Entropic unfolding of DNA molecules in nanofluidic channels.

Single DNA molecules confined to nanoscale fluidic channels extend along the channel axis in order to minimize their conformational free energy. When such molecules are forced into a nanoscale fluidic channel under the application of an external electric field, monomers near the middle of the DNA molecule may enter first, resulting in a folded configuration with less entropy than an unfolded molecule. The increased free energy of a folded molecule results in two effects: an increase in extension factor per unit length for each segment of the molecule, and a spatially localized force that causes the molecule to spontaneously unfold. The ratio of this unfolding force to hydrodynamic friction per DNA contour length is measured in nanochannels with two different diameters.