free
energies associated with exchange of water ligands for channel ligands instead
of free energies of transfer from water to the binding site.
