Many synthetic drugs have molecules that come in left- and right-handed forms; sometimes one is beneficial while its mirror image is harmful. Thalidomide, prescribed to treat nausea in pregnant women in the 1960s, is the most notorious example. One form of the molecule is a sedative, the other a teratogenic that causes gross limb deformities in babies. The drug used 30 years ago was a mixture of both. Nature produces molecules exclusively in one chiral form or another; all enzymes, for example, are "left-handed". By comparison, most chemical reactions in the laboratory have produced equal amounts of the left- and right-handed forms - until now, that is. Dr Steve Davies of Oxford University has learned to mimic nature, using tiny pieces of molecular scaffolding called "chiral auxilliaries" to effectively mould molecules in the correct form.

So far-reaching are the commercial possibilities of Dr Davies's research that many of the world's top drug companies are now beating a path to his door."Each one of the body's chemical reactions is controlled by an enzyme," Dr Davies explains. "When you become ill, one or more of those reactions goes wrong and you have to put something into your body to put them right." That something is a drug.The enzymes in the body are made up of simple amino acid molecules. Nature uses only left-handed amino acids when building enzymes, so enzymes too are "left-handed". For a patient to derive maximum benefit from any drug, it is important that the molecules of the drug are compatible with the enzymes, ie, that they too are left-handed.Clearly, we need drugs whose molecules are all of one "handness" - and that is what Dr Davies's discovery has made possible. As an organic chemist, he is chiefly interested in the vast number of molecules containing the element carbon.

In many of these, each carbon atom is chemically bonded to four other groups of atoms in the structure. It is the geometric arrangement of these groups around the central carbon atom that determines a molecule's handedness.The problem with identifying the handedness of molecules is that they have identical chemical properties, and so react in the same way. If they are solids, they will melt at the same temperature; if they are liquids, they will share the same boiling point. In fact, they only differ from one another in one tiny but important respect. As befits compounds made of mirror-image molecules, they interact with light in totally opposite ways.

Left-handed molecules twist plane-polarised light in one direction, while right-handed molecules twist it by the same amount, but in the opposite direction.If identifying the handedness of chiral molecules is difficult, making them is even tougher. "It's as if you're trying to make a hand from a box of thumbs and fingers," says Dr Davies. "You start from the left, putting the little finger on first and finishing with the thumb. An 'up' palm gives you a right hand while a 'down' palm produces a left hand. The trouble is, you don't know whether the palm is up or down. So you end up with a mixture of right and left hands."To manufacture consistently one type of molecule or the other, an extra piece of information is needed - whether the palm is facing upwards or downwards.