Chemical Knot: Scientists assemble legendary symbol by interlocking molecules

In a feat of chemistry imitating art, researchers have created a molecular version of a Borromean knot, an attractive pattern of three interlocking rings that commonly adorned Viking art and Renaissance architecture. Other chemists have created a multiply linked molecule that looks like an eight-petal flower.

The fascination among chemists with creating interlocking molecules runs deep. For decades, researchers have been coercing molecules into various ringlike structures, primarily as an exercise in gaining better control over chemical building blocks. Making molecular versions of Borromean rings poses formidable challenges for chemists because no pair of rings is linked unless the third ring is present. So, if any one of the rings gets severed, the entire construction falls apart.

“We told ourselves if we could make Borromean rings, we could make just about any kind of interlocking structure,” says Stuart Cantrill of the University of California, Los Angeles. In the May 28 Science, Cantrill and his colleagues, led by UCLA chemist Fraser Stoddart, describe their strategy for producing this complex structure. The researchers designed 12 separate molecular chains, each one representing a quarter of a ring. The chains, made of carbon, hydrogen, nitrogen, and oxygen, were designed such that, once in solution, they would spontaneously assemble into the Borromean configuration.

To guide the assembly, the researchers dissolved a bit of zinc in the solution and heated it. The electrically charged zinc ions served as a template around which the chains organized themselves. The final three-dimensional structure encompassed 6 zinc ions and the 12 chains, all combined into the world’s smallest Borromean rings.

When X-ray crystallographic analysis confirmed that the 2.5-nanometer-wide molecular structures were indeed Borromean rings, Cantrill and the rest of the UCLA team were elated.

“The hard part was coming up with the strategy so that all the pieces would slip into place,” Cantrill says. Repeating the experiment, he adds, is relatively easy.

Reporting in the same issue of Science, a team from Johannes Gutenberg University in Mainz, Germany, describes its synthesis of an eight-ring molecular structure. Volker Böhmer and his colleagues used a multistep process to link two loops, each one made of four rings in a configuration resembling a four-leaf clover. Each ring in one four-ring loop interlocked with two rings in the other loop, and vice versa.

“This is very clever and very elegant work,” says organic chemist Jay Siegel of the University of Zurich. Chemists have “only just begun to explore what kind of functions these ring structures might have,” he notes. He challenges the researchers to find applications for their chemical creations.

Böhmer muses that his eight-ring configuration could serve as a drug-delivery vehicle by encapsulating a medicinal molecule and releasing it on cue. And the UCLA group recently began investigating the electronic and magnetic properties of its rings. By replacing the zinc with another metal, such as copper or cobalt, and exposing the rings to an electric field, scientists might make it possible for the Borromean rings to store bits of computer data in a minuscule space, says Cantrill.