This tiny knot is the tightest ever tied

With eight crossings, it is the most tightly knotted physical structure ever known.

The knot is made using molecular strands that are half a nanometer across, 10,000 times thinner than a human hair, and the achievement opens up the potential for a new generation of advanced materials. The study, published in Science, details the knot, which has a chain 192 atoms long and intersects at eight points.

We spoke with Professor David A Leigh from The University of Manchester about the work.

ResearchGate: What motivated your study?

David A Leigh: Molecular knots occur in biology, in DNA and some proteins, and some six billion different types of knots are known to mathematics. So it might seem surprising that before this work, scientists were only able to construct three of the very simplest types of knots. The eight-crossing knot is the most complex molecular knot made to date and the way it is made, braiding of multiple molecular strands, should enable even more complex and structurally diverse molecular knots to be tied in the future. 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. Interweaving polymer strands has the potential to create much tougher, lighter, and more flexible materials in the same way that weaving threads does in our everyday world.

RG: Could you explain the main findings? Which types of molecular strands can you tie together?

Leigh: What my group and I have been doing is tying knots in molecules. The molecular strands we’re using are really small – half a nanometer across, or 10,000 times thinner than a human hair – and are made of carbon, hydrogen, nitrogen and oxygen atoms. The length of the strands, opened out, is about 20 nm, 500 times smaller than a red blood cell. We’ve recently tied the tightest knot so far. It has eight crossings (where the strand crosses itself in the knot) in a 192-atom long closed loop. That’s just 24 atoms per crossing. It’s the tightest knotted physical structure known.

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. Robert W. McGregor.
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. Robert W. McGregor.

RG: How did you tie this knot? What were the biggest challenges?

Leigh: The strands we’re knotting are so small that you can’t grab the ends and mechanically tie them like you would a shoelace. Instead we use chemistry and a process called self-assembly. We mix organic molecular building blocks with metal ions and chloride ions in a solution. The metal ions are sticky in particular directions (because of the shape of electron orbitals), and the organic building blocks wrap around the metal ions forming crossing points in the right places – just like in knitting. Finally, the ends are fused together by a chemical catalyst to close the loop and form the complete knot. Unlike tying your shoelaces, self-assembly can tie many knots at the same time. We typically make a million of these molecular knots in one go! The biggest challenge is to design the building blocks so that they bind to the metal ions correctly and wrap around them in precisely the way that is required.

RG: Can you give us a brief insight into what this means for your field?

Leigh: To date scientists have only been able to make the simplest types of molecular knot, such as trefoil (three crossings) and pentafoil (five crossing) knots. This is partly because the most widely used methods to make molecular knots twist two molecular strands around one another. The unavailability of all but the simplest molecular knots hinders investigation into the effects of different knot types in materials and for other applications. To make the eight crossing knot we developed a way of braiding multiple molecular strands, rather than just two. This technique should be generally applicable and should enable tighter and more complex knots to be made than has previously been possible.

A 819 knot. Credit: Stuart Jantzen.
A 819 knot. Credit: Stuart Jantzen.

RG: What are some of the applications that most excite you?

Leigh: The principles of knotting are very similar to weaving, so it should be possible to use what we’re learning about knotting molecules to weave molecular strands to make new sorts of materials. In our everyday world we know the benefits of weaving fabrics – you get materials that can stretch in different directions, hold their shape, are light and strong and flexible. Hopefully we will be able to use these concepts to weave molecular strands to make plastics and polymers with similarly advantageous properties. For example Kevlar, used in bullet proof vests, body armor, car brakes, and parts of aircraft bodies, is a super-tough polymer with a chemical structure that consists of tiny straight molecular rods that pack close together, like pencils packed tightly in a pencil case. If we can weave molecular strands into molecular fabrics maybe we will be able to get the same sort of strength with a lighter and more flexible material.

RG: What’s next in your research?

Leigh: We’re trying to apply what we’ve learned from knotting to weaving and we’re investigating what effect knotting has on molecules – does knotting make molecular strands weaker (as it does to fishing lines in our everyday world)? Are knotted molecules more reactive because of the strain of knotting? Can knotting be used to bring about new chemical properties?

Article: Science "Braiding a molecular knot with eight crossings," by J.J. Danon; A. Krüger; D.A. Leigh; J.-F. Lemonnier; A.J. Stephens; I.J. Vitorica-Yrezabal; S.L. Woltering at University of Manchester in Manchester, UK.

Featured image courtesy of flickr.