Enantiomers share identical molecular formulae, bond connectivity, and most physical properties , including melting point and solubility , but rotate plane-polarised light in opposite directions. The enantiomer that rotates light clockwise is designated (R)- or (+)-, whilst the one that rotates it anticlockwise is designated (S)- or (−)-. This optical activity is measured using a polarimeter and expressed as specific rotation.
In biological systems, enantiomers frequently behave very differently from one another. Because biological receptors, enzymes, and transport proteins are themselves chiral structures, they often interact with only one enantiomer of a given molecule. The other enantiomer may be pharmacologically inert, may produce a distinct effect, or , in documented historical cases; may carry markedly different toxicological properties. The thalidomide case, widely cited in pharmacology literature, illustrates how a single chiral centre can separate therapeutic activity from teratogenic activity, though the precise in-vivo relationship remains a subject of continued research.
In drug development and regulation, the two enantiomers of a racemic mixture are increasingly evaluated as separate chemical entities. A racemic drug contains a roughly equal mixture (50:50) of both enantiomers; a single-enantiomer product is termed an enantiopure or eutomer-enriched formulation. The TGA, in line with international guidance from the International Council for Harmonisation (ICH), specifically guideline Q6A, requires sponsors to characterise and justify the stereochemical composition of an active pharmaceutical ingredient during the registration process.
Enantiomeric pairs are sometimes confused with diastereomers, which are stereoisomers that are not mirror images and do differ in most physical properties. The distinction matters in analytical chemistry: enantiomers can only be separated using chiral stationary phases, chiral reagents, or other asymmetric methods, whereas diastereomers can be separated by conventional chromatography or crystallisation.
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