  We carried out dynamic force manipulations $in$ $silico$ on a variety of
superhelical protein fragments from myosin, chemotaxis receptor, vimentin,
fibrin, and phenylalanine zippers that vary in size and topology of their
$\alpha$-helical packing. When stretched along the superhelical axis, all
superhelices show elastic, plastic, and inelastic elongation regimes, and
undergo a dynamic transition from the $\alpha$-helices to the $\beta$-sheets,
which marks the onset of plastic deformation. Using Abeyaratne-Knowles
formulation of phase transitions, we developed a theory to model mechanical and
kinetic properties of protein superhelices under mechanical non-equilibrium
conditions and to map their energy landscapes. The theory was validated by
comparing the simulated and theoretical force-strain spectra. Scaling laws for
the elastic force and the force for $\alpha$-to-$\beta$ transition to plastic
deformation can be used to rationally design new materials of required
mechanical strength with desired balance between stiffness and plasticity.
