70 pN c = 150 mM 65 pN 70 pN 65 pN c = 500 mM c = 50 mM 70 pN 65 pN nick 0 s 10 s 0 s 10 s 0 s 10 s 0 s 10 0 s 10 s 0 s 10 nick nick 0 10 S-DNA B-DNA U-DNA f$_{0}$ FIG. 7: (Color) Molecular compositions at 20 ◦ C as a function of sequence (horizontal axis) for 40 kbp fragmen ts of λ -DNA stretched to two target forces f$_{0}$ at three salt concentrations c . The stretching protocol is as Fig. 3 (force and molecular extension may vary after zeroing the loading rate). Each molecule possesses one free end. Low salt conditions and long waiting times favor unpeeling; at high salt conditions and short waiting times one is more likely to observe S-DNA. 14