“The circuit element we made contains a guanine-quadruplex (G4) motif, which can be used as a connector for multi-ended DNA duplexes,” explains Nadrian Seeman of New York University in the US, who led this research effort along with Nongjian “NJ” Tao of Arizona State University and David Beratan of Duke University. This connector allows charges to enter the structure from one terminal at one end of the three-way G4 motif and exit from one of the two terminals at the other end – something that was not possible with previous such designs.

Nanoscale circuit elements, such as current splitters or combiners, require at least three terminals. A good material for making the building blocks for these elements is DNA. This molecule contains multiple strands that can self-assemble into multi-ended junctions and its nucleobase stacks can transport electric charge over long distances. The problem is, however, that this stacking is often disrupted at junction points, which hampers charge transport between terminals.

“We have now shown that a guanine-quadruplex (G4) motif can be used as a connector element for multi-ended DNA duplexes,” says Seeman. “Without this ability to make three or more branch components, it is not possible to build circuit networks from DNA.”

Alternative strategy

Previous attempts to make such multi-terminal charge-transfer structures from DNA relied on double crossover motifs, but charge cannot flow between the helices of these structures, he explains. To try and overcome this problem, researchers then tried splitting a DNA double helix into two double-stranded helices in a Y-shaped three-way junction, but they found that charge transport through this junction was slow compared to that through a duplex DNA.

“An alternative strategy is to use an extended guanine quadruplex (G4) motif with appended duplexes,” says Seeman. “This motif consists of stacked guanine strands wherein each guanine base forms hydrogen bond pairs with its two neighbours. The structure is stabilized by K+ counterions.”

Splitter/combiner type structure

In this design, charge injected from two different duplexes converges in the G4 motif and exits through one or two duplexes on the other side of the quadruplex without attenuating, which is in fact a splitter/combiner type structure.

In their work, Seeman and colleagues measured the conductance through G4-based nanostructures using the scanning tunnelling microscope break junction (STM-BJ) technique. The structures they studied comprise a G4 core with double helices attached to each side.

The team, detailing its work in Nature Nanotechnology doi:10.1038/s41565-018-0070-x, says that it is now trying to make DNA-based elements that can be controlled, “perhaps like transistors,” Seeman tells nanotechweb.org.