Just 160 atoms have been made to tie themselves into the smallest version of the pentafoil knot ever made. It’s also the most complicated knot ever achieved by a single molecule.
The knot, also known as the cinquefoil or Solomon’s knot, is a “prime” knot”. Its woven star shape contains five crossing points and cannot be built from smaller knots, similar to the way a prime number cannot be the product of smaller numbers. A version of the pentafoil knot features on the flags of Ethiopia and Morocco, giving it cultural as well as mathematical significance.
Chemists have previously created a prime knot called a trefoil, which has three crossing points. and colleagues at the University of Edinburgh, UK, wove the pentafoil using “needles” made of positively charged iron ions attached to long, skinny organic-molecule “threads”.
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When the researchers added negatively charged chloride ions, these ions became hubs, each attracting exactly five needle-and-thread compounds. In the process of arranging themselves around the central hub, the metal ions folded the organic molecules over one another, braiding them into a woven star shape. Finally, chemical bonds formed that connected the strands at the points of the star, turning the whole arrangement into a single molecule.
Some day, the researchers would like to knit a whole surface of knotted molecules. “Perhaps we could make a chain-mail type of material in which, just like a suit of armour, you’ve got a very strong but very flexible material,” says Leigh.