  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.
