Chemicals get tied up in complex knots.

Even the most dexterous Scout would have trouble making a knot in an organic molecule. Drawing on years of experience, however, three chemists in France have done just that, synthesizing a chemical knot that's more complex than earlier ones.

They made a composite knot, a combination of two simple knots, each shaped like a trefoil. Knots provide a basis for studying how a molecule's shape and structure influence its physical and chemical properties.

What's more, mathematical models of knots can help scientists understand and describe those properties (SN: 3/21/92, p. 186). For example, both knots and molecules can exhibit chirality, meaning that like a person's left and right hands, they are mirror images of each other. Proving mathematically that a difference exists is not a trivial problem, says Kenneth C. Millett of the University of California, Santa Barbara.

Riccardo F. Carina, Christiane Dietrich-Buchecker, and Jean-Pierre Sauvage of Louis Pasteur University in Strasbourg, France, report their findings in the Sept. 25 Journal of the American Chemical Society. Dietrich-Buchecker and Sauvage first synthesized the simple trefoil knot in 1989 (SN: 9/8/90, p. 149).

The new composite knot required more than just twisting a molecule and securing the ends. The researchers' approach was first to make the two halves and then to connect them. They took simple, linear organic molecules and bent them around two copper atoms--like wrapping a piece of string around each of two fingers--leaving the ends free. They then attached the ends of two twists together, completing the composite knots. The knots came in three different conformations: two chiral knots that are mirror images of each other and one that is achiral, or the same as its reflection. An equilateral triangle, for example, is achiral.

Other scientists have tied different complicated knots using DNA, a much longer molecule than the one used in the current study. However, "to synthesize a specific knot with a small molecular weight is a real achievement," says Millett, who has created models that relate the length of a molecule to the different ways it can be knotted. "Being able to move from a simple knot to a composite is--from a mathematical point of view--a real step."

It's also a significant step from a chemical point of view, he adds. The researchers got a good yield of knotted molecules, despite using a multistep synthesis that loses a little bit of product after each reaction. "They got more than just the mathematical or statistical analysis would have suggested," he says, which implies that some unknown process is boosting the level of output.

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