Force distribution histograms of fibrinogen binding to purified surface-bound αIIbβ3 preincubated with Ca²⁺ or Mn²⁺. In the experiments shown, fibrinogen-coated beads were oscillated at 5 Hz touching the αIIbβ3-coated pedestals repeatedly. These data represent rupture forces greater than 60 pN, indicative of specific fibrinogen binding to αIIbβ3.¹⁸  The average yield strength in pN and the cumulative probability of specific fibrinogen binding (%) for each histogram are also shown. (A) Fibrinogen binding to αIIbβ3 immobilized in the presence of 1 mM Ca²⁺. (B) Fibrinogen binding to αIIbβ3 immobilized in the presence of 1 mM Mn²⁺. (C) Fibrinogen binding to αIIbβ3 treated with 1 mM Mn²⁺ measured in the presence of 50 μM tirofiban. (D) Fibrinogen binding to αIIbβ3 treated with 1 mM Mn²⁺ measured in the presence of 100 μg/mL abciximab.