Rupture forces of individual protein molecules registered as multiple calibrated signals and arranged into force distribution histograms. (A) Data trace of 9 successive signals produced during repeated contacts of a desAB-fibrin–coated pedestal and fibrinogen-coated latex bead. At the moment of contact the laser trap exerts a small positive, compressive force on the pedestal and the bead. When the pedestal and the bead bind, either specifically or nonspecifically, the force on the bead increases in the negative direction until the pedestal-bead bond is ruptured, and the force rapidly returns to zero. If no attachment occurs, there is no negative rupture force. (B) DesAB-fibrin–fibrinogen rupture forces displayed as a normalized force distribution with 2 force regimes bordering at about 50 pN. The total number of contacts (n = 10 865) is taken to be 100%. (C) A control histogram of exponentially decreasing rupture forces 0 to 40 pN produced by nonspecifically interacting desAB-fibrin–and BSA-coated surfaces (n = 3689). (D) A sample of rupture force distribution of desAB-fibrin–fibrinogen interactions fitted to the Bell function combined with exponential decay for nonspecific forces less than 40 pN (n = 1378). Signals that appeared as forces below 10 pN were considered nonbinding events or zero.