Rb⁺ and Cs⁺ binding in NaK2K. (A) F_o–F_c ion omit map (6σ, blue mesh) of the NaK2K–Rb⁺ complex and the one-dimensional electron density profile (from a 1.85-Å ion omit map) along the central axis of the filter. The electron density distribution of K⁺ ions in the NaK2K–K⁺ complex is also plotted (in gray) for comparison (and for all other one-dimensional profiles). The Rb⁺ ions are modeled as purple spheres. (B) Anomalous difference Fourier map (7σ, red mesh) of the NaK2K–Rb⁺ complex and the one-dimensional anomalous scattering profile at the selectivity filter region. (C) F_o–F_c ion omit map (6σ) of the NaK2K–Cs⁺ complex and the one-dimensional electron density profile along the filter. The Cs⁺ ions are modeled as cyan spheres. The density at site 4 is modeled as a Na⁺ ion (orange sphere). (D) Anomalous difference Fourier map (7σ) of the NaK2K–Cs⁺ complex and the one-dimensional anomalous scattering profile at the selectivity filter region.