betaanomer

A betaanomer, or beta anomer, is one of the two stereoisomeric forms that can arise at the anomeric carbon of a cyclic hemiacetal or hemiketal formed during the ring-closure of a monosaccharide. The other form is the alpha anomer. The distinction lies in the configuration at the anomeric carbon (C1 in aldoses, C2 in ketoses when in the common ring forms).

In the standard Haworth projection for many D-sugars, the beta anomer has the substituent at the anomeric carbon oriented on the same side as the CH2OH group at the far end of the molecule (typically both up). By contrast, the alpha anomer has the anomeric substituent on the opposite side (down in the common projection). For L-sugars, the orientation reverses. The beta designation applies to the cyclic, not to the open-chain form, and it also applies to glycosides where the anomeric carbon is involved in a glycosidic bond.

Mutarotation describes the interconversion between alpha and beta anomers in solution through opening of the ring and reclosing. This process is acid- or base-catalyzed and leads to an equilibrium mixture whose exact ratio depends on the sugar and conditions. In many D-pyranoses, the beta anomer is thermodynamically more stable and thus more abundant at equilibrium.

Beta anomers are important for the biological recognition and chemical reactivity of carbohydrates, influencing enzyme specificity, polymerization, and the properties of disaccharides and polysaccharides.