Distributions of K⁺ and amine energetics of SPM⁴⁺. Histograms, free energy profiles, and structure snapshots from simulations with SPM⁴⁺, at 0 mV (A) and 500 mV (B). As with SPD³⁺, conditional free energy profiles with SPM⁴⁺ are similar at 0 and 500 mV; because the bottommost K⁺ ion is driven outward with depolarization, N_L is driven to occupy a site at the entry to the SF, leading to blockade. Dashed lines on the plots indicate SEM estimates.