Dynamic transition from $α$-helices to $β$-sheets in polypeptide superhelices
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 $α$-helical packing. When stretched along the superhelical axis, all superhelices show elastic, plastic, and inelastic elongation regimes, and undergo a dynamic transition from the $α$-helices to the $β$-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 $α$-to-$β$ transition to plastic deformation can be used to rationally design new materials of required mechanical strength with desired balance between stiffness and plasticity.