Figure 6.

Combining phi and flip. (A) Phi values in diliganded AChR gating (Purohit et al., 2013). There are five populations, the first four of which correspond in decreasing order to purple, blue, green, and red in Fig. 1 A. (B) Simulations based on a model with four intermediate states (center; k = 300 ms⁻¹; see supplemental text). (middle) The TSE are states T_1–4 (all shut); each state is associated with a cartoon structure is in Fig. 7. (top left) Currents and perfect idealization. (top right) Energy landscape corresponding to the model (calibration, 1 kcal/mol; filled circle/‡, committor). (bottom) Histograms; sojourns in the TSE generate five shut components (gray lines), four of which are short-lived (time constants given); open sojourns are described by a single exponential. In standard patch recordings (∼25-µs resolution), the time constant of the open (long-shut) component is the inverse of the C↔O closing (opening) rate constant. (C) Fitting the simulations of panel B by C↔C′↔O after low-pass filtering to 30 kHz. There is only a single, brief component (flip/primed) apparent in the shut interval duration histogram. (D) Simulations using different values for k. Left y axis is the slowest shut interval TSE component (filled circles) calculated from the model, and right y axis (open squares) is the fitted backward/forward exit rate constant ratio from C′ using Scheme 2 (panel C). (E) Energy landscapes. (middle) The model in panel B with k = 300 ms⁻¹; (top) the first two steps of unliganded opening are steeply uphill; (bottom) opening with two bound ACh molecules starts rapidly. In all conditions, the second step of the opening process (ECD twisting) is relatively the most difficult energetically, and the closing process is rate limited by gate bubble formation.