Crystals That Can Switch Configurations

For associate professor of Chemical and Biomolecular Engineering John Crocker, DNA is not just the stuff of genetics, but the makings of the perfect building block.

“The great thing about DNA is that it will only bind to other DNA that has the complementary sequence,” Crocker says. “So engineers have long thought that we could make a whole library of parts, put the appropriate sequences on the outside, then put them in a test tube and they would self-assemble.”

While truly complex construction kits are still a ways off, Crocker and his colleagues in the School of Engineering and Applied Science have been busy making basic structures—crystals—using this process. Crystals are typically made of atoms that are arranged in a repeating 3-D pattern; in the video, DNA-coated plastic balls, 400 nanometers across, play the role of atoms.

Crocker’s team has made a crystal that rearranges itself into a stronger, more stable configuration, all by changing the mix of DNA strands on the spheres’ exterior.

In the first experiment, the team began with two sets of balls, dyed red and green to make tracking easy. Red balls received one sequence of DNA while green balls received the complement. This meant that different-colored balls could bind, but same-colored ones couldn’t, as if one had Velcro hooks and the other had loops.

When put into a hot solution and gradually cooled, the balls assembled into a crystal with a square pattern, which is relatively unstable since squares can stretch and skew.

Next, researchers coated the balls with a mixture of both DNA sequences. The team discovered that if they made it possible for same-colored balls to bind, the balls assembled into a different crystal pattern—one full of triangles, which gave it extra structural support.

Through computer simulations developed by professor Talid Sinno and graduate student Ian Jenkins, the researchers had a theory for what was occurring: The weak crystal was transforming into the strong one.

“All it takes is one of those ‘like’ particle pairs to latch together, and then the entire crystal structure grows around that pattern,” Crocker says.

Text by Evan Lerner
Video by School of Engineering and Applied Science