stereochem+in+biology

Stereochemistry, namely chirality, plays an important role for chemicals involved in biological processes.



For example, all of the **essential amino acids** used and synthesized by higher-order living things are only the L-stereoisomer (enantiomer). The D-stereoisomer amino acids are either not used or converted back to the L-isomer by enzymes. The exception to this is glycine, the simplest amino acid.



Some **vitamins** are also biologically stereospecific. L-ascorbic acid (vitamin C) is an important vitamin, but the D-isomer is useless.



Vitamin E has eight different stereoisomers; only one specific isomer is found in human blood plasma. Synthetic vitamin E, found in many over-the-counter supplements, contains all eight stereoisomers, half of which have no biological activity and are useless.



the terpenoid carvone has two enantiomers. The R isomer has a spearmint odor, while the S isomer has a spicy caraway odor. This implies that olfactory receptors (and likely flavor receptors) are stereospecific.



The R enantiomer of limonene smells like oranges. The S enantiomer smells like lemons.



In pharmacology, stereoisomers play a huge role. Thalidomide, a sedative, was banned by the FDA in 1961 because one enantiomer was found to cause birth defects, while the other was safe.

The NSAID ibuprofen contains a chiral center. Synthetic ibuprofen is therefore a mixture of (R) and (S) enantiomers of which only the (S) isomer is biologically active. However, recent research has shown that an enzyme called an isomerase is able to convert the inactive (R)-ibuprofen into the active (S) enantiomer.



For a very interesting look at the stereochemical world, enter into Mirror Image Land with Alice.