Figure 4.

K⁺ and Cl⁻ densities in the presence of rectification blockers Mg²⁺ and SPM⁴⁺. Density plots of: K⁺ (A) and Cl⁻ (B) with 4 Mg²⁺ ions inside the CD of ROMK (C); K⁺ (D) and Cl⁻ (E) with 4 Mg²⁺ ions inside the CD of IRK (F). Each plot was calculated from an average over three simulations. Density plots of: K⁺ (G), Cl⁻ (H), and SPM⁴⁺ (I) in a single structural model of ROMK; K⁺ (J), Cl⁻ (K), and SPM⁴⁺ (L) in a single structural model of IRK. Averaged density plots for single spermine simulations are shown in Fig. S9. Density plots of K⁺, Cl⁻, and SPM⁴⁺ after an additional spermine molecule is added along the pore from −24 Å < Z < −6 Å and simulated for 10 ns: K⁺ (M), Cl⁻ (N), and SPM⁴⁺ (O) in ROMK, and K⁺ (P), Cl⁻ (Q), and SPM⁴⁺ (R) in IRK. The spermine densities are calculated using the nitrogen atoms (N₁, N₂, N₃, N₄) as the ions. A molecular density of ρ = 6.022 × 10⁻⁴ ions/ų is equivalent to 1 M concentration, and contours are shown for densities corresponding to 1 and 2.5 M. Note that whereas K⁺ and Cl⁻ adequately sample the system and converge to 1 M in the bulk, Mg²⁺ and SPM⁴⁺ remain mainly in the pore throughout the simulations. Therefore, the Mg²⁺ and SPM⁴⁺ distributions can only be interpreted as local densities and do not represent equilibrium.