K⁺ ion hydrodynamic radius. (A) Schematic lateral view of Kv1.2/2.1 paddle chimera structure (PDB accession no. 2R9R) along the pore. The front and back subunits, together with the voltage sensor and T1 domains, were omitted. A schematic representation of the selectivity filter together with two of the pore helices and two Pro475 residues is shown for reference. Also a scheme of the 1 × 1–Å grid and the circles described by r_min and r_max are shown in blue and yellow, respectively. The eye and the arrow drawings indicate the point of view for the following images. (B–E) Surfaces left by rolling a sphere of variable radius onto the van der Waals surface of the protein. Radii varied from 1.3 to 4.6 Å (indicated in each panel). For reference, B also shows the 1 × 1–Å grid used to measure the radii r_min and r_max (represented in blue and yellow, respectively). As the size of the probe increased, the surface left by the probe was less grainy; also, the difference between the two radii decreased. When r_C was ≥4.6 Å (F), the probe was not able to enter the pore. (G) Effective radii of capture, r_C, for r_min and r_max plotted versus r_P, the radius of the probe. The size of the hydrodynamic radius for K⁺ ion was obtained when r_C matched the experimentally obtained capture radius of 0.82 Å (dashed line). The values for the K⁺ ion hydrodynamic radius were 3.8 and 4.1 Å for the comparison with the smaller and the bigger circle data, respectively (vertical arrows).