World’s Smallest, Tightest Knot Ever Created Is Just 54 Atoms Long

Scientists have broken the record for the smallest and tightest molecular knot, creating a chemical that self-assembles into a knot with the formula [Au6{1,2-C6H4(OCH2CC)2}3{Ph2P(CH2)4PPh2}3]. The makers highlight the six gold atoms at the start by referring to the entire molecule as Au6.

The way a molecule affects others depends not just on the elements it’s composed of, but on the shape these take on. Complex molecules can be folded in vast numbers of ways, and sometimes only one of these will produce the desired biological effects. Predicting and controlling such folding is considered one of the hardest problems in science, and one where computers are only just starting to displace humans. 

Knots represent the extreme end of this. It’s not easy to tie even long thin strands of atoms into knots because you can’t just grab the ends like pieces of rope on a sailboat. Finding ways to make molecules knot can help develop humanity’s capacity for more practical knotting. Since DNA, RNA, and many proteins knot without human intervention, performing knotting of our own helps us understand these vital molecules’ behavior.

Knots are categorized by the number of crossings. Au6 forms the simplest kind of non-trivial knot, with three crossings, and is known as a trefoil. 

Chemists cannot measure the tightness of molecular knots in a physical sense, so they use the number of atoms in the knotted strand, dividing by the number of crossings, as a proxy. The fewer atoms per crossing, the tighter a knot is.

Au6, produced by a team including Professor Richard Puddephatt of the University of Western Ontario, has just 54 atoms in its backbone. This compares with the previous record for a metallaknot of 69, while no organic trefoil knot has been achieved with fewer than 76 atoms. Much larger knots, with far more crossings, have achieved tightness scores as low as 24, but Au6’s 54 atoms and tightness score of 18 easily beats all. Theoretical models suggest that 50 atoms may be a minimum for a knot.

Most molecular knot-making involves creating ever more complex knots, but there is a niche subfield in producing ever-tighter knots, as the below video accompanying an earlier record proves. 

        

Small knots have generally been made by using metal ions to draw helical chains into a targeted shape and then pulling the metals out to leave a knot behind. It’s a stepping stone to stronger and lighter plastics, among other things.

Au6, however, is made differently, and Puddephatt told New Scientist it was an accident. They were mixing two liquids containing different molecules to make structures with interlinked, but not knotted, chains called catenanes. Using X-ray crystallography to study the product, they found some of the catenanes had self-assembled into trefoil knots.

“We’ve made many combinations of gold acetylides and phosphine ligands and they’ve never before given a trefoil knot,” said Puddephatt. “We hadn’t predicted that this would happen in this case, so it was serendipity.” He also acknowledged the process was not fully understood, although it is repeatable and has potential for use in more complex situations.

The study is published in Nature Communications.

Leave a Comment