Not Your Average Knot — Journal of Young Investigators

Author: Adam Zhang

Institution: Georgia Institute of Technology

Chemists at the University of Manchester have produced the tightest knot ever created. Published earlier this month in the journal Science, the team led by David Leigh used techniques in synthetic chemistry to braid strands of different molecules into a structure with over eight crossings. The first synthetic chemical knot was created in 1989 by chemist Jean-Pierre Sauvage of Strasbourg University, who later went on to win the 2016 Nobel Prize in Chemistry for molecular machines.

“And then, for the next 25 years, chemists were not able to make any more-complicated knots than that,” says Leigh.

This new knot, itself a type of molecular machine, is a step in the future of nanotechnology. Where the invention of computers brought calculators to the speed of electricity, construction of chemical knots will advance mechanical motors and machinery to a new molecular level. The application of this technology paves the way for an exciting development of new materials within medicine and electronics.

In mathematics, a knot consists of physical threads braided to cross at particular points, which is a similar approach to how you might tie shoe knots. However, the difference occurs in a formal knot, which is closed, meaning there are no ends to untie the knot. Every knot can be formed by braiding strings and joining the ends together; what differentiates the topologies of nontrivial knot types is the number of strings and the braiding technique. Since the beginnings of knot theory in the 1800s, over six-billion knots have been created and listed, and more are developed with growing complexity, but of these only three of the simplest knot types could be chemically recreated. The first and simplest of the three was the trefoil, by Jean-Pierre Sauvage, where the chemical strands cross at exactly three points. Subsequent development of the figure-eight and pentafoil knotanes, with four and five crossings respectively, were successfully created a few years after. However, more complex structures became exponentially more difficult, and progress soon stalled.

Figure 1. The simpler knot types, such as the trefoil and pentafoil, require only two chemical strands to synthesize. Constructing a 819 knot required three distinct strands and a unique braiding pattern. — **Figure 1. The simpler knot types, such as the trefoil and pentafoil, require only two chemical strands to synthesize.** Constructing a 819 knot required three distinct strands and a unique braiding pattern.

This new study depicts the octofoilknot, or 819, which crosses eight times and makes the 19th unique knot with these eight crossings. Though previous attempts at chemical knots used two strands, Leigh’s team employs three strands consisting of Carbon, Hydrogen, and Nitrogen and used a five-step chemical reaction to braid these strands together.

“We ‘tied’ the molecular knot using a technique called ‘self-assembly,’ in which molecular strands are woven around metal ions, forming crossing points in the right places just like in knitting,” says Leigh. “And the ends of the strands were then fused together by a chemical catalyst to close the loop and form the complete knot.”

**Figure 2. Multiple steps are required to chemically assemble the molecular braid for a closed 819 knot.**

The resulting structure crosses strands every 24 atoms, which makes it the “most complex regular woven molecule yet made by scientists.” In total, the structure extends 20 nanometers long and contains 192 total atoms. This exceptionally large length, in conjunction with its complexity, defines the knot’s tightness.

By creating new types of chemical knots, researchers can probe how this molecular architecture affects material strength and elasticity. Just as different sewing patterns create different fabrics, weaving polymer strands creates avenues for entirely new classes of materials.

"Tying knots is a similar process to weaving so the techniques being developed to tie knots in molecules should also be applicable to the weaving of molecular strands,” Leigh says. "For example, bullet-proof vests and body armor 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.”

With advances in molecular architecture, we can begin to dissect the nuances underlying complex chemical bonds. Synthetic chemicals are used everywhere from our cleaning supplies to our computers, and new strategies to create complex chemical structures will lead to lighter, stronger, and more flexible materials. The capabilities of chemical braiding can already be seen today, such as in spider silk, which is twice as strong as steel. Just as the invention of concrete led to the construction of our man-made wonders, further research will uncover bolder and better tools to establish the building blocks to excel the macroscopic world on the nanoscale.

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