Figure 5.

(a) The blue curve is the bare (f = 0) free energy profile of the form given in Eq. A1 and the green curve is the tilted form of G(x) in the presence of force. By fitting the numerically computed (black circles) f^* as a function of r_f to Eq. 5 (red curve), we obtain the parameters shown below. Although the features of original potential G(x) = 100x^1/3 − 10x are reasonably recovered (σ is larger than the value in G(x)) by using Eq. 5, the extracted value of κ is unrealistically large. (b) Equation 5 was used to fit the DFS data of biotin-strepavidin (circles) and biotin-avidin (triangles). Although the quality of fit is excellent, the unrealistically large value of κ, due to the singularity of the hypothesized fractional potential at x = 0, suggests that the potential with a fractional power should not be used for the analysis. In fact, the κ values are comparable to or much greater than the TST estimate k_BT/h (≈6.2 × 10¹²s⁻¹), which of course makes no sense. Hence, we can rule out the free energy profiles of the form given in Eq. A1 to analyze DFS data on protein complexes.