Chirality

Chirality describes a property of certain molecules that exist as two mirror-image forms — called enantiomers — that cannot be superimposed on each other, much like a left and right hand. The term comes from the Greek word for hand, and the two forms are often labelled by their shape (R and S) or by the direction they rotate polarised light (d- or + for right-rotating; l- or - for left-rotating).

A molecule is chiral when it contains at least one carbon atom bonded to four different groups. That asymmetry is enough to produce two structurally identical but spatially distinct versions. In chemistry, chirality is routine; in pharmacology, it is often the difference between a drug that works and one that does not.

How it works · its role

Receptors, enzymes, and transport proteins are themselves built from chiral molecules — amino acids — and are therefore asymmetric. Because of this, they interact with the two enantiomers of a chiral drug as if those enantiomers were genuinely different substances.

One form may bind tightly to a receptor and trigger a strong response. Its mirror image may bind weakly, not at all, or even at a different receptor entirely. Metabolism follows the same logic: enzymes in the liver often process one enantiomer faster than the other, which affects how long each form stays active in the body.

This is why drug developers sometimes produce a single-enantiomer (or enantiopure) formulation of an older racemic mixture drug — a racemic mixture contains both mirror images in equal amounts — to concentrate the therapeutic activity and reduce off-target effects.

Relevance to substances & effects

Chirality is directly relevant to many common substance classes. Amphetamine is a clear example: the d- enantiomer (dextroamphetamine) is several times more potent at releasing dopamine and norepinephrine than its l- counterpart, which is why d-amphetamine dominates stimulant therapeutics.

Ketamine offers another well-studied case. Its S-enantiomer (esketamine) is roughly twice as potent as a dissociative anaesthetic as the R-form; each also has a distinct side-effect and sub-anaesthetic profile, which has driven interest in isolating them for different clinical uses.

For opioids, many naturally occurring and synthetic compounds are chiral, and typically only one enantiomer binds meaningfully to opioid receptors. The other form may be pharmacologically inert or act elsewhere — the antitussive dextromethorphan, for example, is the d-isomer of a compound whose l-isomer is an opioid.

Subjective effects therefore depend not only on dose but on which enantiomer — or which ratio of enantiomers — a product contains. Street samples are rarely enantiopure, and the ratio can shift with different synthesis routes.