Anomer

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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 structure is analogous to one with the hydroxyl group on the anomeric carbon in the axial position of glucose, then the sugar is an alpha anomer. If, however, that hydroxyl is equatorial, the sugar is a beta anomer. For example, α-D-glucopyranose and β-D-glucopyranose, the two cyclic forms of glucose, are anomers.

The α- and β-anomers of D-glucopyranose.

The anomeric effect helps stabilize the α-anomer. The term for interconversion between the two anomers is mutarotation.

The two different anomers are two distinct chemical structures, and thus have different physical and chemical properties, notably optical rotation. 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.