Figure 3.

Parameters describing the buckling transition. Data from simulations that use the WLC model are shown (solid circles, 5-nm segments and open squares, 2.5-nm segments). Experimental data were taken from Brutzer et al. (20) (shown as lines; standard error of the measurements is depicted as shaded area if larger than the line width). The buckling parameters are shown as function of force at 320 mM Na⁺ (a–d) and as a function of ionic strength at constant stretching force of 3.5 pN (e–h). (a and e) Positions of the buckling point N_b. (b and f) Abrupt DNA length jump and (c and g) torque jump at the buckling point. (d and h) Energy difference between the formation energies E_loop and E_helix. E_loop–E_helix represents the energetic penalty to nucleate a plectonemic superhelix and was calculated using Eq. 4.