Anomerer

Anomerer are the stereoisomers that differ in configuration at the anomeric carbon of a cyclic carbohydrate. The anomeric carbon is created when a carbonyl group in the open-chain sugar reacts with a hydroxyl group to form a hemiacetal or hemiketal, producing a new stereocenter at that carbon. The two possible configurations are designated alpha (α) and beta (β).

In common representations, such as Haworth projections, the α and β forms differ in the orientation of the substituent at the anomeric carbon relative to the ring. For many D-sugars, the α anomer has the anomeric hydroxyl down (opposite side to the CH2OH group in the standard convention), while the β anomer has it up (same side as CH2OH). In solution, the two anomerer can interconvert through mutarotation, a process accelerated by acid or base, leading to an equilibrium mixture whose composition depends on the sugar and solvent.

Glycosidic chemistry involves linking anomerer through the anomeric carbon to form glycosides. When the anomeric carbon participates in a glycosidic bond, that center is no longer free, and the sugar typically loses reducing power; such sugars are termed non-reducing. The type of linkage—α or β—affects digestibility, enzyme recognition, and physical properties. Examples include maltose (α(1→4) glucose) and cellobiose (β(1→4) glucose); sucrose contains glycosidic bonds at both anomeric carbons, making it non-reducing.

The term anomerer is used in some languages as the plural of anomer; in English, the common plural is anomers. The underlying concept remains the same: two stereoisomers differing at the anomeric carbon of a cyclic sugar.