Developing Magnetic Tweezers for constant load experiments on microtubule motor proteins

Abstract

This work reports the construction of magnetic tweezers that can apply a spatially uniform magnetic force on superparamagnetic beads viewed in an optical microscope. The apparatus, constructed and assembled on the current microscope setup is composed of 2 electromagnets and can exert a constant force along an axis perpendicular to the generated field. The instrument was calibrated by measuring the velocity of single 2.8μm dynal magnetic beads near the surface since magnetic beads eventually sink to the surface due to their high density (1.6gm/cc). The maximum magnetic force on the beads, determined using Stoke’s law was found to be 7.29pN at 1.6A current with a spatial variation of 4%. However, since beads near the surface experience higher drag force than the value determined using Stoke’s law, the magnetic force on these beads is higher than estimated. Further work aims to calibrate neutrally buoyant beads suspended in a 3.9M Cesium Chloride solution of equal density (1.6gm/cc). The calibrated instrument will then be used to investigate the nature of single and multiple dynein-driven magnetic phagosome motion in response to constant load.