Figure 4.

Folding of barnase measured at high temporal resolution. (a) Example of FTTs recorded at a 50 kHz sampling rate that shows the folding of barnase as a sudden rise in force. (Red) Data for complete trajectory. (Blue) Data used for the alignment with other folding trajectories. (Boxed regions) Data used to compute the power spectrum of fluctuations when barnase is in state F (f_min = 3.8 pN, solid box) or U (f_max = 4.1 pN, dashed box). (b) Force-time trace obtained by aligning and averaging different folding trajectories (obtained at the same value of f_p) at the center of the force jump along a folding event and fits to a sigmoid function (Eq. 3). (Inset) The value τ_TP extracted from fit as a function of f_p, and relaxation time of the experimental setup (made by the bead in the optical trap, the handles, and the protein) as a function of force when barnase is at state F (solid line) or U (dashed line). (c) Power spectrum of the fluctuations of barnase recorded in passive mode experiments at 50 kHz for state F (blue circles) and U (red circles) at 4 pN. In each case, the fit to a double Lorentzian function (solid lines; Eq. S11) is shown, which allows us to determine the values of ν_f and ν_s, characteristic of the fast and slow relaxation modes. (d) Force-dependence of slow and fast frequency modes, ν_s and ν_f, respectively, obtained from the power spectrum of fluctuations measured in passive mode for the folded and unfolded states of barnase. To see this figure in color, go online.