Tightest knotted structure

The tightest knotted structure comprises a loop of just 54 atoms, bonded together in a chain that is entwined in a trefoil knot. The Au6 knot, which was reported in Nature Communications on 2 January 2024, was created by Zhiwen Li, Jingjing Zhang and Gao Li (all China) at the Dalian Institute of Chemical Physics in China, working with remote collaborator Richard J Puddephatt (Canada) at the University of Western Ontario.

In the terminology of knot theory (which is a discipline within mathematics), the formation of these atoms is called a trefoil knot. This is the simplest "non-trivial" knot, with three points at which the chain crosses over itself. It is equivalent to a conventional overhand knot with the two ends of the string bonded together.

The tightest naturally occurring molecular knots have a backbone-to-crossing ratio (BCR) of around 30:1, and the tightest knot previously manufactured under lab conditions had a BCR of 23 (made by a team from Fudan University, China, in June 2020).

The Au6 knot combines three gold acetylide molecules with a structure called a diphosphine ligand, creating a backbone of gold, phosphorus, hydrogen, oxygen and carbon atoms.

What makes the new holder unique is that it appears to be self-assembling. Previous holders of the title were all created through complex procedures that used charged ions to entwine chains of atoms. The Au6 knot, on the other hand, formed spontaneously and unexpectedly during lab experiments. Shortly after the paper's publication, chemist Richard J Puddephatt told New Scientist: "It's quite a complicated system and, honestly, we don't know how it happens."