Anomer

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In sugar chemistry, an anomer is a special type of epimer. It is a stereoisomer (diastereomer, more exactly) of a saccharide (in the cyclic form) that differs only in its configuration at the hemiacetal or hemiketal carbon, also called the anomeric carbon. If the anomeric carbon's hydroxy group is trans to the <math>CH_2OH</math> group, then the sugar is an alpha anomer. If, however, the anomeric carbon's hydroxy group is cis to the <math>CH_2OH</math> group, the sugar is a beta anomer.

For example, α-D-glucopyranose and β-D-glucopyranose, the two cyclic forms of glucose, are anomers.

The term for interconversion between the two anomers is mutarotation.

Image:Glucoseab.png

Anomers exist in dynamic equilibrium with each other. All solutions of the same sugar will have the same ratio of α and β anomers under identical conditions of temperature, solvent, etc.

The two different anomers have different optical rotations. For example, α-D-glucose has an optical rotation of +112 degrees and its anomer, β-D-glucose, has an optical rotation of +19 degrees (http://www.cem.msu.edu/~reusch/VirtualText/carbhyd.htm)