Strongly stretched semiflexible extensible polyelectrolytes and DNA

The stretching response of a single charged semiflexible and extensible polymer in the limit of large tensile forces is calculated. We consider the effects of W the coupling of bending and elongational fluctuations, (ii) the electrostatic contribution to the bending and elongational energies, and (iii) nonlinear bare elastic elongational energies. We show that the electrostatic repulsion between charged monomers leads to an intrinsic stretching force, which cannot be neglected for low salt concentrations. For DNA this effect shifts the B-DNA-S-DNA transition to lower external forces as the salt concentration is decreased, in agreement with experiments. Because of the scale dependence of the electrostatic contribution to the persistence length, the effective persistence length (obtained from fitting the high-force stretching response to a semiflexible chain model) acquires a force dependence which follows a universal heuristic scaling function. As a consequence, flexible (synthetic) polyelectrolytes are characterized by their bare persistence length in the piconewton force range, while the effective persistence length of double-stranded DNA contains additional electrostatic contributions in the same force range as was found in recent experiments. The coupling between elongational and bending fluctuations leads to a force renormalization which is particularly important close to conformational phase transitions or bond failure. Finally, these perturbative results are supplemented by quantum-chemical ab initio calculations for the stretching response of polymers, giving an elastic modulus of 28 nN for fully saturated carbon-backbone polymers (plus nonlinear corrections which are dominant close to bond failure).