Elastically coupled two-level systems as a model for biopolymer extensibility

We present Monte Carlo simulations for the elasticity of biopolymers consisting of segments that can undergo conformational transitions. Based on the thermodynamics of an elastically coupled two-level system, the probability for a transition and a related change in length of each segment was calculated. Good agreement between this model description and measured data was found for both the polysaccharide dextran where the conformational changes are fast and the muscle protein titin where the marked rate dependence of the transition forces could be explained by nonequilibrium processes.