New molecular knot is most complex yet

One hundred and ninety-two atoms have tied the knot.

Chains of carbon, hydrogen, oxygen and nitrogen atoms, woven together in a triple braid, form the most complex molecular knot ever described, chemists from the University of Manchester in England report in the Jan. 13 Science.

Learning how to tie such knots could one day help researchers weave molecular fabrics with all sorts of snazzy properties. “We might get the strength of Kevlar with a lighter and more flexible material,” says study coauthor David Leigh.
That’s still a long way away, but molecular knot tying has an appeal that’s purely intellectual, too, says University of Cambridge chemist Jeremy Sanders. “It’s like the answer to why you climb Everest,” he says. “It’s a challenge.”

Mathematicians know of more than six billion types of prime knots, which, like prime numbers, cannot be broken down into simpler components. “Prime knots can’t be built up by sticking other knots together,” Leigh explains. For years, chemists were able to synthesize just one type of prime knot out of small molecules. “We thought that was pretty ridiculous,” says Leigh.

That molecular knot was a trefoil, like a three-leaf clover. Jean-Pierre Sauvage and colleagues wove it from chemical strands in 1989. Sauvage won a Nobel Prize in 2016 for earlier work that used the same principles explored in his knots (SN: 10/29/16, p. 6).

In the decades since Sauvage’s trefoil, chemists have tried to synthesize other types of molecular knots, but “they’ve always found it incredibly difficult,” says chemist Sophie Jackson, also at the University of Cambridge.

Persuading nanoscale strands to interlock together in an orderly fashion isn’t simple. “You can’t just grab the ends and tie them like you would a shoelace,” Leigh says. Instead, scientists choose molecular ingredients that assemble themselves.
In 2012, Leigh and colleagues used the self-assembly technique to make a molecular pentafoil knot, a star-shaped structure made up of 160 atoms and with strands that cross five times (SN: 1/28/12, p. 12). This latest knot, with eight crossing points, is even more intricate.

Leigh’s team mixed together building blocks containing carbon, hydrogen, oxygen and nitrogen atoms with iron ions and chloride ions. “You dump them all in, heat them all up and they self-assemble,” he says. Sticky metal ions hold the building blocks in the correct position, and a single chloride ion sitting in the middle of the structure anchors it all together. Then, a chemical catalyst links the building blocks, forming the completed knot. The new knot is the tightest ever created, Leigh says, with just 24 atoms between each crossing point.

It’s beautiful, Sanders says. “It’s a string of atoms rolled up in a spherical shape, with an astonishing amount of symmetry.” Sanders is reluctant to speculate how such a knot might be used, but it’s round and very dense, he says. That could give it some interesting materials properties.

Leigh suspects that different molecular knots might behave differently, like the various knots used by fishermen and sailors. “We want to make specific knots, see what they do and then figure out how to best exploit that,” he says.