Theory of single-molecule experiments in the overstretching force regime
  We present a statistical mechanics analysis of the finite-size elasticity of biopolymers, consisting of domains which can exhibit transitions between more than one stable state at large applied force. The constant-force (Gibbs) and constant-displacement (Helmholtz) formulations of single molecule stretching experiments are shown to converge in the thermodynamic limit. Monte Carlo simulations of continuous three dimensional polymers of variable length are carried out, based on this formulation. We demonstrate that the experimental force-extension curves for short and long chain polymers are described by a unique universal model, despite the differences in chemistry and rate-dependence of transition forces. 
Fabio Manca, Stefano Giordano, Pier Luca Palla, Fabrizio Cleri and   Luciano Colombo
physics.bio-ph cond-mat.soft