Force-Induced Dissociation of Single Protein A−IgG Bonds

We investigated the force-induced dissociation of single specific bonds between biomolecules. For these experiments we used a micropipet-based picoforce transducer. It consisted of a red blood cell tensed by micropipet suction. The stiffness of this “spring” was tuned by suction pressure. The proteins under investigation were immobilized onto microbeads. One type of bead carried the adhesion proteins and was biochemically attached to the red blood cell membrane. The second type, carrying the respective ligand, was held in a second micropipet which was moved by a piezoelectric transducer. Complementary beads were repeatedly brought into contact in order to form and break bonds. Yield forces exceeding 3 pN could be detected. For protein binding, the microbeads were covalently coated with a hydrogel of several nanometer thickness onto which the proteins were bound. This preparation resulted in a low frequency of nonspecific interactions between the beads. Linkages between beads and proteins were of sufficient strength for mechanical experiments on single molecules. We controlled the number of available binding sites by competitive blocking with soluble ligand so that only single molecular bonds were present between the microbeads. Protein A−IgG bonds were studied. Here we found a marked dependence of bond strength on the rate of force application. This effect is due to a force dependent rate of bond dissociation.