FIGURE 7.

Force-melting mode in the absence of nicks: Effect of varying loop exponent. Simulated force-extension curves for un-nicked λ-phage DNA at 150 mM NaCl, T = 25°C, and pulling rate 1.8 μm/s. The curves correspond to loop exponents c = 0, 1.25, 1.5, and 1.8 (offset horizontally), and we show equilibrium data in three cases (symbols). Hysteresis increases as c increases, even though the equilibrium fraction of M-DNA decreases with increasing c. Note that the cooperativity (or sharpness) of the force-melting transition decreases as c decreases. For the largest value of c, we show the effect of pulling rate upon the degree of hysteresis.