Although converting a molecule from one enantiomer to the other seems like only a small change in the structure, it can provide a significant impact on the way the molecule interacts with its surroundings, and especially other chiral compounds. Many of the molecules that are important in nature are chiral, these include proteins (and their constituent amino acids), which control most processes within biological systems, and the nucleic acids DNA and RNA which are responsible for holding the information necessary for proteins to be synthesised.
For this reason, if a chiral compound interacts with a protein to induce a specific response in a biological organism, it is likely that its enantiomer will either not interact or produce a completely different response. Some of these differences can be quite startling, for example limonene contains a chiral carbon atom. One enantiomer produces the smell of oranges whereas the other gives rise to the smell of lemons.
Understanding chirality is extremely important in the preparation of therapeutic drugs. For example, one enantiomer of penicillamine is a potent anti-arthritic agent whereas the other enantiomer is highly toxic. Perhaps the most startling example of the difference in activity between enantiomers is Thalidomide. This drug was seen as a panacea for the treatment of morning sickness in pregnant women, and indeed one enantiomer reliably has this effect. The other enantiomer, unfortunately, has been associated with the well-characterised birth defects that arose from use of Thalidomide.
One further difference between enantiomers is the way that they interact with light. Light is an electromagnetic radiation. This means that it consists of electronic and magnetic components. The electronic components of light interact with electrons, such as bonds, within a molecule. Changing the arrangement of the bonds changes the way that light interacts with the molecule. This difference in chiral molecules only becomes apparent when polarised light is shone through a solution of the molecule. In polarised light, all the electronic components are aligned. As the polarised light passes through the solution of a chiral compound the polarised light is twisted, with the plane of polarisation being rotated.
Two enantiomers rotate the plane of polarised light by equal amounts but in opposite directions. For this reason, stereoisomerism is also sometimes referred to as optical isomerism.