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See the ‘periodic table’ of molecular knots

Fashioning these structures is a way for chemists to test their mettle

7:00am, August 27, 2018
illustrations of molecular knots

FIT TO BE TIED  Researchers cataloged the possible types of molecular knots that can be made, including the already-synthesized trefoil knot (3-D illustration at left) and 10124 (right), which is yet to be created.

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Like a scouting handbook for the molecular realm, a new chart reveals how to tie molecules up in knots of increasing complexity.

Mathematicians have cataloged billions of distinct knot types, but researchers have been able to make only a few molecular versions. Scientists craft the minuscule knots using a solution filled with building blocks of curved strings of atoms, which glom onto one another.

Now, using computer simulations, physicist Cristian Micheletti of the International School for Advanced Studies in Trieste, Italy, and colleagues have created a “periodic table” of the molecular pretzels. The table reveals which molecular knots are able to be created and arranges them in order of increasing complexity, the researchers report August 3 in Nature Communications.

The team organized the table based on the realization that two characteristics predict how difficult it is to create a molecular knot: the number of molecular building blocks needed to construct each pretzel shape and the number of times each knot’s strands loop around the knot’s center.

Lots of knots

In the “periodic table” below, knots that scientists have already synthesized are in orange and knots yet to be made are in blue. Knots increase in complexity as you go down the table and to the right, offering a road map to creating more knots. Ellipses indicate that the table continues.

Hover over the black circles for 3-D knot renderings and/or images of created knots’ molecular structures.

C. Chang, E. Otwell, T. Tibbitts

Fashioning these knots is a challenge, and provides a way for chemists to strengthen their skills for manipulating molecules. The new table offers those scientists a blueprint to figuring out which molecular knots to make next.

These knots eventually could lead to useful new materials. For example, the tiny knots could serve as nanocages, structures that could store chemicals such as drugs for release when needed.


M. Marenda, E. Orlandini and C. Micheletti. Discovering privileged topologies of molecular knots with self-assembling models. Nature Communications. Published online August 3, 2018. doi:10.1038/s41467-018-05413-z.

D.H. Kim et al. Coordination‐driven self‐assembly of a molecular knot comprising sixteen crossings. Angewandte Chemie International Edition. Vol. 57, March 22, 2018, p. 5,669. doi:10.1002/anie.201800638.

Further Reading

M. Rosen. New molecular knot is most complex yet. Science News. Vol. 191, February 18, 2017, p. 8.

A. Witze. Molecule ties itself in a complex knot. Science News. Vol. 181, January 28, 2012, p. 12.

A. Goho. Chemical Knot: Scientists assemble legendary symbol by interlocking molecules. Science News. Vol. 165, May 29, 2004, p. 342.

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