Single Molecule Manipulations in Biophysics 2

Torque Measurements on single DNA Molecules

The physical properties of the DNA double helix are unlike those of any other natural or synthetic polymer. The molecule’s characteristic base stacking and braided architecture lend it unusual stiffness: It takes about 50 times more energy to bend a double-stranded DNA molecule into a circle than to perform the same operation on single-stranded DNA. Moreover, the phosphates in DNA’s backbone make it one of the most highly charged polymers known.

To perform dynamic torque measurements on single DNA molecules, molecular constructs were made.This kind of experiments have been performed in the Prof. Bustamante's laboratory at Berkeley University.

The use of three distinct chemical modifications of DNA allows for oriented tethering of the ends of the molecule and the subsequent attachment of a rotor to a third, internal position (shown on the figure). A site-specific nick in the duplex DNA is engineered adjacent to the rotor attachment point; this design allows covalent bonds in the intact strand to serve as free swivels, preventing torque from accumulating in the "lower" DNA segment. Thus, torque stored in the "upper" segment can drive the rotation of a submicron object on a low-friction molecular bearing. At low Reynolds numbers, the magnitude of the torque can be measured by multiplying the observed angular velocity by the rotational drag of the rotor.

To be continued

reference: Bustamante, C.Of torques, forces, and protein machines(2004) Protein Science, 13 (11), pp. 3061-3065.doi: 10.1110/ps.041064504