Force and torque measurements using magnetic micro beads for single molecule biophysics

Characterization of optical trapping with laser tweezers of super-paramagnetic beads has been performed for the first time. In particular, we have compared the force action in optical trapping on super-paramagnetic beads with respect to the polystyrene ones. Using a novel magneto-optical manipulator we have developed a direct rotation detection scheme for trapped beads characterized by a high sensitivity and fast time response. Finally a calibration procedure able to directly measure an applied external torque on the trapped bead is shown. This demonstrates the capability of this apparatus to perform direct torque measurement (torsional rigidity) on a single bio-molecule attached to a super-paramagnetic bead.