 Spectrin unfolding by force spectroscopy

A modular protein can release additional contour length when a folded domain crosses a mechanical unfolding barrier. At almost fixed end separation this lowers fractional extension and tension, producing a saw-tooth force record. Successive worm-like chain fits can share backbone persistence length while their contour lengths increase. Rupture force and hysteresis depend on loading history; peak force alone is not an equilibrium unfolding free energy.