Epimer


In stereochemistry, an epimer is one of a pair of diastereomers. The two epimers have opposite configuration at only one stereogenic center out of at least two.. All other stereogenic center in the molecules are the same in each. Epimerization is the interconversion of one epimer to the other epimer.
Doxorubicin and epirubicin are two epimers that are used as drugs.
Doxorubicin–epirubicin comparison

Examples

The stereoisomers 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, while in β-D-mannopyranose the C-2 hydroxyl group is axial. These two molecules are epimers, but because they are not mirror images of each other, are not enantiomers. They are also not sugar anomers, since it is not the anomeric carbon involved in the stereochemistry. Similarly, β-D-glucopyranose and β-D-galactopyranose are epimers that differ at the C-4 position, with the former being equatorial and the latter being axial.
β-D-glucopyranose

β-D-mannopyranose

In the case that the difference is the -OH groups on C-1, the anomeric carbon, such as in the case of α-D-glucopyranose and β-D-glucopyranose, the molecules are both epimers and anomers.
α-D-glucopyranose

β-D-glucopyranose

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

Epimerization

Epimerization is a chemical process where an epimer is converted to its chiral counterpart. It can happen in condensed tannins depolymerisation reactions. Epimerisation can be spontaneous, or catalyzed by enzymes, e.g. the epimerization between the sugars N-acetylglucosamine and N-acetylmannosamine, which is catalyzed by renin-binding protein.