Isomer

An isomer is any compound that shares an identical molecular formula with one or more other compounds but differs in how its atoms are connected or arranged in space. Because the arrangement of atoms determines a molecule's three-dimensional shape, isomers can behave almost like entirely different substances even though they contain exactly the same elements in the same proportions.

There are two broad families. Structural isomers differ in which atoms are bonded to which — the connectivity is different. Stereoisomers share the same connectivity but differ in the spatial orientation of their atoms. The most pharmacologically significant stereoisomers are enantiomers: non-superimposable mirror images of each other, like a left and right hand.

How it works · its role

Biological receptors and enzymes are themselves three-dimensional, and they interact with molecules the way a lock interacts with a key — shape matters. Two enantiomers of the same compound can therefore fit a given receptor very differently: one may bind tightly and produce a strong effect, while the other binds weakly, binds to a different receptor entirely, or is rapidly cleared without acting at all.

This sensitivity to shape is called chirality. A molecule with a chirality centre — typically a carbon atom bonded to four distinct groups — exists in two mirror-image forms, conventionally labelled R and S, or sometimes d and l. The body's enzymes often process each form at different rates, which can also affect how quickly a substance is metabolised and for how long it remains active.

Relevance to substances & effects

Isomerism is central to understanding why closely related substances can produce strikingly different effects. Amphetamine exists as two enantiomers: dextroamphetamine (d-amphetamine) acts strongly on dopamine pathways and drives the stimulant and cognitive effects associated with the class; levoamphetamine is less centrally active and is more prominent in peripheral effects such as increased heart rate.

Ketamine similarly has two enantiomers. The S-form binds more potently to the NMDA that underlies ketamine's dissociative and anaesthetic properties; the isolated S-enantiomer (esketamine) is now used clinically at lower doses than the racemic mixture.

Many substituted phenethylamines and tryptamines also exist as positional or structural isomers — compounds where a substituent sits at a different location on the ring. Small shifts in position can move a molecule from psychedelic activity to stimulant activity or render it largely inactive.

When a substance is sold or prescribed as a racemate, it is an equal mixture of both enantiomers. Pharmaceutical development sometimes isolates a single enantiomer to increase potency, reduce unwanted effects, or extend patent protection — a process called chiral switching.