Octa-Coordination and the Aqueous Ba2+ Ion

The hydration structure of Ba2+ ion is important for understanding blocking mechanisms in potassium ion channels. Here, we combine statistical mechanical theory, ab initio molecular dynamics simulations, and electronic structure methods to calculate the hydration free energy and local hydration structure of Ba2+(aq). The predicted hydration free energy (-304 +/- 1 kcal/mol) agrees with the experimental value (-303 kcal/mol) when a maximally occupied, unimodal inner solvation shell is treated. In the local environment defined by the first shell of hydrating waters, Ba2+ is directly and stably coordinated by eight (8) waters. Octa-coordination resembles the crystal structure of Ba2+ and K+ bound in potassium ion channels, but differs from the local hydration structure of K+(aq) determined earlier.