Ba²⁺ binding in the NaK2K filter in the presence of K⁺. (A) F_o–F_c ion omit map (6σ) of the NaK2K–Ba²⁺ complex and the one-dimensional electron density profile along the filter. The Ba²⁺ ions are modeled as yellow spheres. The density at sites 1, 2, and 4 is modeled as K⁺ ions (green spheres) for simplicity. The inset (boxed) is a magnified view of the external Ba²⁺ with the F_o–F_c ion omit map contoured at 3σ to show the density of water molecules. (B) Anomalous difference Fourier map (7σ) of the NaK2K–Ba²⁺ complex and the one-dimensional anomalous scattering profile at the selectivity filter region.