A chiral isomer of a molecule. The connectivity of atoms and bonds is identical between two enantiomers, but they differ in the spatial orientation about a bond or atom (the chiral centre).

An example might clear things up.
If a carbon atom has four different groups attached to it, then there are two different ways in which these groups can be arranged around the carbon atom. Such a carbon atom is known as an asymmetric carbon atom. The two different forms of the molecule are known as enantiomers. They are mirror images of each other but cannot be superimposed on each other. The molecules below are both alanine (2-aminopropanoic acid), yet they possess unique bonding arrangements.

     NH2                    NH2
      |                      |
      |                      |
CH3---C---COOH         CH3---C---H
      |                      |
      |                      |
      H                      COOH

Enantiomers of chiral molecules can have extremely different biological properties. Two enantiomers of the same chemical can have completely unlike or entirely opposite tastes, odors, medicinal properties, or toxicity.

Enantiomers are designated R- or S- depending on whether the 3 functional groups of highest molecular weight go clockwise, R-, or anticlockwise, S-. These designations are contractions of the latin "rectus", right, and "sinister", left.

The amino acid (R)-asparagine tastes sweet while (S)-asparagine is bitter, (R)-carvone smells of spearmint and (S)-carvone provides the odor of caraway, (R)-naproxen is a liver toxin while the (S) form is an widely used anti-inflammatory drug. (R,R)-Chloramphenicol is a useful antibiotic while its enantiomers are harmless to bacteria, (R,R) paclobutrazol is a fungicide while its enantiomer is a plant growth regulator, (R,R,S)-deltamethrin is an insecticide but its enantiomers are inactive.

Biological activity of such molecules is initiated by the binding of the molecule into a highly specific receptor complex. Such receptors are usually chiral, non-racemic molecular complexes such as proteins, nucleic acids or complex carbohydrates that bind to only one of a pair of enantiomers, due to their different three-dimensional shapes. The obvious analogy is to a pair of shoes - your left foot will not fit happily in a shoe shaped for a right foot - if it fits at all it will do so uncomfortably, eliciting a different response to that it would give in the more appropriate left shoe. Most biological receptors for a molecule only have one side shoe, so to speak.

This biological specificity has important consequences for drug design, as quite frequently one enantiomer of a chiral molecule has a desirable property, while the other is highly undesirable, or even dangerous. A controversial example is Thalidomide, originally sold as a racemic mixture of two enantiomers, one of which was later found to cause birth defects. Non-racemic forms of the drug are now being proposed for re-release. This principle has stimulated the development of new techniques to produce only one enantiomer of a pair, known as asymmetric synthesis, as well as the purification of one enantiomer from a racemic solution, a process called resolution.

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