Dissociation of ligand-receptor complexes using magnetic tweezers.

We present a new tool for measuring ligand-receptor complex bonds at the single-molecule level using magnetic tweezers. Our apparatus allows massively parallel (100-1000) measurements on many single complexes perturbed by constant forces. Compared to other single-molecule techniques, our method is simple, inexpensive, robust, and widely compatible with other techniques. We immobilized specific receptor molecules on the surface of superparamagnetic beads and corresponding ligand molecules on a fixed surface. The beads were allowed to contact the surface so that ligand-receptor binding occurred. A permanent magnet then generated a constant force that pulled the receptors away from the ligands. The rates at which bound species separated at various forces allowed us to characterize the potential energy landscape of the bond and extrapolate bulk solution kinetic rates and transition-state distances. These values agreed with those obtained using bulk and single-molecule methods.