anômeros

Anomers are stereoisomers of a cyclic saccharide that differ in configuration at the anomeric carbon. The anomeric carbon is the carbon atom that was the carbonyl carbon in the open-chain form of the sugar. When a sugar cyclizes, this carbon becomes chiral and can exist in two different orientations, denoted as alpha (α) and beta (β).

In the alpha anomer, the hydroxyl group (-OH) attached to the anomeric carbon is positioned below the plane of the ring in a Haworth projection, or is trans to the CH2OH group in a chair conformation of a six-membered ring. In the beta anomer, the hydroxyl group is positioned above the plane of the ring in a Haworth projection, or is cis to the CH2OH group in a chair conformation.

The interconversion between alpha and beta anomers occurs through a process called mutarotation. In aqueous solution, the cyclic form of a monosaccharide can open to its linear form and then re-cyclize. During this opening and closing process, the configuration at the anomeric carbon can change, leading to an equilibrium mixture of the alpha and beta anomers. The relative proportions of the anomers at equilibrium depend on the specific sugar. For example, glucose typically exists as approximately 36% alpha-anomer and 64% beta-anomer at equilibrium.

Anomers are important in the chemistry of carbohydrates, particularly in the formation of glycosidic bonds, which link monosaccharides together to form disaccharides, oligosaccharides, and polysaccharides. The configuration at the anomeric carbon determines the type of glycosidic linkage and influences the properties and biological functions of the resulting carbohydrate.