Figure 3.

Influence of coordination number and ligand charge on ion selectivity. (A) Selectivity between K⁺ and Na⁺ is shown with 2 kcal/mol contour differences. Regions that yield K⁺ selectivity are indicated in red, Na⁺ selectivity in blue, and regions where ions will not leave water to enter the site in black. The numbers in circles correspond to the molecules in Table 2: 1. K⁺ channel model; 2. Na⁺ channel model; 7. DNA quadruplex; and 8. valinomycin. (B) Selectivity map for multiple group I ions with regions shown that are selective for Li⁺ (green), Na⁺ (blue), K⁺ (red), Rb⁺ (cyan), and Cs⁺ (yellow). The percentage contribution to selectivity between K⁺ and Na⁺ by (C) the restriction of coordination number, χ_n and (D) the partial charge on the coordinating ligands, χ_q. Color indicates whether this contribution is toward K⁺ (reds) or Na⁺ (blues) selectivity. (E) The effect on Na⁺/K⁺ selectivity when two of the n ligands are water molecules. (F) The effect on Na⁺/K⁺ selectivity when the coordination number of Na⁺ can be different to that of K⁺ with q = 0.5.