Enantiomer

What are enantiomers?

Enantiomers are a pair of molecules that are mirror images of one another but cannot be superimposed, much like left and right hands. This property arises from the presence of a chiral centre—typically a carbon atom bonded to four different substituents. While enantiomers share identical melting points, boiling points, solubility, and reactivity in achiral environments, they can behave very differently in chiral environments, such as biological systems. For example, one enantiomer of a drug may be therapeutically active, while its mirror image could be inactive or even toxic.

Why do enantiomers matter in pharmaceuticals?

In drug development, enantiomers can have vastly different pharmacokinetics, pharmacodynamics, and toxicological profiles. Regulatory agencies such as the MHRA, EMA, and FDA often require rigorous characterisation of enantiomeric purity. Techniques like chiral HPLC, GC-MS, and NMR with chiral shift reagents are used to assess enantiomeric excess (ee). The production of single enantiomers—known as enantioselective synthesis—is critical in ensuring drug safety and efficacy. Regulatory frameworks such as ICH Q6A and ISO 17025 guide the validation of analytical methods for enantiomeric purity.

How are enantiomers separated and characterised?

Common separation methods include chiral chromatography (e.g., chiral HPLC columns), crystallisation with chiral resolving agents, and enzymatic resolution. Characterisation relies on polarimetry (measuring optical rotation), circular dichroism (CD), and chiral NMR. The absolute configuration is assigned using the Cahn-Ingold-Prelog (CIP) rules. Ensuring enantiomeric purity is essential for compliance with pharmacopoeial standards such as the British Pharmacopoeia (BP), European Pharmacopoeia (EP), and USP.

Related concepts

Enantiomeric purity, chiral centre, racemate, optical activity, chiral synthesis, enantioselective catalysis, ICH guidelines, chiral stationary phases.