ScienceWise - Sep/Oct 2008

Molecules with muscles

Article Illustration
Illustration of the two interlocking cyclodextrin rings that make up the molecular muscle
Article Illustration
Karen Zhang

cyclodextrin molecular muscle offers smart material potential

Karen Zhang came to ANU as an international student from China back in 2004 and is now completing her honours year in the PhB program - a unique degree structure that enables exceptional students to undertake research as part of their undergraduate studies. "As a PhB scholar, I love the opportunity to combine research with my studies. I find it really opens your mind and helps you get ahead" she says.

Karen is working in the Research School of Chemistry's Biochemical Reactions and Molecular Recognition group headed by Professor Chris Easton. Her area of interest is molecular muscles, a term given to a molecule that is able to change its physical dimensions when subjected to an appropriate stimulus. Earlier this year, Professor Easton's group achieved a world first by creating a cyclodextrin-based molecular muscle that can expand and contract when exposed to different wavelengths of light.

Cyclodextrins are complex toroidal molecules that can be created by the appropriate actions of enzymes on starch. One of their key features is that the exterior of the ring is hydrophilic, meaning they are highly soluble in water. The hollow interior is much less hydrophilic than the exterior, making cyclodextrins a good host for other smaller hydrophobic molecules that like to sit inside them.

The molecular muscles use two cyclodextrin rings. Through the centre of each ring passes a stilbene complex with a blocking group sealing the end to hold it in place. "You can imagine two molecular doughnuts each with a tail. Each ones tail passes through the hole of the other molecule forming a two ring complex" Karen explains. "Illumination with ultraviolet light at 350nm causes isomerisation of the stilbene resulting in a contraction of the system. The molecular complex can be re-expanded using light at 254nm."

Although the muscles can be cycled many times cis-stilbenes undergo competing reactions that have been suspected to cause the decomposition of the molecular muscle. Karen's honours project centres on trying to avoid this problem by substituting azobenzene as the photosensitive component.

Part of the drive to develop artificial molecular muscles is the potential to use them in smart materials that can adapt to different environments. One example might be skin tight sports clothing that contracts around the wearer's body when in use then relaxes to ease getting in or out of the suit. Another might be pressure suits for pilots of high performance fighter aircraft, the suit contracting around the lower body to prevent blood draining from the brain in high G manoeuvres.
When asked why she chose to study chemistry Karen says, "Chemistry deals with fundamental particles. OK, maybe not so fundamental as physics, but atoms. And atoms are the building blocks of everything. I'm constantly amazed by the new and creative ways we can assemble atoms into all sorts of cool molecules with interesting and useful properties."

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