Epimer

In chemistry, epimers are diastereomers that differ in configuration of only one stereogenic center. Diastereomers are a class of stereoisomers that are non-superposable, non-mirror images of one another.

In chemical nomenclature, one of the epimeric pairs is given the prefix epi- for example in quinine and epi-quinine. When the pairs are enantiomers, the prefix becomes ent-.

Examples

The sugars α-glucose and β-glucose are epimers. In α-glucose, the -OH group on the first (anomeric) carbon is in the direction opposite the methylene group on carbon C-6 (in the axial position). In β-glucose, the -OH group is oriented in the same direction as the methylene group (in the equatorial position). These two molecules are both epimers and anomers.

α-D-glucopyranose	β-D-glucopyranose

β-D-glucopyranose and β-D-mannopyranose are epimers because they differ only in the stereochemistry at the C-2 position. The hydroxyl group in β-D-glucopyranose is equatorial (in the "plane" of the ring) while in β-D-mannopyranose the C-2 hydroxyl group is axial (up from the "plane" of the ring). These two molecules are epimers but not anomers.

β-D-glucopyranose	β-D-mannopyranose

Doxorubicin and epirubicin are two closely related drugs and epimers.

Doxorubicin–epirubicin comparison

Other closely related compounds are epi-inositol and inositol and lipoxin and epilipoxin.

Epi-inositol	Inositol	Lipoxin	Epilipoxin

Epimerisation

Epimerisation is a chemical process where an epimer is transformed into its chiral counterpart. It can happen in condensed tannins depolymerisation reactions. Epimerisation can be spontaneous (generally a slow process), or catalyzed by enzymes, e.g. the epimerization between the sugars N-acetylglucosamine and N-acetylmannosamine, which is catalyzed by RENBP.