A knot made from gold that crosses over itself three times, known as a trefoil knot, is both the tiniest and tightest knot ever made. The knot contains just 54 atoms, including six gold atoms that form its backbone, and could help us understand how knots form in biological systems.
Knots have been studied by mathematicians for centuries, but it wasn鈥檛 until the late 1980s that people first made really tiny ones: molecular knots made from tangled chains of atoms. They have since been shown to have interesting properties, such as well-defined structures that fit together like Lego blocks.
The smallest knot previously known was reported in 2020 and was made with .
Advertisement
This one and the new smallest knot both beat the current Guinness World Records holder, a longer one made from 192 atoms crossed over eight times, because tightness is measured by dividing the number of atoms in the knot by the number of crossings, giving you a measure called the backbone crossing ratio. The smaller the ratio, the tighter the knot. The 69-atom chain had a crossing ratio of 23, beating the 192-atom knot鈥檚 crossing ratio of 24.
at the University of Western Ontario in Canada and his colleagues have now produced a knot that is 54 atoms long with its three crossings giving it a ratio of 18.
They did it by mixing a liquid containing molecules of two gold atoms linked by carbon rings 鈥� known as gold acetylide 鈥� with another liquid consisting of pairs of phosphorus atoms linked by a different assortment of carbon rings, or diphosphine ligand.
Puddephatt and his team had previously found that mixing these two liquids produced a kind of molecule called a catenane 鈥� which is two interlinked chains 鈥� containing two gold atoms but they had seen no evidence of a knot. Now they have characterised the products in the mix using X-ray crystallography, and found that some of them were trefoil knots containing three of these catenanes linked together, with six gold atoms.
鈥淲e鈥檝e made many combinations of gold acetylides and phosphine ligands and they鈥檝e never before given a trefoil knot,鈥� says Puddephatt. 鈥淲e hadn鈥檛 predicted that this would happen in this case, so it was serendipity.鈥�
While it seems the catenane rings are combining to make the knot, what makes them take this form isn鈥檛 clear, says Puddephatt. 鈥淚t鈥檚 quite a complicated system and, honestly, we don鈥檛 know how it happens,鈥� he says.
Figuring out the way in which this knot forms could help us make more complex structures using gold atoms鈥� tendency to stick together, says at the University of Cambridge. 鈥淭he rules work beyond the simple example that they initially started out with.鈥�
Puddephatt and his colleagues haven鈥檛 yet measured other physical properties of the knot, but he says gold-containing compounds often have interesting optical properties, such as reflectiveness, and could find use in optical systems.
Understanding how molecular knots form is also relevant for biological systems, because proteins often form knots in ways we don鈥檛 have a good grasp of, says Puddephatt.
Whether an even smaller knot can be made remains to be seen, but Puddephatt says theoretical calculations suggest that knots can鈥檛 be made with less than about 50 atoms. 鈥淥urs is fairly close to the limit,鈥� he says.
Nature Communications
