What is "enantiomer"

Enantiomer \E*nan"ti*o*mer\, n. [Gr. 'enanti`os opposite + -mer.] (Chem.) either of a pair of optical isomers of a chemical substance whose molecules are mirror images of each other, i. e., are related to each other as a right-handed to a left-handed glove; -- meaningful only for structures which have internal asymmetry. -- E*nan`ti*o*mer"ic, a.

n. (context chemistry English) One of a pair of stereoisomers that is the mirror image of the other, but may not be superimposed on this other stereoisomer. Almost always, a pair of enantiomers contain at least one chiral center, and a sample of either enantiomer will be optically active.

n. either one of a pair of compounds (crystals or molecules) that are mirror images on each other but are not identical [syn: enantiomorph]

In chemistry, an enantiomer ( ; ), also known as an optical isomer, is one of two stereoisomers that are mirror images of each other that are non-superposable (not identical), much as one's left and right hands are the same except for being reversed along one axis (the hands cannot be made to appear identical simply by reorientation). Organic compounds that contain a chiral carbon usually have two non-superposable structures. These two structures are mirror images of each other and are, thus, commonly called enantiomorphs (enantio = opposite ; morph = form), hence this structural property is now commonly referred to as enantiomerism.

Enantiopure compounds refer to samples having, within the limits of detection, molecules of only one chirality.

When present in a symmetric environment, enantiomers have identical chemical and physical properties except for their ability to rotate plane- polarized light (+/−) by equal amounts but in opposite directions (although the polarized light can be considered an asymmetric medium). They are sometimes called optical isomers for this reason. A mixture of equal parts of an optically active isomer and its enantiomer is termed racemic and has zero net rotation of plane-polarized light because the positive rotation of each (+) form is exactly counteracted by the negative rotation of a (−) one.

Enantiomer members often have different chemical reactions with other enantiomer substances. Since many biological molecules are enantiomers, there is sometimes a marked difference in the effects of two enantiomers on biological organisms. In drugs, for example, often only one of a drug's enantiomers is responsible for the desired physiologic effects, while the other enantiomer is less active, inactive, or sometimes even productive of adverse effects. Owing to this discovery, drugs composed of only one enantiomer ("enantiopure") can be developed to enhance the pharmacological efficacy and sometimes eliminate some side effects. An example is eszopiclone (Lunesta), which is enantiopure and therefore administered in doses that are exactly 1/2 of the older, racemic mixture called zopiclone. In the case of eszopiclone, the S enantiomer is responsible for all the desired effects, while the other enantiomer seems to be inactive. A dose of 2 mg of zopiclone must be administered to produce the same therapeutic effect as 1 mg of eszopiclone, and that appears to be the only difference between the two drugs.

In chemical synthesis of enantiomeric substances, non-enantiomeric precursors inevitably produce racemic mixtures. In the absence of an effective enantiomeric environment ( precursor, chiral catalyst, or kinetic resolution), separation of a racemic mixture into its enantiomeric components is impossible.