Anomerisen

Anomerisen, or anomerization, is the interconversion between the alpha and beta forms of the cyclic form of a monosaccharide at the anomeric carbon. In solution, many sugars adopt cyclic structures (pyranose or furanose forms) that can open to the linear carbonyl form and then reclose, yielding either configuration at the anomeric center. This process is commonly observed as mutarotation, the change in optical rotation as an equilibrium mixture of anomers is established.

The anomeric carbon is the carbon bearing the newly formed hydroxyl in the cyclic hemiacetal or hemiketal. For aldoses, this is C1; for ketoses, it is C2. The relative abundance of alpha and beta forms at equilibrium depends on temperature, solvent, and the specific sugar. For example, in aqueous solution at room temperature, D-glucose at equilibrium is typically about 36% alpha and 64% beta, though the ratio shifts with conditions.

Anomerization requires partial ring opening, which is facilitated by acidic or basic catalysis and by higher temperatures. When the anomeric carbon is involved in a glycosidic bond (as in many disaccharides and polysaccharides), interconversion at that center is blocked, preventing mutarotation.

The phenomenon has practical implications for reactivity, enzymatic recognition, and properties such as sweetness and digestibility, as many biological processes are stereospecific for a given anomer.