Cutting edge

July 16, 1999

Enter the world of the supramolecular chemist, the scientist whose task it is to decipher nature's most complex codes

The public perception of chemists is the crazy bespectacled professors of 1950s sitcoms, but what are those comedians in the white coats really up to these days?

Well, we have worked out how to synthesise useful things such as drugs, plastics, paints and superconductors. Designer chemicals are tailor-made to give the optimum environmental, mechanical or biological properties. And with a little bit of effort and grant money, there are not many molecular structures we cannot build in our modern equivalents of the alchemist's bubbling glassware.

So that is it, right? Put three smart blokes on the production line and we can all go home for tea?

Almost, but not quite. While we can make most molecules, there is still the question of size. Your average garden-variety molecule is about one or two nanometres across. That is a thousandth of a millionth of a metre. But that is OK because we can make a few billion trillion of them to bulk things up a bit. The problem comes in the amount of usefulness you can get out of one of these molecules. Let us compare the best we can do with what nature has been able to achieve. Chemists can make molecules that give attractive colours, are selectively toxic to certain bugs, and we do a heck of a line in disposable carrier bags. Nature, on the other hand, has produced sophisticated molecules that send nerve signals, that harvest, store and consume energy, "calculate" sophisticated equations, reproduce and generally get up to all sorts of things that would impress even the most deep-thinking computer.

The problem we have in duplicating these results is that we cannot make computer chips much smaller (and therefore faster, more sophisticated, better) because the way they are made (lithography) is just not capable of chopping up silicon that finely and evenly. On the other hand, large multifunctional molecules could be made by "engineering up" using chemistry. And while at a basic level this is plausible, ask a molecular scientist to create 10,000 atoms and while they are at it engineer up a reversible and readable "on-off" capability and a sophisticated bit of self-repair functionality and they will die of old age trying. The challenge for chemists today is to bridge the gap between the molecular and engineering disciplines. "To boldly go" as the famous split infinitive says, where only nature has gone before.

This new breed of chemists - supra molecular chemists - do chemistry "beyond the molecule". In fact, a French, Swedish and American trio won the Nobel prize for the topic in 1987. This kind of chemistry is pretty much borrowed from nature. It uses lots of words beginning with "self": self-assembly, self-replication, self-organisation, self-recognition. The whole self thing is all about putting the ball firmly back in the molecules' court.

The theory goes that if you can make a few small ordinary molecules with manageable numbers of atoms and you make them just right, then when you throw all of them together they will all self-assemble to give you the large, functional device you were after. Science fiction? No: that is exactly how DNA zips up into a double helix; it can be done and it is being done. Molecules have very definite modes of interaction with one another - controllable attractive forces that can be manipulated by production of suitable pre-programmed building blocks that know what to do as soon as the start gate lifts. Chemists are just beginning to gain an understanding and control of these phenomena. The challenge has only just begun.

Jonathan Steed, department of chemistry, King's

College, London.

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