Three-dimensional bead position histograms reveal single-molecule nanomechanics

We describe a method to investigate the structure and elasticity of macromolecules by a combination of single molecule experiments and kinematic modeling. With a photonic force microscope, we recorded spatial position histograms of a fluctuating microsphere tethered to full-length myosin-II. Assuming only that the molecule consists of concatenated rigid segments, a model derived from robot kinematics allows us to relate these histograms to the molecule's segment lengths and bending stiffnesses. Both our calculated position distributions and the experimental data show an asymmetry characteristic of a mixed entropic-enthalpic spring. Our model that fits best to experimental line profiles has two intramolecular hinges, one at the bound head domain, and another about 50 nm down the myosin tail, with a summed bending stiffness of about 3 k(B)T/rad.