Stretching helical semiflexible polymers

We study the elastic behavior of a helical semiflexible polymer under extension. The polymer is described with a coarse-grained model recently developed by us (V. Varshney, et al. Macromolecules 2004, 37, 8794) where the chain is modeled using the freely rotating chain model and each bead can be in one of two possible conformations, helix or coil, depending on the value of its torsion. In this study, we extend the original model to include the effect of external mechanical forces and solve it using a Monte Carlo simulation approach based on the Wang-Landau sampling scheme. We found that the application of a mechanical force first increases the helix-to-coil transition temperature and then decreases it. This nonmonotonic behavior is a consequence of a change in the nature of the helix-coil transition which becomes a helix-extended coil transition for strong forces. We also found that the force-elongation curve at constant temperature displays three different behaviors depending on the temperature of the system. At temperatures below or slightly above the helix-coil transition temperature the force-elongation curve shows one or two coexistence regions, respectively. In these regions, helical sequences and random coil domains coexist while the strength of the force and temperature determine the fraction of the polymer adopting each conformation. At high temperatures, our model recovers the elastic behavior of a random coil. We also present a quantitative comparison of our simulation results with the theoretical ones obtained by Buhot and Halperin, and very good agreement is found. A qualitative comparison with experimental data for Xanthan is also presented.