Chengde Mao / Purdue
|DNA strands can be programmed to
create the scaffolding for complex 3-D
structures, as shown in this artwork.
Click on the image for a larger version.
The DNA double-helix molecule serves not only as an excellent construction manual for life as we know it, but also as a pretty good construction material in its own right. Scientists can bend the twisty stuff into two-dimensional shapes, including a "happy face" design - but three-dimensional shapes are much trickier.
In this week's issue of the journal Nature, researchers describe how sticky bits of DNA can put themselves together like Lego blocks to build up hollow geometrical shapes - ranging from pyramids to soccer balls less than a micron wide.
These DNA structures aren't just for kicking around: In the future, they could be used to deliver drugs, build nano-machines ... or hold prize molecular catches.
One of the researchers behind the Nature study, Purdue chemist Chengde Mao, told me today that the first applications for such structures would likely be as nano-containers.
Scientists could use such containers as cages for interesting molecular-scale structures, Mao said. Let's say you want to study the machinery of cellular factories known as ribosomes: You want to keep the ribosome in a medium where it can process proteins, but you don't want it to float away. In the future, you could hold the ribosome - or other interesting molecular structures - inside a DNA buckyball, Mao said.
"Think of it as a porous fish basket," he explained. "If you catch a fish, you can hold the fish in this confined space, but water can move in and out. ... The fish will be pretty happy."
The beauty part is that the DNA baskets build themselves out of smaller Lego-like structures, which Mao and his colleagues call "stars."
"We bought this DNA from a commercial company," Mao said. The trick is to mix the DNA and other ingredients in the right proportions and concentrations, heat the solution up to near the boiling point, then cool it down slowly.
The simplest star is a short three-pointed tripod with chemically sticky ends. Under the right conditions, the ends of three stars connect with each other to form a tetrahedron. A slightly bigger star has two sticky ends pointing out from each side of a bar. Depending on the recipe, the stars come together to form either a 12-sided dodecahedron or a 32-sided buckyball (which looks like the outline of a soccer ball).
The tetrahedra measure about 10 nanometers wide, the dodecahedra are about 70 nanometers wide, and the buckyballs are about 110 nanometers wide. The smaller the nano-gadget, the easier it is to self-assemble: Mao and his colleagues found that their recipe for tetrahedra yielded a 90 percent success rate - but the yield was only 76 percent for dodecahedra, and 69 percent for the buckyballs. The rest of the time, the DNA bits built themselves into less useful shapes, such as two-dimensional crystals.
This means picking out the keepers from a batch of DNA might have to be a little bit like picking out the consonants from a bowl of Alpha-Bits cereal. But there might be ways to streamline the filtering process - and Mao's group is also looking for different kinds of stars that can build even more complex structures.
Theoretically, the DNA structures could be used as cogs or ball bearings for nano-machinery. Or they could serve as packages for drug molecules, meant for unwrapping inside cells. That was one of the goals behind earlier research into DNA buckyballs, conducted by Cornell University's Dan Luo and colleagues. But Mao as well as Luo cautioned that the drug-delivery application was still far off.
In an e-mail, Luo said he was "quite impressed by the design" described in the Nature paper:
"DNA is indeed becoming a true designer polymer, and this work further demonstrates the power of DNA."
"As for applications, I, along with many people in the field, firmly believe that real-world applications will be realized in the near future with DNA materials such as reported by Chengde's group. The challenges are how to design and control DNA materials (Chengde's work clearly is a progress toward that challenge), how to expand DNA materials to wider fields and ultimately to consumers, and how to interface DNA materials with biology. In particular for drug delivery, it is still a black box in terms of how our body will interact with and react to complicated DNA tiles and closed 3-D structures as reported in the article."
We've already discussed the promise and potential peril of nano-machinery this week, but feel free to comment on this fresh facet of DNA technology.
Mao's colleagues in the Nature research are Yu He, Tao Ye, Min Su, Chuan Zhang, Alexander Ribbe and Wen Jiang, all of Purdue University.
Update for 12:20 a.m. March 13: I revised the first reference to the DNA buckyballs in response to a comment arguing that a molecule of buckminsterfullerene had a better claim on the title of "world's smallest soccer ball."