Unbinding process of adsorbed proteins under external stress studied by atomic force microscopy spectroscopy

We report the study of the dynamics of the unbinding process under a force load f of adsorbed proteins (fibrinogen) on a solid surface (hydrophilic silica) by means of atomic force microscopy spectroscopy. By varying the loading rate r(f), defined by f = r(f) t, t being the time, we find that, as for specific interactions, the mean rupture force increases with r(f). This unbinding process is analyzed in the framework of the widely used Bell model. The typical dissociation rate at zero force entering in the model lies between 0.02 and 0.6 s(-1). Each measured rupture is characterized by a force f(0), which appears to be quantized in integer multiples of 180-200 pN.