Nonlinear low-force elasticity of single-stranded DNA molecules.

We reconcile single-molecule force-extension data with scaling theories of polymer elasticity: measurements of denatured single-stranded DNA show a regime where the extension grows as a nonlinear power law with force, in accordance with "tensile blob" models. Analysis of the salt dependence of this regime indicates that the polymer's Kuhn length is proportional to the Debye length; this contradicts the Odijk-Skolnick-Fixman theory, but agrees with other predictions. Finally, we identify a Theta condition of the polymer, and find that the wormlike chain model best describes the polymer's elasticity at this point.