Epimer

In chemistry, epimers are diastereomers that differ in configuration of only one stereogenic center. Diastereomers are a class of stereoisomers that are non-superimposable, non-mirror images of one another, unlike enantiomers which are non-superimposable mirror images of one another. [March, Jerry and Smith, Michael B.. March's Advanced Organic Chemistry: "Reactions, Mechanisms and Structure". "6th ed." Hoboken, NJ: John Wiley & Sons, Inc., 2007.]

For example, 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 (in the axial position). In β-glucose, the -OH group is oriented in the same direction as the methylene group (in the equatorial position). [ [http://www.biotopics.co.uk/as/glucose2.html Structure of the glucose molecule ] ] These two molecules are both epimers and anomers.

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

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-.

References

External links

* IUPAC Gold Book [http://www.iupac.org/goldbook/E02167.pdf definition]