Figure 2.

Titin refolding under mechanical load. a. Time dependence of titin refolding in force‐clamp experiments. Force was quenched (to 0.8 pN in this example) for varying amounts of time to allow for refolding to occur. Then the molecule was rapidly stretched to a high force level (115 pN) so as to assess the extent of refolding quantitated as the length of the refolded segment. Traces containing 1‐, 5‐ and 10‐second‐long refolding periods are shown. The extent of refolding was measured as the end‐to‐end length difference (ΔZ) of the manipulated molecular segment. ΔZ is the difference in extension just prior to force quench and the onset of the first unfolding step during re‐stretch (gray double arrows). Inset, Refolded titin length as a function of time in the quenched‐force state (1pN in this example). Data points were fitted with the equation ΔZ=ΔZ ₀ − Ae ^{− t/τ}, where ΔZ ₀ is maximal refolded length, the pre‐exponential factor A is the apparent refolded length at t ₀, and τ is the time constant of refolding. From the fit τ = 28.4 s was obtained. b. Extension versus time traces recorded at various refolding clamp forces. Traces recorded at clamp forces of 1, 3 and 10 pN are shown. Refolding of titin occurred only in the case of 1 and 3 pN clamp forces as indicated by the presence of unfolding steps in the successive high‐clamp‐force phase.