Scientists working to harness DNA to construct and power microscopic devices have gained an important tool.

Researchers at Lucent Technologies' Bell Labs and the University of Oxford in England have found a way to keep their DNA motor running continuously. Previously, the researchers had to add particular strands of DNA at different steps in the motor's cycle to keep it going.

The free-running DNA motor could eventually power microscopic machines capable of constructing and transporting chemicals and materials molecule by molecule.

Each strand of DNA contains four types of bases--adenine, cytosine, guanine and thymine--attached to a sugar-phosphate backbone. Complementary bases--adenine with thymine, and cytosine with guanine--combine to zip a pair of single DNA strands into a double helix.

A motor works by changing shape, then changing back again. In previous work, the researchers realized that a strand of DNA could be coaxed into a hairpin-like shape by causing complementary base sequences near the middle of the strand to combine with each other. This was the first step in constructing a motor from the molecule. "We realized that the nanostructure could be restored to its initial configuration by the complement, or opposite base sequence, of the strand of DNA that was used to induce the shape change," said Bernard Yurke, a distinguished member of technical staff at Bell Labs.

The key to making a free-running motor rather than a device that required the addition of new base sequences between movements, however, was finding a way to prevent the portion of the strand that formed a hairpin--the fuel strand--and the complement that removed the hairpin shape from simply combining with each other.

The researchers accomplished this by forcing the fuel strand of DNA into a tiny loop to make it physically impossible for the removal strand to combine directly with the fuel strand. "The removal strand, because it is long and only its middle portion can [combine] with the unpaired bases of the loop, has difficulty threading its way through the loop to form a double helix," he said.

The researchers added a shorter strand of DNA, dubbed the motor strand, that was able to combine with some of the unpaired bases of the loop to open it. "Once a loop is opened, the removal strand can [combine] with unpaired bases on the fuel strand and displace the [motor] strand from the loop," said Yurke. The displaced motor strand is then free to attach itself to a new loop to start the process over again.

If the motor strand were part of a nanostructure, the structure could be repetitively switched between two states as long as a few fuel loops and removal strands were present, Yurke said.

DNA can be used to impart force due to a pair of useful properties, said Yurke. First, when double-stranded DNA is less than 50 nanometers long, it behaves like a rigid rod, while a single strand of DNA behaves more like a floppy string, he said. Second, when strands of DNA are zipping up to form double-strand DNA, they can impart a force of up to 15 piconewtons.

A nanometer is one millionth of a millimeter, and 50 nanometers is about one 20th the diameter of E. coli bacteria. A piconewton is one trillionth of a Newton, which is about the force of a falling apple.

"The change in stiffness as a single-strand of DNA is transformed into double-strand DNA and the force that can be developed during the transformation can be used to drive shape changes in nanostructures," said Yurke.

The method is the first that uses DNA to power a free running system, according to Nadrian Seeman, a professor of chemistry at New York University. "To date, all DNA-based nanomechanical devices have required [input] to change state," he said. "These [researchers] have put together a system that will, in principle, allow for a free-running machine."

Such a machine could provide power for devices like nanorobots and nanomechanical computers, Seeman said. "This is cutting-edge work that advances DNA nanotechnology," he added.

The DNA motor could eventually power nanomachines for use in medicine, chemistry and materials science, said Yurke. "Molecular motors ought to be extremely useful. Living cells are loaded with molecular motors and these serve crucial roles in turning out a wide range of life's functions, including cell movement, molecular transport, replication and even chemical synthesis," he said.

Devising DNA-based molecular motors was an exercise in getting DNA to assemble itself into a useful device, said Yurke. Eventually it may be possible to coax DNA to assemble into much more complicated structures, including molecular-scale electronic circuits, he said.

It is too early to tell when synthetic molecular motors will be used practically, said Yurke.

Yurke's research colleagues were Andrew J. Turberfield and J. C. Mitchell at the University of Oxford, A. P. Mills, Jr., formerly at Bell Labs and now the University of California at Riverside, F. C. Simmel, formerly at Bell Labs and now at Ludwig Maximilians University in Germany, and M. I. Blakey at Bell Labs. The work appeared in the March 18, 2003 issue of Physical Review Letters. The research was funded by Lucent Technologies.

Technology Research News April 15, 2003