In the field of stereochemistry, the word Epimer refers to one of two stereoisomers. According to the stereogenic centre, the two isomers present in the molecules differ from one another while the remaining is similar. Some of the forms of glucose epimers include starch, glycogen, glucose, polysaccharides, and oligosaccharides.
The stereoisomers -D-mannopyranose and -D-glucopyranose are known as epimers since their sole difference in stereochemistry is in the C-2 position. The hydroxyl group in the -D-glucopyranose molecule is equatorial (in the “plane” of the ring), but the C-2 hydroxyl group in the -D-mannopyranose molecule is axial (up from the “plane” of the ring). These two molecules are known as epimers, however, they are not enantiomers since they do not mirror reflections of each other.
What are Epimers?
Epimers are a form of stereoisomer that has numerous stereocenters but differs only in the arrangement of one of the stereogenic centres. A stereocenter, also known as a stereogenic centre, is a carbon atom that is bonded to four separate atoms or groups of atoms. Consider the chemical ephedrine, which is used to treat asthma and to treat low blood pressure.
Take note of how the carbon atom is identified, one is connected to hydrogen, an alcohol group, a benzene ring, and another carbon atom. Likewise, the carbon atom labelled two is connected to hydrogen, nitrogen, a methyl group, and another carbon atom. Because they have four distinct groups linked to them, the atoms labelled one and two are both stereocenters.
Example
D-glucose and D-galactose are the best examples. Because both monosaccharides are D-sugars, the -OH group on carbon-5 of both hexoses is on the right in Fischer Projection. The sole difference between D-glucose and D-galactose is the carbon-4 position. In Fischer Projection, the -OH group for D-glucose is on the right, while the -OH group for D-galactose is on the left. This single difference results in the formation of D-glucose and D-galactose epimers. They are simply epimers, not enantiomers, diastereomers, or isomers.