Torque measurements during the spontaneous unbraiding of DNA molecules in the absence of pulling forces

- Direct observation of the unwinding dynamics of pairs of DNA molecules against different amount of drag.
- Determination of the relationship between tether geometry and the viscous drag.
- Identification of probe-surface interactions.
- Estimation of the torque exerted by braided DNA molecules in absence of pulling forces.
- Maximum torque resolution in the order of 102 pN nm.

We present an improved version of the magnetic tweezers/viscous drag (MTVD) hybrid assay that allows dynamic torque measurements during the spontaneous relaxation of DNA braids. The method relies on the ability of pairs of DNA molecules to rotate a microscopic probe attached to their upper ends using the elastic energy stored when braided. Digital tracking of the rotation of the dumbbell against the viscous drag allowed estimation of the torque exerted by the molecules. The unwinding dynamics of DNA braids against different amounts of viscous drag was characterized employing probes of two different sizes. We identified and characterized different perturbations that may affect the unbraiding dynamics. Estimates of the torques associated to the process of DNA unbraiding are reported.

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