Figure 4.

Mechanical characterization of P-DNA. (A) Elasticity curves showing two sharp transitions at 3 and 25 pN. The first transition (not shown for all curves) is associated with the disappearance of plectonemes in B-DNA and the formation of P-DNA. The second transition, showing hysteresis, is attributed to the disappearance of plectonemes in the P-DNA sub-phase. Note that, because of the possibility of stabilizing interactions between exposed bases, these plectonemes should be more stable than with B-DNA. The existence of these plectonemic structures also might explain the shortening of the molecule at relatively low forces 3 pN < F < 10 pN. At high force, these curves show that P-DNA is actually longer than B-DNA. (B) A detailed view of the curves from A for decreasing forces (each curve corresponds to σ_i = i × 0.343). (C) Rescaling, following Eq. 1, enabled all of the curves shown in B to be collapsed to a single curve l_p(F) that describes the extension vs. force behavior of a pure P-DNA. The full line is a fit to the model for P-DNA (Eq. 2) with l_p,0 = 1.75, ξ_p = 19 nm, and ɛ = 0.12 k_BT/nm (see text for details).