Position of binding groups

The molecule may have the correct binding groups, but if they are in the wrong positions, they will not all be able to form bonds at the same time. As a result, bonding would be weak and the molecule would very quickly drift away. Result—no activity. 

Compounds which have non-superimposable mirror images are termed chiral or asymmetric. There are only two detectable differences between the two mirror images (or enantiomers) of a chiral compound. They rotate plane polarized light in opposite directions and they interact differently with other chiral systems such as enzymes. This has very important consequences for the pharmaceutical industry. 

At best, it floats about in the body doing nothing. At worst, it interacts with a totally different receptor and results in an undesired side-effect. Herein lies the explanation for the thalidomide tragedy. One of the enantiomers was an excellent sedative. The other reacted elsewhere in the body as a poison and was teratogenic (induced abnormalities in human embryos). If the two enantiomers had been separated, then the tragedy would not have occurred. 

Even if the wrong enantiomer does not do any harm, it seems to be a great waste of time, money and effort to synthesize drugs which are only 50 per cent efficient. That is why one of the biggest areas of chemical research in recent years has been in the field of asymmetric synthesis—the synthesis in the laboratory of a single enantiomer of a chiral compound. 

Of course, nature has been at it for millions of years. Since nature has chosen to work with only the left-handed enantiomer of amino acids,2 enzymes (made up of 

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