We have constructed a molecular bipedal walker out of DNA.1 The system consists of a biped, a "footpath" for it to walk on, and a series of fuel strands that are introduced to control the movement. The biped is reminiscent of naturally occurring motor proteins such as kinesin or myosin V. Such proteins, however, typically run in only one direction, and it is difficult to target them to stop at a specific point on a track. Our system provides convenient control of the rise and fall of each foot via the simple introduction of "set" strands and "unset" strands - types of fuel strands similar to those developed by Yurke et al.2 This allows directing movement forward or backward along the footpath to specific destinations as desired. We have made the first prototype entirely of DNA except for 2 psoralen molecules attached to the bottom of each "foot" of the biped. Upon ultra-violet irradiation, the psoralen forms covalent bonds between the foot, the foothold, and the set strand connecting them. This allows clear determination of the state of the biped using denaturing gel electrophoresis.
There are a large number of potentially useful variations on this prototype. Taking advantage of the power of structural nucleic acid nanotechnology to produce complex structures with addressable, nano-scale features allows the construction of a broad variety of walker systems.3 A circular footpath can be built, which would allow a walker to generate rotational as well as linear movement.4 A two- or three-dimensional footpath would allow for the operation of multiple, independently addressable bipeds (such bipeds would have approximately identical structures, but would be distinguished by their unique base sequences). Potential applications of such systems include precise transport of loads, as well as winding or threading polymers which have one end attached to a footpath, and the other end to a biped.
1 Sherman, W. B.; Seeman, N. C. Nano Letters, June 2004.
2 Yurke, B.; Turberfield, A.J.; Mills, A.P., Jr.; Simmel, F.C.; Neumann, J.L. Nature, 2000, 406, 605-608.
3 Seeman, N. C. Nature, 2003, 421, 427-431.
4 Mathieu, F.; Mao, C.; Seeman, N. C. J. Biomol. Struct. Dyn., 2001, 18, 907-908.