Magnetization Properties of Superparamagnetic Beads

Abstract

Magnetic beads are frequently used in molecular biology and biophysics. In particular, nanometer- to micrometer-sized magnetic beads are a key component in magnetic tweezers and enable the application of precisely calibrated forces and torques to individual molecules of interest. Interestingly, despite the extensive use of magnetic beads, the origin of some of their apparent magnetic properties remains controversial. While the forces exerted on the particles in magnetic tweezers are well understood and have been quantitatively modeled from first principles (Lipfert et al., Biophys J., 2009), the torques are less well understood. Application of torque in magnetic tweezers requires the magnetization of the beads to exhibit some amount of anisotropy, the exact mechanism, however, remains poorly understood. Two models have been put forward: one relies on a small ferromagnetic component in addition to a larger paramagnetic component, while the other model postulates an ‘easy’ magnetization axis. We have performed two separate experiments to address this issue. First, we used direct angle tracking (Lipfert et al., Rev. Sci. Instrum., 2011) of DNA-tethered beads held in conventional magnetic tweezers to determine the magnetic field dependence of the rotational trap stiffness. The results are well described by a model proposed by Pavone and coworkers (Normanno et al., PRA, 2004) and argue in favor of a weak axis model. Second, we probed beads tethered to rotating flagellar motors of fixed E. coli cells to determine the field-dependent angular speed. Again, the studies suggest that the beads possess an ‘easy’ magnetization axis. Further insight and understanding of the intrinsic magnetization properties is desirable to improve the sensitivity of single-molecule magnetic tweezers and will be important for a quantitative understanding and optimization of the torque exerted in magnetic tweezers.