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Subpiconewton Dynamic Force Spectroscopy Using Magnetic Tweezers

Leiden Institute of Physics, 2333 CA Leiden, The Netherlands

Correspondence: Address reprint requests to John van Noort, Leiden University, Leiden Institute of Physics, Lorentzweg 1, Delft 2628 CJ, The Netherlands. Tel.: 31-71-527-5980; E-mail: noort{at}physics.leidenuniv.nl.

We introduce a simple method for dynamic force spectroscopy with magnetic tweezers. This method allows application of subpiconewton force and twist control by calibration of the applied force from the height of the magnets. Initial dynamic force spectroscopy experiments on DNA molecules revealed a large hysteresis that is caused by viscous drag on the magnetic bead and will conceal weak interactions. When smaller beads are used, this hysteresis is sufficiently reduced to reveal intramolecular interactions at subpiconewton forces. Compared with typical quasistatic force spectroscopy, a significant reduction of measurement time is achieved, allowing the real-time study of transient structures and reaction intermediates. As a proof of principle, nucleosome-nucleosome interactions on a subsaturated chromatin fiber were analyzed.