Stretching single polypeptides: The effect of rotational constraints in the backbone

Polypeptides consist of three types of backbone bonds that are distinguished by the degree to which their dihedral rotation is hindered. We discuss the effect of this inhomogeneous rotational restriction on the stretching response as measurable by single-molecule force spectroscopic experiments. The ab initio-derived molecular backbone structure yields geometric input parameters. For the cases where one or two out of three bonds are rotationally frozen, which are two extreme cases of the actual peptidic backbone statistics, we derive analytic and numerical results and show that the Kuhn and persistence lengths are substantially increased. A simple explicit formula is derived that accurately describes the extension over the entire force range.