Chemists learn to knot their molecules.

Chemists learn to knot their molecules

The quest to make molecules shaped like knots gives flight to chemists' imaginations. It also challenges their molecule-building ingenuity, a basic exercise that can later prove useful for more practical pursuits such as drug sesign.

After four years of laboratory effort, chemist Qun Yi Zheng of the University of Colorado in Boulder has come up with a "hook and ladder" approach for making knotted compounds. At last week's meeting of the American Chemical Society in Washington, D.C., he reported that his procedure has yielded a candidate molecular knot. Zheng suspects it may have a clover-like trefoil shape, similar to a molecule synthesized last year by French chemists using a different method, or an even twistier figure-8 shape. He is now performing structural analyses to determine the actual conformation.

The making of a molecular knot involves sequences of reactions in which molecular laces thread through molecular loops and then tie together, sometimes with twists, to form finished knots. The same assemblies can tie together with different twists to yield several related structures. Zheng and his supervisor, David M. Walba, refer to these products as topological isomers.

More familiar to chemists are geometric isomers, in which the same atoms arrange in different spatial patterns, and stereoisomers, in which atoms arrange range in different orientations around an atomic hub.

Zheng begins his synthesis of molecular knots by constructing a "hook," which looks like a ring with two dangling chemical groups. He temporarily "caps" these to protect them during further reactions. Next, he threads a semicircular molecular segment through the hook's opening and joins it with another semicircular segment, which has a pair of alcohol groups at the far end. He then elongates these with flexible molecular segments.

To set the stage for the final tying of the knot, Zheng tips the elongated segments with iodine atoms and replaces the protective caps on the hook's far end with reactive alcohol groups. The final steps involve linking the sets of iodine-and alcohol-tipped ends and breaking a couple of rigid double bonds, which earlier served structure-directing roles and which look like ladder rungs in sketches of the molecules. The four ends link in any of three possible ways that differ in the number of half-twists the ends make before joining. Molecules with no, one and two twists yield a ring, a trefoil knot or a figure-8 knot, respectively.