In order to fully understand what a stereoisomer is, the distinction between chiral and achiral objects must be made. A chiral object cannot be superimposed on its mirror image. An object that can be superimposed on its mirror image is considered achiral. Our hands are an example of chiral objects because they are mirror images of one another and are not superimposable. As a set, the left and right hands are different and asymmetric, causing them to be considered enantiomers. A chiral molecule and its superimposable mirror-image molecule are considered enantiomers. Enantiomers are a type of stereoisomer. A stereoisomer is a molecule with the same molecular formula but different three-dimensional structures. Biomolecules sometimes have a stereoisomer that is more commonly used or found in nature. Naproxen is a medication that shows the possibility of a “good” and “bad” enantiomer. In naproxen, the “good” enantiomer is a pain reliever and the “bad” enantiomer causes liver damage. Therefore the “good” enantiomer is sold alone as a chiral drug. The same can be seen with the stereoisomers of metorphan. Its optically active stereoisomer, dextramethorphan, is a nonadictive opiate, while its mirror-image stereoisomer, levomethorpan, is an addictive opiate. Dextramethorphan is used to prevent coughing.
Thalidomide is a drug that has been around for decades and has somehow been useful in one way or another. This drug was used in 1963 by pregnant women. Thalidomide, being a chiral drug, has two enantiomers. Its first enantiomer was useful for treating morning sickness, while the other was a teratogen. A teratogen is a chemical or factor that causes malformation of an embryo. When women took this drug during the first trimester of their pregnancy, their children were sometimes forced to live with stunted flippers for arms or legs. Luckily women in the U.S. were not affected by this tragedy due to the fact that Dr. Frances Kelsey, of the U.S. Food and...
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