Molecule ties itself in a complex knot

Chemists synthesize a five-crossing structure

Chemists have tangled themselves a complicated knot: a molecule whose 160 atoms loop over one another like a five-pointed star.

A single chloride ion (green ball) anchors this newly made “pentafoil” chemical knot that also stars iron (purple), oxygen (red), nitrogen (dark blue) and carbon (grey and light blue). Robert W. McGregor

The molecule’s design, called a pentafoil, is the most complex knot synthesized from building blocks other than DNA. Knowing how to make a pentafoil, its discoverers say, could lead to ways to make materials lighter, stronger or more flexible than before.

“By knowing how to design types of knots, hopefully we can optimize these properties,” says David Leigh, a chemist at the University of Edinburgh. He and his colleagues report the new knot in the January issue of Nature Chemistry.

The simplest knot is the “unknot,” a loop that doesn’t cross over itself. The next simplest — a “trefoil,” with three crossing points — was first made out of a molecule in 1989. Leigh’s team decided to take things a step further and aim for the five-crossing pentafoil. 

The scientists took negatively charged chloride ions and added ingredients such as positively charged iron ions and long chains of carbon and other atoms, then chemically programmed the whole thing to assemble itself. Five of the chains looped over one another and hooked up, along with five irons, with each chloride to create the pentafoil.

This molecule turns out to be interesting for more than just its shape, Leigh says. “The pocket where the chloride ion sits turns out to be a perfect fit, and if you remove it, the molecule is desperate to bind chloride back in there,” he says. So the molecule could be used as a sensor to help detect chlorine in its surroundings, he says.

Building bigger knots could also help scientists uncover general rules of knotted molecules. Rubber, for instance, gets much of its stretchiness from knots within its polymer chains.

But given that mathematicians know of more than 6 billion kinds of knots, chemists still have a long way to go.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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