Scientists tie the world's tightest knot

Researchers at the University of Manchester hope their chemically produced, three-strand molecular knot will some day form the foundation for very light and strong materials.

Courtesy of Robert W. McGregor/University of Manchester
The X-ray crystal structure of a 192-atom-loop molecular 819 knot featuring iron ions (shown in purple), oxygen atoms (red), nitrogen atoms (dark blue), carbon atoms (shown in metallic grey, with one of the building blocks shown in light blue) and a single chloride ion (green) at the center of the structure.

Knots have been around for thousands of years. But now, a team of researchers from the University of Manchester has added a new twist to one of the most basic technologies known to humans.

Using cutting-edge chemical techniques, the researchers have created the tightest knot ever made, woven on a molecular level. The new knot is a circular triple helix only 20 nanometers long, containing only 192 atoms, the researchers report in a paper published in the Jan. 13 issue of Science magazine.

While scientists have known for decades that molecular knots like this one are theoretically possible, it has proven difficult to create knots of such complexity, with previous molecular knots using only two strands woven together in very basic patterns. This is the first molecule to incorporate three strands into its structure, in a leap forward for a technology that the researchers suggest could eventually lead to a new generation of super-light and super-strong knotted materials.

Until now, there had only been three molecular knots created by scientists at this scale: the trefoil, the figure-eight, and the pentafoil, a paltry amount compared to the 6 billion known prime knot formations, Science's Jessica Boddy explains in a brief. But this new circular triple helix is the most complex knot ever created on this scale, with a total of 8 strand crossings, with each crossing only 24 atoms apart.

"That's very, very tight indeed," David Leigh, a professor of chemistry at the University of Manchester and study leader, told The Guardian. "It is definitely the most tightly knotted physical structure known."

The entire structure is about 200,000 times thinner than a human hair and requires much more chemical finesse to weave than a macro-scale knot.

"These strands we are knotting are so small that you can't grab the ends and tie them like you would a shoelace," Dr. Leigh told The Guardian. "Instead we use a chemical process called self assembly, where we mix the organic building blocks with ions that the building blocks then wrap around to make crossing points in the right places."

In order to create the new knot, the team first created a solution of strands of carbon, nitrogen, and oxygen atoms, according to the Guardian. When mixed with chloride and iron ions in a heated solvent, the strands wove themselves together over about 24 hours. Then, the ends of each strand were fused together in a continuous loop, and the ions were washed away, leaving only the knot. X-ray crystallography images confirmed that the knot had formed as the researchers had expected.

"It's fantastic," Edward Fenlon, a chemist at Pennsylvania's Franklin & Marshall College, who was not part of the study, told NPR. "It's really impressive that they've been able to go beyond some of the more simple knots with just three crossings."

Leigh hopes that building increasingly complex molecular knots will eventually lead to weaving, which could, in turn, lead to new technologies.

"Bullet-proof vests and body armour are made of kevlar, a plastic that consists of rigid molecular rods aligned in a parallel structure - however, interweaving polymer strands have the potential to create much tougher, lighter and more flexible materials in the same way that weaving threads does in our everyday world," said Leigh in a statement from Manchester University. "Some polymers, such as spider silk, can be twice as strong as steel so braiding polymer strands may lead to new generations of light, super-strong and flexible materials for fabrication and construction."

Of course, there is still a long way to go before this kind of micro-weaving is a reality.

"Historically, knotting and weaving have led to all kinds of breakthrough technologies," Leigh told NPR. "Knots should be just as important at the molecular level, but we can't exploit that until we learn how to make them, and that's really what we're beginning to do."

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