Anomer

An anomer is one of the two stereoisomers of a cyclic monosaccharide that differ only at the anomeric carbon. The anomeric carbon is the carbon that was the carbonyl carbon in the open-chain form (C1 in aldoses, C2 in ketoses). When a sugar cyclizes to form a hemiacetal or hemiketal, this carbon becomes a new stereocenter, yielding two possible configurations known as alpha and beta.

In common Haworth projections, the alpha and beta descriptors refer to the orientation of the anomeric substituent relative to the CH2OH group at the far end of the ring. For many aldopyranoses, alpha means the anomeric OH is on the opposite side of the ring from the CH2OH group, while beta means it is on the same side. The exact orientation can vary with ring size (pyranose versus furanose forms).

In solution, alpha and beta anomers readily interconvert through the open-chain form in a process called mutarotation. This equilibration produces a characteristic ratio of anomers that depends on the sugar and conditions; for glucose, the equilibrium mixture is typically about 36% alpha and 64% beta at room temperature.

In glycosides and acetals, the anomeric carbon forms a bond to another group, and such anomeric centers are no longer reducing. Anomerism is a specific type of stereoisomerism distinguished from epimerism, which involves differences at other stereocenters.

Common examples include alpha- and beta-D-glucopyranose and alpha- and beta-D-galactopyranose. Anomerism profoundly influences sugar reactivity, mutarotation, and enzyme recognition.