When a long thin polymer such as DNA is forced into a confined space — say a small membrane channel — it loses some of its freedom, and hence its entropy. Regaining that entropy is a powerful driving force for escape.

Chia-Fu Chou at the Academia Sinica in Taipei and his colleagues used an electric pulse to force a single DNA molecule to extend from one microchannel to another through a restrictive gap just nanometres high. When the electric field was turned off, a tug-of-war lasting from seconds to minutes occurred as both ends of the DNA tried to pull out of the nanometre-sized space. Eventually, one side won and the DNA retracted.

The forces acting on the DNA depended only on the height of the confined passage between channels, and not on its length or the length of DNA passing through it. This understanding could aid applications from molecular filters to nanopore transporters, the authors say.

Nano Lett. http://dx.doi.org/10.1021/nl2045292 (2012)