Figure 5.

Force dependence of the buckling transition at 320 mM Na⁺ for the 1.9 kbp (black) and the 10.9 kbp DNA molecule (blue). Experimental values are shown as solid circles, predictions from the end-loop model as solid lines. In the case of an overlap, only the black curve is depicted. (A) Jump size at buckling equilibrium. (B) Position of the buckling equilibrium. To highlight the DNA length dependence, the right axis, corresponding to the 10.9 kbp DNA molecule, was scaled linearly with DNA length compared to the left axis corresponding to the 1.9 kbp DNA molecule. (C) Change of twist transferred into writhe during buckling $\Delta N_{\text{b}}^{\text{p}}$ at buckling equilibrium and corresponding torque change $\Delta {\Gamma }_{\text{b}}=2\pi C_{\text{s}}/L\times \Delta N_{\text{b}}^{\text{p}}$. The expected scaling of $\Delta N_{\text{b}}^{\text{p}}$ with the square root of the DNA length is highlighted by scaling the axes accordingly (Eq. 13). (D) (Top) Superhelix-formation energy E₂ and inferred postbuckling torque. (Bottom) Energy difference between the initial-loop formation energy E₁ and E₂. Error bars represent the statistical error of the data.