Anomers

Anomers are stereoisomers of cyclic monosaccharides that differ in configuration at the anomeric carbon, the carbon that was the carbonyl carbon in the open-chain form and becomes a stereocenter upon cyclization to a hemiacetal (aldoses) or hemiketal (ketoses). For aldoses the anomeric carbon is C1; for ketoses it is C2.

Two anomers exist for each cyclic sugar: alpha and beta. In common convention for D-sugars, the alpha anomer has the substituent at the anomeric carbon (usually the hydroxyl) oriented opposite to the CH2OH group, while the beta anomer has it on the same side. In Haworth projections of D-glucopyranose, the alpha anomer has the anomeric OH pointing down and the beta anomer has it pointing up.

Anomers interconvert in solution through mutarotation, a process that involves ring opening to reform the carbonyl compound and subsequent reclosure to either configuration. The relative amounts of alpha and beta at equilibrium depend on the sugar and the conditions (pH, solvent, temperature).

The anomeric configuration influences properties such as reactivity at the anomeric carbon, the ability to form glycosidic bonds, and the reducing character of the sugar. When the anomeric carbon participates in a glycosidic linkage (as in disaccharides and polysaccharides), that end may become non-reducing if the ring can no longer reopen at the anomeric center. Examples include alpha- and beta-D-glucopyranose, alpha- and beta-D-galactopyranose, and alpha- and beta-D-fructofuranose.

Anomers are distinct from epimers, which differ at other stereocenters besides the anomeric carbon.