Rupture force histograms for KKO and RTO binding to PF4. (A) Frequency of events in each 5-pN bin was plotted against the average force for that bin after normalizing for the total number of interaction cycles. PF4 tetramers were attached covalently to pedestals and cross-linked with glutaraldehyde, and Ab was covalently attached to latex beads in the absence of heparin. Each curve represents ∼ 10 000 contact cycles of bead to pedestal. The probability of KKO binding to PF4 is much greater than for RTO, and the binding strength is slightly higher. Inset shows the cumulative binding probability for KKO and RTO. (B) Similar analysis of KKO and RTO interactions with the glutaraldehyde-treated PF4^K50E mutant that does not form tetramers. In this case, the probability of binding of KKO is lower and comparable with that of RTO.