Preferred binding sites of Rb ⁺ within the SF. (A–C) Conduction trajectory for Rb⁺ along the SF axis from simulations in mixed-ionic solutions with K⁺, at different voltages, and for different initial cation placements within the SF: Rb⁺ is preplaced in p23 and K⁺ in p34 at simulations with an applied voltage of (A) −150 mV, (B) −500 mV, and (C) −700 mV. The arrowhead in A indicates a direct Sa–S3 transfer. (D and E) Details of conduction mechanisms at −500 mV are shown in D with corresponding snapshots (E) taken at times indicated by arrowheads in D.