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Enantiomer (or optical isomer) is the name for the two stereoisomers of an asymmetric molecule. This asymmetric molecule has a chiral carbon with four different atoms or group of atoms attached. The molecule has a tetrahedral shape with a bond angle of 109.5 degrees. The two enantiomers are non-superimposable mirror images of each other. Enantiomers are chemically identical but rotate plane-polarised light in different directions. A solution of one enantiomer will rotate plane-polarised light in a clockwise direction whilst a solution of the other enantiomer will rotate plane-polarised light in an anti-clockwise direction. A mixture containing equal amounts of each enantiomer (a racemic mixture) will have no effect on plane-polarised light because the rotations are cancelled out. All amino acids are optically active with the exception of glycine, which has two hydrogen atoms attached to the central alpha carbon. It is also frequently found in carbohydrates. Two enantiomers can be L or D form and in proteins, only the L form can be found in nature[1]

The main difference between two enantiomers is their response to plane-polarised light. However, even though enantiomers are chemically identical, they may show different chemical reactivity.

In the 1960's, Thalidomide was a drug prescribed for morning sickness during pregnancy. However, many of the woman gave birth to children with undeveloped limbs and deformities. Thalidomide is a chiral molecule and can exist as two enantiomers. Effects on these children proved that only one enantiomer was safe and effective and that the other prevented limb development in the baby. Even though the drug affected thousands of babies, it was not banned until 1961, since its release in 1957.


  1. Stryer L., Berg J., Tymoczko J. (2006) Biochemistry, 6th Edition, New York: WH Freeman and Company. Page 304
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