Researchers at Arizona State University (ASU) have revealed, for the first time, three-dimensional characters of DNA nanotubes, rings and spirals, each a few hundred thousandths the diameter of a human hair. According to Hao Yan and Yan Liu, researchers at ASU's Biodesign Institute and faculty in the Department of Chemistry and Biochemistry, these DNA nanostructures may soon find their way into a new generation of ultratiny electronic and biomedical innovations. 5-nm gold nanoparticles wrap around the perimeter of a DNA nanotube in a spiral pattern. The 3-D structures have been re-created from cryoelectron tomographic imaging (image: ASU).Yan and Liu are working in the rapidly proliferating field of structural DNA nanotechnology. Inspired by nanoscale ingenuity in the natural world, they capitalize on the DNA molecule’s remarkable properties of self-assembly. When ribbonlike strands of the molecule are brought together, they fasten to each other like strips of Velcro. DNA accomplishes this through the cellular synthesis of structural proteins, coded for by specific sequences of the bases. Such proteins are fundamental constituents of living matter, forming cell walls, vessels, tissues and organs. But DNA itself also can form stable architectural structures, and may be artificially cajoled into doing so. "Unicellular creatures like oceanic diatoms contain self-assembled protein architectures," Yan said. These diverse forms of enormous delicacy and organismic practicality are frequently the result of the orchestrated self-assembly of both organic and inorganic material. Scientists in the field of structural DNA nanotechnology, including Yan's team, previously demonstrated that prefab DNA elements could be induced to self-assemble, forming useful nanostructural platforms, or "tiles." Such tiles can snap together – with jigsaw puzzle-piece specificity – through base pairing, forming larger arrays. Yan and Liu's work responds to one of the fundamental challenges in nanotechnology and materials science – the construction of molecular-level forms in three dimensions. To do so, the team uses gold nanoparticles, which can be placed on single-stranded DNA, compelling these flexible molecular tile arrays to bend away from the nanoparticles, curling into closed loops or forming springlike spirals or nested rings, roughly 30 to 180 nm in diameter. The gold nanoparticles, which coerce DNA strands to arc back on themselves, produce a force known as "steric hindrance," whose magnitude depends on the size of particle used. Using this steric hindrance, Yan and Liu have shown for the first time that DNA nanotubules can be specifically directed to curl into closed rings with high yield. When 5-nm gold particles were used, a milder steric hindrance directed the DNA tiles to curl up and join complementary neighboring segments, often forming spirals of varying diameter in addition to closed rings. A 10-nm gold particle, however, exerted greater steric hindrance, directing a more tightly constrained curling that produced mostly closed tubules. Yan stressed that the particle not only participates in the self-assembly process as the directed material but also as an active agent, inducing and guiding formation of the nanotube. With the assistance of Anchi Cheng and Jonanthan Brownell at the Scripps Research Institute, they have used an imaging technique known as electron cryotomography to provide the first glimpses of the elusive 3-D architecture of DNA nanotubules. "You quickly freeze the sample in vitreous ice," explained Yan. "This will preserve the native conformation of the structure." Subsequent imaging at various tilted angles allows the reconstruction of the 3-D nanostructure, with the gold particles providing enough electron density for crisp visualization. DNA nanotubules soon will be ready to join their carbon nanotube cousins, providing flexible, resilient and manipulatable structures at the molecular level. Extending control over 3-D architectures will lay the foundation for future applications in photometry, photovoltaics, touch screen and flexible displays as well as for far-reaching biomedical advancements. "The ability to build three-dimensional structures through self-assembly is really exciting," Yan said. "It's massively parallel. You can simultaneously produce millions or trillions of copies."Yan and Liu believe that controlled tubular nanostructures bearing nanoparticles may be applied to the design of electrical channels for cell-cell communication or used in the construction of various nanoelectrical devices. Yan and Liu’s research is published in the January 2009 issue of Science.For more information, visit: www.asu.edu