Permeant ion-dependent kinetics of SF conformational dynamics in KirBac1.1 WT and I131M. (A) smFRET trajectories for all WT (gray) and I131M (green) traces (n traces given in Figs. 6 and 7) in 150 mM KCl and 150 mM NaCl were concatenated and idealized into three (low, middle, and high) FRET states (red). (B) Lifetime probability density function of low (L) and high (H) FRET states as a function of [K⁺]/[Na⁺], from idealized trajectories as in A (and Figs. S3 and S4), fitted with a single exponential distribution in each case. (C) WT and I131M lifetimes (from fits in B) reveal qualitative dependence on ionic conditions, with H state lifetimes increasing, and L state lifetimes generally decreasing, as Na⁺ is replaced by K⁺. For the time constants, a monoexponential decay function was fitted to the full dataset without splitting. The time constants are represented as mean ± 95% confidence interval of the fitting.