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ScienceWise - Jul/Aug 2006

Chemistry in the Superbowl

Article Illustration
Members of Michael Sherburn’s research group (from left to right) Alistair Longshaw, Laurence Kwan, Gomotsang Bojase-Moleta and Emma Pearson ponder the purple superbowl’s possibilities. (Photo by Tim Wetherell)
Article Illustration
A model of the superbowl holding five guest molecules (coloured gold).

Building container molecules

Building molecules that can carry other molecules inside them has enormous potential in drug delivery and chemical catalysis. Scientists at the Research School of Chemistry have now built molecular host containers that they believe are good enough to be known as superbowls.

Back in 1987, Donald Cram, Jean-Marie Lehn and Charles Pedersen were awarded the Nobel Prize in chemistry for their pioneering chemical syntheses of molecules that mimic important biological processes. Cram’s most significant work involved the development of the first fully encapsulating molecules, spherical molecules which were assembled by uniting two hemispheres. These ‘host’ spheres had the interesting property of being able to trap a small ‘guest’ molecule inside when the two halves came together. Since the sphere-trapped guest molecule is isolated from the surrounding medium, it exhibits different physical and chemical properties from the solid, liquid and gas phases. This observation led Cram to describe the interior of a host molecule as a new phase of matter.

Although brilliant in conception, these early sphere hosts were only able to hold small guest molecules of up to a dozen or so atoms. Further, once they were closed, by joining the two hemispheres, the hosts were difficult to open again so the trapped guest molecule was not available for use in chemical and biological processes. Associate Professor Mick Sherburn’s group at the Research School of Chemistry has spent many years studying container molecules looking for better ways to overcome these twin problems.

Recently, by combining six bowl shaped molecules, rather than just two, Sherburn and his group have been able to create a very large (on the molecular scale) spherical host molecule. The molecule is capable of containing much larger guest molecules, up to 100 atoms in size. The problem with release has been solved by removing one of the six bowls to leave five in the form of a vessel with a hole at one end, rather like a microscopic jam jar, which Sherburn calls a ‘superbowl’.

The stopper is formed by an ingenious set of smaller molecular groups which surround the hole and can, in effect, vary its aperture depending on their size and orientation. But unlike the familiar jam jars of everyday use, these microscopic molecular versions are quite particular about their contents. By slightly varying their internal structure, scientists are able to create sites that are attractive to some molecular groups while being repulsive to others. This opens up many exciting technological possibilities.

The larger internal volume of the ANU superbowl molecules is well suited to the scale of many important medicinal compounds such as the new cancer treatment Paclitaxel (Taxol). One of the problems with Taxol is its low solubility in water which makes delivery within the body problematic. The ANU researchers are hopeful that they will be able to engineer their superbowl so that it will hold a molecule of Taxol on the inside yet itself be highly soluble in water. Once at the required site, the door could be opened possibly using laser light or radio waves, and the Taxol could be released in high doses right where it’s needed.

Superbowls also have potential applications in removing contaminants from the environment such as the PCBs used in computer manufacture. Molecules similar to PCBs show a strong affinity for superbowl’s hydrophobic interior but also can be readily removed from the host. These are the necessary attributes for separation technologies.

A third exciting possibility involves using superbowl host molecules to catalyse chemical reactions between small guest molecules. The ANU group has already shown that up to five different molecules are bound simultaneously in fixed locations inside the superbowl. They are looking at ways to chemically bond these guest molecules together and then release them to form products that would be difficult or impossible to create by other means. In this way, the superbowl acts like a microscopic molecular assembly plant.

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