Twisted DNA under the loop with new Delft instrument
A team of researchers at the TU Delft, led by FOM workgroup leader and member of the FOM Executive Board Prof.dr. Nynke Dekker, has developed a new instrument for high-resolution torque measurements on biological molecules such as DNA. This instrument makes it possible to gain new insight into biological phenomena such as the copying and repair of DNA. This research appears on the Nano Letters website this week.
Flagella for Swimming
The new instrument, termed electromagnetic torque tweezers (eMTT; see figure below), combines permanent magnets for force application with electromagnets that afford sensitive and independent control over rotation and torque. This makes it possible for scientists to study how the physical parameter torque (force times distance) acts in a range of biological contexts. Examples of these include the rotary motors that power the spinning of bacterial flagella and the precise role played by supercoils in DNA that is undergoing copying or repair.
Phone cords
Using the new eMTT instrument, Xander Janssen, Jan Lipfert, and their colleagues of the Kavli Institute of Nanosciecen of the TU Delft performed high-resolution torque measurements on double-stranded DNA. Their experiments directly observed an effect that had previously been predicted theoretically. When a DNA strand is twisted, the torque first increases linearly with the added number of turns; at some point, the molecule undergoes a buckling transition and form extra windings called supercoils. Similar effects occur in familiar objects such as rubber bands or old-fashioned telephone cords. However, at this buckling transition, the torque is found to actually 'overshoot', i.e. it reaches a maximum and then suddenly drops to a final plateau value, an effect clearly visible in the experimental data.
Magnets
The eMTT is based on magnetic tweezers, which are a powerful technique to study such processes at the single-molecule level. They work by tethering molecules between a glass surface and micrometer-sized magnetic particles. The molecules' response to forces and torques can then be monitored. Typically, permanent magnets are employed to carry this out, as these allow for the application of a large range of forces. However, such magnets make it difficult to sensitively control and measure torque.
The new instrument introduces a novel magnet configuration in which modified permanent magnets for force application are integrated with Helmholtz coils for full control over rotation and torque.
This research appears this week online in Nano Letters. It was co-financed by FOM and NWO through a VENI grant to researcher Jan Lipfert.
Reference
Xander J.A. Janssen†, Jan Lipfert†, Tessa jager, Renier Daudey, Jaap Beekman, and Nynke H. Dekker (†Equal contribution) 'ElectromagneticTorque Tweezers: A Versatile Approach for Measurement of Single-Molecule Twistand Torque' Nano Letters (2012)
More information
Dr. Xander Janssen; +31 (0)15 278 5394, Dr. Jan Lipfert +31 (0)15 278 3219, Prof.dr. Nynke Dekker; +31 (0)15 278 3219.