Epimers and anomers are types of stereoisomers of carbohydrates that differ in the position at a single carbon atom.
Epimers are stereoisomers that differ in the configuration of atoms attached to a chiral carbon. In the examples below, the difference in the position of the hydroxyl (OH) at one chiral carbon creates a pair of epimers. Glucose and mannose are epimers that differ at the C-2 carbon, while glucose and galactose are epimers that differ at the C-4 carbon, as shown below.
When a molecule such as glucose converts to a cyclic form, it generates a new chiral center at C-1. The carbon atom that forms the new chiral center (C-1) is called the anomeric carbon.
Anomers are special cases of epimers that differ in position at the anomeric carbon in particular. For example, α-D-glucose and β-D-glucose below are anomers.
The α form has the anomeric OH group at C-1 on the opposite side of the ring from the CH₂OH group at C-5. The β form has the anomeric OH group on the same side as the CH₂OH.
Practice Questions
Khan Academy
The structure of monosaccharides
Hemiacetal formation of carbohydrates
MCAT Official Prep (AAMC)
Key Points
• Epimers differ in the position of the atoms attached at one chiral carbon.
• Anomers differ in position at the anomeric carbon; they are a special type of epimer.
• The α form of glucose has the anomeric OH group opposite from the CH₂OH group, while the β form has the anomeric OH group on the same side as the CH₂OH group.
Key Terms
Epimers: A type of stereoisomer that differs in configuration at a single stereogenic center (the anomeric carbon).
Anomers: A type of stereoisomer that differs in configuration at the hemiacetal or acetal carbon; they are a specific type of epimer.
Stereoisomers or optical isomers: Molecules that differ three-dimensionally by the placement of substituents around one or more atoms in a molecule.
Chiral carbon (asymmetric carbon):A carbon that is attached to four different types of atoms or groups of atoms.
Anomeric carbon: A carbon derived from the carbonyl carbon (the ketone or aldehyde functional group) of the open-chain form of the carbohydrate molecule.
α form: The configuration of a cyclic monosaccharide where the oxygen attached to the anomeric carbon is on the opposite side of the ring from the CH₂OH group.
β form: The configuration of a cyclic monosaccharide where the oxygen attached to the anomeric carbon is on the same side of the ring as the CH₂OH group.
