Figure 2.

(A) Time courses of the position of the S4 segment (x_S4) obtained from stochastic simulations at four different applied voltages. (B) Amplitude histograms of x_S4, obtained from 100 ms simulations at the indicated voltages. The arrows, corresponding to the amplitude histogram peaks, indicate the five positions where the S4 segment spends most of its time. (C) Plot of the mean x_S4 as a function of the applied voltage, obtained from 100 ms simulations at the indicated voltages. The curve represents a fit of the model data with a Boltzmann relationship, and the best-fit parameters are indicated in the panel. (D) Plot showing the gating charge profiles for the five positions where the S4 segment spends most of its time, corresponding to x_S4 = −1.33, −0.62, 0.17, 0.62, and 1.53 nm. Also shown are the fixed charge profile (Z_F, same as that shown in Fig. 1B) and the radius profile of the VSD. The gray region represents the GCTC of (12), i.e., the region laying between F290 and the first negative peak of fixed charges corresponding to the two closest intracellular negative residues. (E) Graphs showing the electrostatic energy associated to the S4 segment, assessed as $G_{el}=\int Z_{S4}{e}_0\left(dV\left(x\right)/dx\right)dx$, as a function of the S4 segment position (x_S4) at three applied voltages (indicated). The dashed vertical lines indicate the S4 segment position corresponding to x_S4 = 0.