Electrostatic interaction free energy of ions along the central pathways in zfP2X4 and rP2X2. Results for zfP2X4 are shown in A and B, whereas those for rP2X2 are shown in C and D. (A and C) ΔΔG_int of Na⁺ (black), Ca²⁺ (green), and Cl⁻ (blue), calculated along the central pathway, with the transmembrane region depicted with the yellow slab and the center of the membrane at Z = 0 Å. The pore radius profile is shown in the bar graph on top. (B and D) ΔΔG_int of Na⁺ along the central pathway of the original closed model (black), and an artificially opened pore widened to 5-Å radius from the molecular threefold symmetry axis (red). The surface representations of the original closed (white) and the forced open (red) conformations are shown.