Stereoisomer

Two broad classes divide the stereoisomers. Enantiomers are non-superimposable mirror images, arising most commonly from a carbon bearing four different substituents; they share every scalar physical property and are distinguished only by their opposite rotation of plane-polarised light and by their behaviour toward other chiral species. Diastereomers are stereoisomers that are not mirror images, a category that includes cis-trans isomers about a double bond or ring and compounds with more than one stereocentre; unlike enantiomers they differ in melting point, solubility and spectra, and can therefore be separated by ordinary means. The distinction has direct pharmacological weight, since receptors are themselves chiral and frequently bind one enantiomer of a drug while ignoring or antagonising the other.