Diffusion of microspheres in shear flow near a wall: Use to measure binding rates between attached molecules

The rate and distance-dependence of association between surface-attached mo lecules may be determined by monitoring the motion of receptor-bearing sphe res along ligand-coated surfaces in a flow chamber (Pierres et al., Proc. N atl. Acad. Sci. U.S.A. 95:9256-9261, 1998). Particle arrests reveal bond fo rmation, and the particle-to-surface distance may be estimated from the rat io between the velocity and the wall shear rate. However, several problems are raised. First, data interpretation requires extensive computer simulati ons. Second, the relevance of standard results from fluid mechanics to micr ometer-size particles separated from surfaces by nanometer distances is not fully demonstrated. Third, the wall shear rate must be known with high acc uracy. Here we present a simple derivation of an algorithm permitting one t o simulate the motion of spheres near a plane in shear flow. We check that theoretical predictions are consistent with the experimental dependence of motion on medium viscosity or particle size, and the requirement for equili brium particle height distribution to follow Boltzman's law. The determinat ion of the statistical relationship between particle velocity and accelerat ion allows one to derive the wall shear rate with 1-s(-1) accuracy and the Hamaker constant of interaction between the particle and the wall with a se nsitivity better than 10(-21) J. It is demonstrated that the correlation be tween particle height and mean velocity during a time interval Deltat is ma ximal when Deltat is about 0.1-0.2 s for a particle of 1.4-mum radius. When the particle-to-surface distance ranges between 10 and 40 nm, the particle height distribution may be obtained with a standard deviation ranging betw een 8 and 25 nm, provided the average velocity during a 160-ms period of ti me is determined with 10% accuracy. It is concluded that the flow chamber a llows one to detect the formation of individual bonds with a minimal lifeti me of 40 ms in presence of a disruptive force of similar to5 pN and to asse ss the distance dependence within the tens of nanometer range.