FIGURE 4.

Energy landscape of the streptavidin-biotin bond. The landscape used to obtain the probabilities in Fig. 5 with the parameters from Table 1 (shaded line) is superimposed to the one predicted by molecular dynamics (solid lines, given in Merkel et al. (2) and deduced from original data given in Izrailev et al. (17)). In the simulations, the instantaneous energy was computed over a half-nanosecond extraction from the biotin-avidin binding pocket. The denser regions with rapid fluctuations correspond to the minima in the energy landscape, while the heights of the barriers (maxima in the energy landscape) cannot be found. The shaded dashed line represents the inmost barrier that is seen in the DNA experiments but not in the rupture force measurements. The values x_m1(0), x_m2(0), x_m3(0), x_b1(0), x_b2(0), and x_b3(0), are, respectively, the positions of the first, second, and third minimums and of the first, second, and third barriers under zero force. These positions will move when a force is applied.