Use of thermal fluctuations to study the length and flexibility of ligand-receptor bonds

We describe an experimental approach yielding new information on the behavior of ligand-receptor bonds. Spherical particles of 1.4 microns radius were coated with anti-rabbit immunoglobulin monoclonal antibodies and deposited on surfaces derivatized with rabbit immunoglobulins. Brownian motion was studied. When particles where bound through multiple bonds, the mean square displacement during a 1 s interval was 0.0038 micron2 as compared to 0.245 micron2 for unbound particles. Under the same conditions, the mean square displacement of particules coated with limiting dilutions of binding sites and bound by single molecular bonds was 0.0774 micron2. Results are compatible with the concept that the latter particles behaved as spheres transiently bound to the substratum by links of 2.7 nm length, allowing brownian oscillations with an angular amplitude of 0.062 radian.