DNA is often called the “building block of life”. It consists of two linear strands wound into a double helix with one of four different “bases” attached to every sugar group along the strands. DNA is an attractive component for use in molecular machines because it can recognize specific base sequences. It self-assembles easily and complex molecular structures can be made from simple double helices. In addition, DNA can change its shape, which further expands the number of nanostructures possible.

Alberti and Mergny used an unusual “quadruplex” DNA structure, which contains four strands with twenty-one bases, folded in a special way. The structure is made to unfold by adding a fuel DNA strand, creating a “duplex” structure that resembles the more conventional double helix. To re-fold the duplex, the researchers add an “anti-fuel”, which combines with the fuel to form a waste product. The folding-unfolding cycle takes only a few seconds and fluorescence resonance energy-transfer spectroscopy shows that the expansion and contraction occurs over a distance of 5 to 6 nanometres.

The device oscillates between two well-defined states and can be compared to the movement of a piston in a cylinder, the researchers say. “This new type of extension-contraction movement ties in well with work by other groups who observe rotation and scissor-like opening and closing,” Mergny told PhysicsWeb. “From a nanotechnology point of view, it is possible to finely control the structure by the addition of strands with specific sequences.”

The sequence of bases along the chain chosen by the researchers is important biologically and the team now hopes to look at other sequences that exhibit the same type of movement. “We would also like to know if quadruplexes are able to form inside a human cell,” Mergny added.