λ − DNA ( c ) 0 50 10 20 30 40 S-DNA B-DNA U-DNA FIG. 1: (Color) The thermo dynamics implied by the S-DNA picture of overstretching for salt concen- tration c = 150 mM. (a) Carto on sketc hes of phases B, M, U and S (see text). (b) Equilibrium phase stabilities (see Appendix) in the temp erature-pulling force plane for phases U, B and S. We consider a 40 kbp fragmen t of λ -DNA (left), a similar length of ‘av eraged’ CG-DNA (center; see Appendix for details of sequence averaging), and averaged AT-DNA (right). The model of S-DNA described in the main text is not thermodynamically stable at any temperature or stretching force within the first 40 kbps of λ -DNA as a whole. However, its considerable stability against unpeeling in CG-rich regions suggests the possibil- ity of long-lived metastable ‘pockets’ of S-DNA during dynamic stretching experiments if nicks are few. Compare these data with the ‘dynamic phase diagram’ of Fig. 3. 8 0 50 65 100 150 10 20 30 40 0 50 65 100 150 10 20 30 40 S-DNA B-DNA U-DNA 0 50 65 100 150 10 20 30 40 T ( ◦ C) T ( ◦ C) T ( ◦ C) f (pN) 〈 AT 〉 〈 CG 〉 λ − DNA ( a ) ( b ) 65 100 150 B M U S 0 50 65 100 150 10 2 0 30 40 S-DNA