Simulations of Stretching a Strong, Flexible Polyelectrolyte

We present simulations of the stretching of a single strongly charged, flexible polyelectrolyte chain. Over a range of salt concentrations and valences, our results quantitatively match recent single-molecule measurements of the elastic response of single-stranded DNA. Of particular interest is a heretofore unexplained high-force regime of logarithmic elasticity seen in experiments with monovalent salts. We investigate this by calculating the force-dependent structure factor of the charged chain. We find distinct structures at long and short length scales and show that the compliance of the short-scale crumpled structure underlies the logarithmic elasticity regime. This is corroborated by results in divalent salts, in which stronger electrostatics leads to more short-scale crumpling and a corresponding increase in the high-force compliance. Thus, we show that the high-force elasticity of a charged chain is a signature of a structural regime that exists in flexible polyelectrolytes, but not neutral polymers.