Theory of Biopolymer Stretching at High Forces

We provide a unified theory for the high force entropic elasticity of biopolymers solely in terms of the persistence length, xi(p), and the monomer spacing, a. When the force f > F-h similar to k(B)T xi(p)/a(2) the biopolymers behave as freely jointed chains (FJCs) while in the range F-1 similar to k(B)T/xi(p) < f < F-h the worm-like chain (WLC) is a better model. We show that xi(p) can be estimated from the force extension curve (FEC) at the extension x approximate to 1/2 (normalized by the contour length of the biopolymer). After validating the theory using simulations, we provide a quantitative analysis of the FECs for a diverse set of biopolymers (dsDNA, ssRNA, ssDNA, polysaccharides, and unstructured PEVK domain of titin) for x >= 1/2. The success of a specific polymer model (FJC or WLC) to describe the FEC of a given biopolymer is naturally explained by the theory. Only by probing the response of biopolymers over a wide range of forces can the f-dependent elasticity be fully described.