Anomer

An anomer is one of two cyclic sugar stereoisomers that differ only in configuration at the anomeric carbon, formed when a sugar cyclizes.

Anomeric carbon: The carbon derived from a sugar’s carbonyl carbon when the sugar forms a cyclic hemiacetal or hemiketal. Its configuration is the sole stereochemical difference between a sugar’s anomers.
Mutarotation: The change in optical rotation that occurs as a sugar solution reaches an equilibrium mixture of anomers. It is the observable consequence of anomers interconverting through the open-chain form.
Glycosidic bond: A covalent linkage joining a sugar’s anomeric carbon to an oxygen, nitrogen, sulfur, or carbon atom. The configuration fixed at an anomeric carbon shapes the resulting sugar linkage.
Alpha anomer: The anomer of a cyclic sugar whose anomeric substituent has the alpha configuration under the applicable stereochemical convention. It is one of the two configurations that define an anomeric pair.
Cyclic hemiacetal: A ring structure formed when a molecule’s hydroxyl group adds to its aldehyde group. Aldose cyclization creates the anomeric carbon whose configuration defines the anomers.
Open-chain form: The acyclic form of a monosaccharide, containing its aldehyde or ketone carbonyl group. Ring opening passes through this form before the sugar can close as the other anomer.
Reducing sugar: A carbohydrate with a free anomeric center that can access an open-chain carbonyl form and act as a reducing agent. A free anomeric center allows ring opening and interconversion of anomers.
Beta anomer: The anomer of a cyclic sugar whose anomeric substituent has the beta configuration under the applicable stereochemical convention. It is the alternative configuration to the alpha form at the same anomeric carbon.
Cyclic hemiketal: A ring structure formed when a molecule’s hydroxyl group adds to its ketone group. Ketose cyclization likewise creates an anomeric carbon with two possible configurations.
Glycosylation: The formation of a glycosidic bond by transferring a sugar-derived group to a nucleophile. Bond formation at the anomeric center can preserve or alter its configuration.

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