Fig. 2.

Force histograms for PSGL-1–P-selectin bonds ruptured under steady ramps (A–C) and under jump/ramps (D–F). Forces in the first bin include tests without detection of an attachment, which causes the first bin (magenta) to rise off-scale. As shown by a 40–45% attachment frequency for D–F in contrast to <10% for A, ≈25% for B, and ≈33% for C, the quick initital jumps in force to ≈20–30 pN captured numerous bonds otherwise missed because of fast failure along a low-impedance pathway within the time increment defined by the first bin (see Fig. 1D). (The few bonds that broke during the jump phase are shown by the dark bins in D–F.) After force jumps or under fast steady ramps, the force distributions are seen to agree with the probability densities (dashed red curves) predicted for kinetically limited failure along a single pathway (labeled 2) defined by the failure rate, k_2rup ≈ (0.37 ± 0.07pN/sec) exp(f/18 ± 0.5 pN). Superimposed as solid black curves are the probability distributions computed by using the master equation for the two-pathway switch described in the text. To match the histograms for both steady ramp and jump/ramp modes at all loading rates, the two-pathway dissociation was modeled by a fast rate of k_1rup ≈ 8–12 pN/sec along low-impedance pathway 1 and a switch to high-impedance pathway 2 in the range of ≈20–25 pN. [It appears that a few (<20% in all cases) double bonds led to the small tails of very high forces, as shown by the predicted distribution added to the histogram in F.]