Biological Magnetometry

(by: Communications TU Delft/TNW)
In biophysics, usually tools from physics are used to study biology, for example by applying physical models to biological systems or by employing measurement techniques from the physical sciences to answer questions of biological relevance. A team of researchers led by Jan Lipfert (LMU Munich) and Nynke H. Dekker (Kavli/BN) has now taken the opposite route: they have used tools from biology to study a physical problem.

They were interested in the magnetic properties of small, micrometer sized, so-called superparamagnetic beads. Such beads are used routinely in biotechnological and diagnostic applications; in addition, they are at the heart of single-molecule measurements in magnetic tweezers. In magnetic tweezers, a molecule of interest, such as double-stranded DNA, is tethered between a glass slide and a small magnetic bead. By applying external magnetic fields, it is possible to both pull on the beads (and thus exerting a stretching force on the attached molecules) and to rotate the beads (and thus exert torque on the molecules). While the overall magnetization of the beads and the resulting forces were well understood (in part from previous work by the same lab), there was controversy about how the beads can be rotated by an external field. The ability to rotate the particles by rotating the external field requires some form of anisotropy or asymmetry in the magnetization, yet the nature of this anisotropy had been debated with several competing models proposed in the literature.

In order to probe the nature of the magnetic anisotropy, the authors carried out two types of experiments. In the first experiment, they attached cells of the bacterium E. coli to the glass surface and attached the superparamagnetic beads to the flagellar motors of the cells (see Figure 1). The flagellar motor is a powerful rotational molecular motor that allows E. coli cells to swim by rotating their flagella, which are long filaments extruding from the cell that propel the swimming bacteria similar to the propeller screws powering modern ships. In this experiment, the flagellar motors provided a convenient way to apply an approximately constant torque and to rotate the micrometer sized particles. By studying how externally applied magnetic fields slow down and eventually stall the motor, the authors could quantitatively test and rule out some of the previously proposed models.

In a second experiment, the beads were not actively rotated, but attached to the surface via double-stranded DNA molecules, such that thermal fluctuations lead to rotational fluctuations. These rotational fluctuations were measured again as a function of the applied magnetic field and provided further quantitative tests of the magnetic models.

Together, the two measurements ruled out models that proposed that the beads behave in part as permanent (ferro-)magnets, i.e. like compass needles, and support models that feature an anisotropy in the induced magnetization, either through small deviations from exactly spherical shape or through an inhomogeneous distribution of the magnetic material.

The results provide a quantitative understanding of the torques in magnetic tweezers that will help to further optimize magnetic tweezers measurements. In addition, they suggest that even small beads and relatively modest fields that are easily generated in the laboratory can generate torques large enough to stall even powerful molecular motors. 

Publication

“Biological Magnetometry: Torque on Superparamagnetic Beads in Magnetic Fields” Maarten M. van Oene, Laura E. Dickinson, Francesco Pedaci, Mariana Köber, David Dulin, Jan Lipfert, and Nynke H. Dekker, Physical Review Letters 114, 218301 (2015)