Stereochemistry

Stereochemistry describes the three-dimensional arrangement of atoms in a molecule. Two compounds can share identical atoms and bonds — the same molecular formula and connectivity — yet differ only in how those atoms are oriented in space. That spatial difference alone is enough to change how the molecule behaves at a receptor.

The most pharmacologically important case is chirality: a molecule has a chirality center when one atom (usually carbon) is bonded to four different groups, making it non-superimposable on its mirror image. The two mirror-image forms are called enantiomers. Like left and right hands, they are related but not identical — and a receptor, which is itself built from chiral protein chains, can tell the difference.

Many substances exist as an equal mixture of both enantiomers, called a racemate or racemic mixture. When a record does not specify configuration, it may reflect the racemic mixture, or simply that the configuration was not documented. Either way, the pharmacology is incompletely described, because the two forms can act quite differently.

How it works · its role

Receptors and enzymes are built from amino acids, which are themselves chiral. A receptor's binding site has a specific three-dimensional shape, so it responds differently to each enantiomer — one may fit tightly and produce a strong effect; the other may fit poorly, produce a weak effect, or engage a different receptor entirely.

Enantiomers are labelled by several conventions: R/S (based on a priority ranking of the groups around the chiral center), d/l or +/− (based on how the compound rotates polarised light), and prefixes like dextro- and levo-. These systems do not map consistently onto each other — the d form is not always the R form — so reading them requires knowing which convention a source is using.

Relevance to substances & effects

Stereochemistry runs through many substance classes covered in these pages. Amphetamine is a clear example: the dextro form (d-amphetamine) crosses the blood-brain barrier readily and drives the well-known stimulant effect; the levo form has far weaker CNS activity. Pharmaceutical formulations exist as the racemate, as d-amphetamine alone, or as defined ratios — and those formulations are not equivalent in effect.

Ketamine is dispensed clinically as a racemate. S-ketamine (esketamine) has higher affinity for the NMDA and is available as a separate preparation; the two enantiomers differ in potency, duration, and subjective character. Citalopram and escitalopram follow the same pattern: escitalopram is the active enantiomer isolated from the racemate, developed because the other form contributes adverse effects without adding therapeutic benefit.

For substances on this site without a specified configuration — particularly research chemicals where analytical data is limited — the entry reflects what is known, which may be incomplete. Effects attributed to a substance may in practice belong predominantly to one enantiomer.