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Nanoscale chains forged from a simple molecule

Assembly process can link up to 22 rings using noncovalent interactions

by Bethany Halford
July 15, 2020 | A version of this story appeared in Volume 98, Issue 28

An atomic force micrograph shows a chain with links that are approximately 20 nm in diameter.
Credit: Nature
This atomic force micrograph shows a nanoscale chain with 22 links.

In an Olympic-level feat of chemical choreography, scientists have assembled nanoscale chains from a simple monomer held together only by noncovalent interactions. Making such a complex supramolecular structure from such a simple building block takes self-assembly to a new level, experts say.

The monomer used to build nanoscale chains has a polar group that makes hydrogen bonds attached to a rigid region and a nonpolar tail.

Shiki Yagai, a chemist at Chiba University who led the research effort, says the nanoscale chains, though intricately forged, are the result of an accidental discovery. “We were originally investigating methods to efficiently synthesize ring-shaped molecular assemblies,” Yagai says. A simple monomer Yagai’s team created can assemble via hydrogen bonding to make a six-membered supermacrocycle. These snowflake-like structures then stack one on top of the other, curving as they do so to make snakelike structures or helical coils. Under the right conditions, the snowflake stacks will curve into rings.

To their surprise, Yagai and coworkers observed that when they mixed certain polar and nonpolar solvents, the rings would link together. When they investigated the phenomenon, they found that the inside of the ring seeds the formation of another ring around itself. By sequentially adding more monomer to a solution of rings, they were able to drive the formation of long supramolecular chains of up to 22 links (Nature 2020, DOI: 10.1038/s41586-020-2445-z).

Six monomers assemble into a snowflake-shaped supermacrocycle.

David A. Leigh, an expert in complex molecular topology at the University of Manchester, says it’s remarkable to see a process that threads and cyclizes such large, regularly sized rings. “It opens the door for future exploration of the properties mechanically interlocked molecular structures can have at these larger length scales,” he says.

“This is a remarkable example of topologically complex supramolecular materials,” adds M. Reza Ghadiri, who studies self-assembly at Scripps Research, California. “Given better understanding of the basic principles governing such topological self-assembly, a new generation of materials could be rationally fabricated and studied for their potentially unique properties.”

Yagai first realized the power this technique when he saw that it could make a chain with five links that resembled the symbol for the Olympic Games. “I seriously thought that this result should be published in the year of the Tokyo Olympics,” he says. “Unfortunately, the Olympics have been postponed, but I am very happy to leave a trace in 2020.”



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