Base Load: Currents add detail to DNA structure

By Davide Castelvecchi
From Science News, December 1st, 2007; Vol.172 #22 (p. 342)

Researchers have made the first precise measurements of DNA’s ability to conduct electricity laterally, across its double helix structure. The team’s newly improved methods confirm that DNA has some properties in common with those of semiconductors and might help in the development of new genome-sequencing technology as well as DNA-based electronics.

Life’s double helix is not a metal, so it has no freely roaming electrons to carry currents. But DNA has “excited” states that electrons can hop between if they have sufficient energy.

DNA’s complexity and flexibility have made it hard, however, to analyze the detailed structure of its excited states. It’s a “finicky, difficult molecule,” says Stuart Lindsay, a chemist at Arizona State University in Tempe.

Over the past decade, scientists have obtained inconsistent results when running electrical currents along DNA’s length, Lindsay says. Different experiments have suggested that DNA was a conductor, an insulator, a semiconductor—or even a superconductor. A breakthrough came in 2004, when scientists showed that DNA can act either as a conductor or as a semiconductor, depending on its sequence of bases, which are chemical units denoted by A, C, T, and G.

Danny Porath of the Hebrew University in Jerusalem and his collaborators have now tested DNA’s conduction properties transversely, through single base pairs—the rungs in the double helix formed by links between bases in the two strands.

Porath’s collaborators at the University of Regensburg in Germany prepared artificial DNA consisting, for simplicity, of one strand containing only G’s paired with another of all C’s. The synthetic DNA had none of the A-T pairs that occur in natural DNA.

In Jerusalem, Porath’s team immobilized the strands on a metal surface kept under vacuum at about –200°C. The researchers then suspended the tip of a scanning tunneling microscope 1 nanometer above the strands. When they applied a voltage between the tip and the metal surface, electrons started flowing, crossing a single base pair from side to side.

As the researchers ramped up the voltage from zero to a few volts, the current increased in discrete jumps. Such jumps, reminiscent of semiconductor behavior, unmistakably reveal the activation of excited states, Porath says. Moreover, for repeated measurements of different sites on each strand, and on strands that lay in slightly different positions, the researchers observed jumps at the same voltages.

The team’s findings will appear in an upcoming Nature Materials.

The results don’t overturn the known picture of DNA, comments Lindsay. But the fact that they are reliable and reproducible makes them “an impressive achievement,” he says, helping to move the field past “voodoo science” status.

A more interesting test, Lindsay says, will be to compare the behavior of C-G pairs with that of A-T pairs. Porath says his team is working toward that goal. Such knowledge could help in the effort to develop faster, cheaper DNA-sequencing technologies. Some researchers, including Lindsay, are hoping to sequence strands by passing them through a hole in a membrane while running a current across them.

Porath says that controlling the motion of electrons in DNA might also have applications in future computers. With their self-assembling skills, DNA molecules show promise as components for electronics circuits with features just a few nm wide, compared to the 45 nm in state-of-the-art silicon-based chips.

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