Enantiomer

An enantiomer is one of a pair of molecules that are exact mirror images of each other but cannot be overlaid — much like a left hand and a right hand. The two forms share the same atoms and bonds; they differ only in the three-dimensional orientation of groups around one or more carbon atoms, a property called chirality.

Because biological receptors and enzymes are themselves three-dimensional structures, they often interact with one mirror-image form very differently from the other. A molecule that is pharmacologically potent as one enantiomer may be weak, inert, or even produce opposing effects as the other.

How it works · its role

Chirality arises at a stereogenic centre — usually a carbon atom bonded to four different chemical groups. The two resulting mirror-image forms are labelled by convention: R and S (from the Latin rectus and sinister), or sometimes as d/l (dextro/levo) or (+)/-(-), referring to the direction in which each form rotates polarised light.

When a drug binds to a receptor, it slots into a pocket shaped by the receptor's own folded protein. If the drug's spatial geometry fits that pocket well, it binds tightly and triggers a strong response. The mirror-image version may fit poorly, bind weakly, or even block the site without activating it — acting as an antagonist rather than an agonist.

Many pharmaceutical syntheses produce both enantiomers in equal amounts (a racemate). Separating them — a process called chiral resolution — can yield a cleaner pharmacological profile.

Relevance to substances & effects

Enantiomer differences are clinically significant across several substance classes. Ketamine is a racemic mixture; its S-enantiomer (esketamine) binds the NMDA with roughly twice the potency of the R-form and has been developed as a standalone antidepressant under a different trade name, while R-ketamine is being investigated for potentially longer-lasting effects through a different mechanism.

In stimulants, amphetamine is sold as both the racemate and as the isolated d-enantiomer (dextroamphetamine). The d-form crosses into the central nervous system more readily and produces stronger stimulant effects; the l-form acts more on peripheral sympathetic pathways.

With antidepressants, citalopram is a racemate in which the S-enantiomer carries nearly all the serotonin-reuptake inhibitor; the isolated S-form is marketed separately as escitalopram at roughly half the dose.

For MDMA, the two enantiomers differ in their ratio of serotonin to dopamine and norepinephrine release, which is thought to shape the balance between empathogenic and stimulant qualities, though both are pharmacologically active.

This pattern — where one mirror image dominates the desired effect and the other contributes side effects or a different pharmacology — is a recurring reason why a purified single enantiomer sometimes replaces an older racemic drug in clinical use.