Enantiomers ( also called optical isomers or chirality) are isomers that are mirror images of each other and that differ in shape due to the presence of an asymmetric (chiral) carbon, one that is attached to four different atoms or groups of atoms. The four groups can be arranged in space around the asymmetric carbon in two different ways that are mirror images. This structural property is termed as enantiomerism and the presence of multiple chiral features in a given compound increases the number of geometric forms possible, though there may be some perfect-mirror-image pairs.
Enantiomers are, in a way, left-handed and right-handed versions of the molecule – (levo and dextro). Just as your right hand won’t fit into a left-handed glove, a “right-handed” molecule won’t fit into the same space as the “left-handed” version. Compounds that are enantiomers of each other have the same physical properties except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds. Usually, only one isomer is biologically active because only that form can bind to specific molecules in an organism.
The amino acid alanine is example of an entantiomer. The two structures, D-alanine and L-alanine, are non-superimposable. In nature, only the L-forms of amino acids are used to make proteins. Some D forms of amino acids are seen in the cell walls of bacteria, but never in their proteins. Similarly, the D-form of glucose is the main product of photosynthesis and the L-form of the molecule is rarely seen in nature.
The concept of enantiomers is important in the pharmaceutical industry because the two enantiomers of a drug may not be equally effective, as is the case for both ibuprofen and the asthma medication albuterol. Methamphetamine also occurs in two enantiomers that have very different effects. One enantiomer is the highly addictive stimulant drug known as “crank,” sold illegally in the street drug trade. The other has a much weaker effect and is even found as an ingredient in an over-the-counter vapor inhaler for treatment of nasal congestion. The differing effects of enantiomers in the body demonstrate that organisms are sensitive to even the most subtle variations in molecular architecture.