Stretching a macromolecule in an atomic force microscope: Statistical mechanical analysis - art. no. 021906

We formulate the proper statistical mechanics to describe the stretching of a macromolecule under a force provided by the cantilever of an atomic forc e microscope. In the limit of a soft cantilever the generalized ensemble of the coupled molecule/cantilever system reduces to the Gibbs ensemble for a n isolated molecule subject to a constant force in which the extension is f luctuating. For a stiff cantilever we obtain the Helmholtz ensemble for an isolated molecule held at a fixed extension with the force fluctuating. Num erical examples are given for poly (ethylene glycol) chains.