Objectives: To investigate the kinetics of the adsorption process of fibrinogen (Fib) molecules. Methods: Atomic force microscope (AFM) operated in force-spectroscopy mode was employed to measure the adhesion forces. The Fib-coated tip was brought into contact with the hydrophilic silica surface for different interaction times varying from 5 to 2000 ms. Results: Multiple consecutive ruptures are observed. The histograms of the distances between two consecutive ruptures in one "approach/retraction"cycle exhibit maxima around 20-25 nm, suggesting that a Fib molecule adsorbs through its D and E globules. In a second stage, we studied the dynamics of the unbinding process of Fib from a silica surface under an increasing force load. We found that, as for specific interactions, the mean rupture force increases with the retraction velocity of the surface. Each measured rupture is characterized by a force which appears to be quantized in integer multiples of 180-200 pN. Conclusions: Our AFM data suggest that a Fib molecule interacts with the surface through various domains, each interacting with the surface by several amino acids groups. The protein can unbind if all of these groups unbind simultaneously. In absence of applied force, these different amino acids interact with the surface "reversibly" and the probability that all of them unbind simultaneously becomes extremely small. On the other hand, if a significant force is applied to the protein, it is expected that, because of the elasticity of the protein, once an amino acid unbinds, the on rate to reestablish the connection with the surface becomes small. The unbinding is then constituted of a sequence of individual "irreversible unbinding" processes of the different amino acids interacting with the surface. Theoretical simulations showed that adhesion of Fib can be modelized by a simple mechanical system: 8 beads connected by "molecular" stiff springs to the surface.

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