FIGURE 4.

Force-extension curves for pulling on a normal (n = 2) polymorphic form filament. The upper panel shows 10 force-extension measurements perfomed at an extension rate of 0.4 μm/s. During 6 of the 10 trials shown, the filament followed a simple elastic force-extension curve (upper trace, labeled A–E). During the other trials a polymorphic transformation to the hyperextended (n = 1) form occurred during the extension of the filament, dropping the force onto a lower curve. When the same filament was pulled at a 10-fold slower extension rate, it transformed during every trial (inset) at about half the force required during rapid pulling. Stills of the same filament (lower panels) were extracted from video at points on the force-extension curve labeled A–E. The flat near-zero portion corresponds to filament buckling (A and B). Trap stiffness was 90 pN/μm. The location of zero displacement is arbitrary. The trapped bead is visible at the left end of the filament.