Enantiomers of drug compounds can produce different pharmacological effects. For example, in the compound methorphan, the dextromethorphan enantiomer acts as a cough suppressant (antitussive), whereas the levomethorphan enantiomer acts as an analgesic similar in effect to codeine.

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Enantiomers, also called optical isomers, are molecules characterized by chirality, meaning their structural configurations are nonsuperimposable mirror images of each other. They are referred to as optical isomers due to their ability to rotate the plane of polarized light.

Enantiomers

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In 1848, Louis Pasteur discovered the properties of enantiomers while using a microscope to analyze crystallized fermentation products of wine.

Discovery of Enantiomers by Louis Pasteur

Enantiomeric forms, or optical isomers, often exhibit distinct biological properties and activities. For instance, specific species of molds (Rhizopus), yeasts (Yarrowia), and bacteria (Lactobacillus) can only metabolize a single type of optical isomer, finding the opposite form unsuitable as nutrients. Furthermore, in pharmaceutical treatments, certain microorganisms are only susceptible to one specific optical isomer of a drug.

Biological Significance of Enantiomers

Certain enantiomeric forms of amino acids exhibit distinct tastes and smells when consumed. For example, L-aspartame tastes sweet, whereas its mirror image D-aspartame has no taste.

Examples of Taste and Smell Differences in Amino Acid Enantiomers

Pharmacologic Differences in Drug Enantiomers

Chirality is a structural characteristic of molecules in which their structures are nonsuperimposable mirror images of each other.

Chirality

Enantiomers are classified into $$d$$ and $$l$$ forms based on how they rotate polarized light. The $$d$$ form rotates the light clockwise and is designated with a plus ($$+$$) sign, from the Latin *dexter* (right). The $$l$$ form, which is the mirror image, rotates the light counterclockwise and is designated with a minus ($$-$$) sign, from the Latin *laevus* (left).

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