FIGURE 6.

Force-melting mode: Effect of temperature. Simulated force-extension data for once-nicked λ-phage DNA at 500 mM NaCl, chosen to model experiments presented in Mao et al. (13). (Left panel) We show data for pulling rate 1000 nm/s at T = 10, 20, 30, 40, and 50°C (offset left to right panel), together with equilibrium data for 10 and 30°C (circles). Overstretching corresponds to the conversion of B- to U-DNA, with accompanying hysteresis. This hysteresis is symmetric, in the sense that the stretching transition lies as far from equilibrium as does the shortening transition. There is no appreciable change with temperature in the degree of hysteresis, because no competition between U- and S-DNA can arise (we do not permit the latter state in force-melting mode). (Right panel) For 30°C we show data obtained at pulling rates of 1000, 2000, and 4000 nm/s, together with the percentage of unpeeled DNA during stretching and shortening at 1000 nm/s (top panel; arrows and lines correspond to those on force-extension curve). The slowness of unpeeling confers a rate-dependence upon the overstretching plateau.