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Chemists tie the smallest ever knot

1 February 2017
Researchers have used molecules to tie the tightest knot ever

Dr Alice Williamson from the School of Chemistry looks at a recent breakthrough where scientists have used strings of molecules to form a chemical knot two hundred thousand times thinner than a human hair.

X-ray crystal structure of a molecular knot with eight crossings

Take a look down at your shoes and you’ll probably see a version of the very first knot that you learnt to tie. The trusty bowknot that still keeps your shoelaces safely fastened is just one of many that we wear everyday.

Since the Stone Age, humans have spun fibres into yarns and then woven them into cloth using different types of knots.

But knots aren’t only of interest to textile manufacturers and fashion designers, or sailors and scouts for that matter. Knots have intrigued mathematicians and physicists for centuries and now, chemists have also been roped in. A team from the University of Manchester have just tied the tightest physical knot ever.

The scientists from Professor David Leigh’s group didn’t tie a knot from string, but rather from strings of molecules, to form a knot that’s two hundred thousand times thinner than a human hair.

It isn’t possible to tie the ends of a molecular strand together in the same way we tie our laces though. Instead the ends of the strands react with each other chemically to form bonds. And unlike the bowknot, a mathematical knot doesn’t have any free ends. Leigh’s closed knot is made of a strand of 192 atoms that twists around itself in a triple loop with eight separate crossings.

The knot began as four strings of molecules that were carefully designed to include nitrogen at fixed points along the string. When the molecular strings were heated in a solvent containing ions of iron and chloride, the nitrogen atoms were attracted to the iron. This causes each strand to weave around the metal ions, crossing each other and self assembling to form much of the knot’s structure. The chemists then used a catalyst to close the loop by forming the bonds needed to zip up the structure into a mathematical knot that resembles a four-leaf clover.

The team have just beaten their own knotty world record, but they weren’t the first group to cook up a chemical knot in the laboratory. In the late 80s, Jean-Pierre Sauvage tied a simpler knot – more like a three-leaf clover. Sauvage went on to share the 2016 Chemistry Nobel Prize for related discoveries.

Tiny chemical knots may also become key to the design of materials with new properties.

So why are chemical knots important?

If we think of synthetic chemists as molecular architects, then they need to be able to control the way that atoms join together to make molecules. Forming knots in this regard goes even further, because the molecules are designed to assemble by themselves – imagine a house that spontaneously builds itself when all of the structural elements are combined under the right conditions!

Tiny chemical knots may also become key to the design of materials with new properties. Take bulletproof vests for instance. It might be possible to weave plastic fibres together to make a tougher but lighter material.

Knots are also found in some proteins and DNA and might be important for the properties of other biochemical or chemical structures. Leigh’s group for example have previously found that one of the knots makes a good catalyst for chemical transformations.

However, it may be that the best reason to synthesise chemical knots is because they haven’t been made before. Until chemists know how to make new structures, it isn’t possible to test and know of all their possible applications.

And there are lots of knots to keep them occupied – a whopping 6 billion unique possibilities are known. So far only four have been created in the lab.

About the author

Dr Alice E Williamson is a research chemist and lecturer based at the University of Sydney. Originally from the North West of England, Alice completed her PhD at The University of Cambridge, where she worked with colleagues to develop two new chemical reactions.

Alice moved to Australia to take up a position as the principal synthetic chemist for the Open Source Malaria (OSM) consortium. OSM are pioneering an open source drug discovery project and are trying to prove that science is better and more efficient when all data and results are shared. The team won't patent any of their findings and publish all of their work online in real time so that anyone can access the research. Alice makes new medicines in the lab, helps to coordinate the international team and tries to encourage people to join the project. She has also set up some unusual collaborations with high school students and undergraduates who have made new drugs designed to kill the malaria parasite.

In addition to her research, Alice has been lecturing chemistry for the past three years and developing skills as a science communicator. In February 2015, she was named as one of ABC RN and UNSW's Top 5 Under 40 in recognition of her passion for sharing science stories. She has been an active participant in science outreach events across Sydney and in June 2015, Alice took up a weekly science slot on FBi Radio's breakfast show.