A force calibration standard for magnetic tweezers

Abstract

To study the behavior of biological macromolecules and enzymatic reactions under force, advances in single-molecule force spectroscopy have proven instrumental. Magnetic tweezers form one of the most powerful of these techniques, due to their overall simplicity, non-invasive character, potential for high throughput measurements, and large force range. Drawbacks of magnetic tweezers, however, are that accurate determination of the applied forces can be challenging for short biomolecules at high forces and very time-consuming for long tethers at low forces below ∼1 piconewton. Here, we address these drawbacks by presenting a calibration standard for magnetic tweezers consisting of measured forces for four magnet configurations. Each such configuration is calibrated for two commonly employed commercially available magnetic microspheres. We calculate forces in both time and spectral domains by analyzing bead fluctuations. The resulting calibration curves, validated through the use of different algorithms that yield close agreement in their determination of the applied forces, span a range from 100 piconewtons down to tens of femtonewtons. These generalized force calibrations will serve as a convenient resource for magnetic tweezers users and diminish variations between different experimental configurations or laboratories.

Stokes trap for multiplexed particle manipulation and assembly using fluidics

“…3B). We fit the PSD with a Lorentzian and a maximum-likelihood estimator (26,27), and the corner frequency f c was determined to be 0.64 and 0.82 Hz using these methods, respectively. We determined the trap stiffness κ = 2πζf c as 1.1 × 10 −3 pN/nm and 1.3 × 10 −3 pN/nm, where ζ is the Stokes drag of the bead.…”

Hysteresis in DNA compaction by Dps is described by an Ising model

“…We developed a magnetic tweezer assay (17)(18)(19) that allowed us to modulate the force applied to individual DNA molecules in the presence of Dps (Fig. S1C).…”

Biological Magnetometry: Torque on Superparamagnetic Beads in Magnetic Fields

“…Interestingly, the effective volume appears to be an approximately fixed proportion of the magnetic particle content, ≈3%. However, the bead-to-bead variation in NV is considerable (≥30%) for both types of beads investigated here, much larger than the variation in forces (≈10% for both MyOne and M-270 beads [5,39,42]). Possible causes of the variation include the size-, shape-, and orientation-distributions (Section 5 of [17]) of the NPs inside the bead.…”

“…For the M-270 beads, the fit yielded C ¼ 8.1 AE 7.1 kJ=m 3 and NV ¼ ð2.8 AE 2.3Þ × 10 −2 μm 3 , which corresponds to ∼0.2% of the total volume of a M-270 bead. We estimate the magnetic NP content for M-270 beads to be approximately twofold lower than for MyOne beads, i.e., ≈6% (v=v), from the fact that M-270 beads experience only about tenfold higher forces than MyOne beads [39], compared to the ð1.4=0.5Þ…”