See the ‘periodic table’ of molecular knots

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

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.

Mattia Marenda and C. Micheletti

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.

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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