A new kind of magnetic tweezers can make torque measurements on single molecules.

Sean Sun, a professor of engineering at Johns Hopkins University, had what seemed to be a straightforward goal: to build a standard magnetic tweezers setup to measure the torsional rigidity of chromatin — a linear biomolecule that is similar to DNA. However, as it became clear that this would not be so straightforward, the project began to gain collaborators. Greg Bowman, an expert in chromatin biochemistry who had just arrived at Hopkins, was introduced to the project by Sun's graduate student, Alfredo Celedon. And when Sun realized that standard probes would not be sufficient, he contacted two other Hopkins professors: Peter Searson, who had worked on magnetic probes, and Denis Wirtz, who had experience building magnetic tweezers.

In the experiments the molecule of interest is strung between a substrate and a probe. The probe is rotated by a magnetic field, twisting the molecule. The size of the thermally induced fluctuations in the twist angle is used to calculate torque. The problem with the standard tweezers approach is that the probe is held too tightly. This constrains the minimum resolvable torque to about 40 pN nm — too high to be useful for single DNA or chromatin molecules.

Sun and his collaborators were able to demonstrate a new kind of torsion measurement in which the probe is more weakly held by the magnetic field. Its resolution is better than 1 pN nm, which allowed torque measurements to be made on a single chromatin fibre for the first time (Nano Lett. 9, 1720–1725; 2009).

One of the challenges in bringing the collaboration together, according to Sun, was getting people to step out of their comfort zone and interested in the problem. “Collaborations are successful if the people involved are not 100% occupied with advancing their own agenda and careers, but are genuinely interested in good science” he says. “There must also be a person that takes ownership, and provides the driving force to bring everything together.”