Figure 4.

Measured supercoiling curves and predictions from the initial-loop model. (A and B) Supercoiling curves for a 1.9 kbp DNA molecule at 3.0 pN in a buffer containing 320 mM and 60 mM Na⁺ (as indicated). Data were taken at 300 Hz (dark shading) and smoothed to 20 Hz (light shading). The solid red line is the prediction from the initial-loop model according to Eq. 15 using estimates for the plectoneme formation energies E₁ and E₂ (Eqs. 16 and 17). (C) Salt dependence of supercoiling curves and (D) torque development as predicted by the initial-loop model. The shaded area between the torque overshoot and the postbuckling torque under the black curve corresponds to the difference between the initial loop energy E₁ and superhelix formation energy E₂.