Single-channel recording of ion permeation from experiment and simulation. Electrophysiological recording of current through a potassium channel (top) and recording of ion permeation from an all-atom simulation of the Kv1.2 potassium channel (bottom; cf. Jensen et al., 2010). (Bottom left) The simulated model, consisting of lipid molecules (spheres), Kv1.2 (orange; only the pore domain is shown), potassium ions (shown as green spheres within the selectivity filter [white box]; remaining K⁺ and Cl⁻ ions not shown), lipid molecules (chains of spheres), and water molecules (red/white spheres and gray surface). (Bottom middle) Magnification of the selectivity filter. (Bottom right) Positions of K⁺ ions permeating the selectivity filter (left ordinate; drawn to scale with the selectivity filter) versus simulation time (abcissa) at a depolarizing voltage of ∼25 mV. Traces of individual ion positions are shown in various colors. Red line and circles indicate accumulated permeation events of individual K⁺ ions (right ordinate). The experimental channel recording was originally published by Li et al. (2006) and is reprinted here with permission from Proceedings of the National Academy of Sciences of the United States of America (PNAS). Use of this figure is gratefully acknowledged.