Coarse-grained simulations of DNA overstretching
  We use a recently developed coarse-grained model to simulate the overstretching of duplex DNA. Overstretching at 23C occurs at 74 pN in the model, about 6-7 pN higher than the experimental value at equivalent salt conditions. Furthermore, the model reproduces the temperature dependence of the overstretching force well. The mechanism of overstretching is always force-induced melting by unpeeling from the free ends. That we never see S-DNA (overstretched duplex DNA), even though there is clear experimental evidence for this mode of overstretching under certain conditions, suggests that S-DNA is not simply an unstacked but hydrogen-bonded duplex, but instead probably has a more exotic structure. 
Flavio Romano, Debayan Chakraborty, Jonathan P. K. Doye, Thomas E.   Ouldridge and Ard. A. Louis
q-bio.BM cond-mat.soft physics.bio-ph