Figure 6.

(a) Numerical simulations of diffusive barrier crossing in a simple potential described by sin(x) − x indicate that it is the diffusive nature of motions at or near the transition state that are monitored by the experiments described in this paper. The native and unfolded states are indicated by x_f and x_u, respectively. (b) The dependence of folding time constants viscosity. From the left, the histograms represent relative reaction time constants when viscosity is fixed at 1 cP over the folding path, is fixed at 2 cP, is 2 cP before the barrier (to the left of the dotted line in Fig. 4a), or is 2 cP at the barrier (to the right of the dotted line in Fig. 4a). The near equivalence of the second and fourth bars indicates that the viscosity dependence of motions at or near the transition state is the primary determinant of the viscosity dependence of protein folding rates.