The smallest known molecular knot is made of just 54 atoms

Imagine a knot so small that it can’t be seen with the naked eye. Then think even smaller.

Chemists have tied together just 54 atoms to form the smallest molecular knot yet. Described January 2 in Nature Communications, the knot is a chain of gold, phosphorus, oxygen and carbon atoms that crosses itself three times, forming a pretzel shape called a trefoil. The previous smallest molecular knot, reported in 2020, contained 69 atoms.

Chemist Richard Puddephatt, working with colleagues at the Chinese Academy of Sciences in Dalian, created the new knot by accident while attempting to build complex structures of interlocked ring molecules, or catenanes. Someday catenanes could be used in molecular machines — essentially, switches and motors at the molecular scale — but for now scientists are still figuring out how they work, which, in this case, resulted in producing something else by mistake.

“It was just serendipity really, one of those lucky moments in research that balances out all the hard knocks that you take,” says Puddephatt, of the University of Western Ontario in London, Canada.

The new trefoil knot is also the tightest of its kind. Researchers calculate a molecular knot’s tightness by dividing the number of atoms in the chain by the number of chain crossings to get what’s called the backbone crossing ratio, or BCR. The smaller the BCR, the tighter the knot. The new knot has a BCR of 18. The previous tightest trefoil knot had a BCR of 23.

Studying small molecular knots could someday lead to new materials (SN: 8/27/18). But for now, the team is still trying to determine why this combination of atoms results in a knot at all.