Drug Enantiomers and Chirality

Introduction & definitions

• Chirality — (Greek cheir, "hand") the property of a molecule of being non-superimposable on its own mirror image — exactly as a left hand is not superimposable on a right hand. Chirality is the most pharmacologically important form of stereoisomerism.
• Structural origin — an asymmetric (chiral) centre — most commonly a carbon atom bearing four different substituents, around which the groups can adopt two distinct spatial arrangements.
• Enantiomers — the two stereoisomers of a chiral molecule that are non-superimposable mirror images; they have identical connectivity (same bonds & groups) and differ only in 3-D arrangement about the chiral centre.
• Clinical scale — chirality is extremely common — more than half of all useful drugs are chiral molecules, because the receptors, enzymes & transporters drugs act on are themselves built from chiral building blocks (L-amino acids, D-sugars) and present an asymmetric environment.
• Why it matters — chirality is one of the physicochemical properties (with size, shape, charge, configuration) that determine selectivity of drug–target binding — making it a cross-cutting topic spanning both pharmacodynamics and pharmacokinetics.