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||This section may be too technical for most readers to understand. (May 2011)|
In carbohydrate chemistry, an anomer is a special type of epimer. It is one of two stereoisomers of a cyclic saccharide that differs only in its configuration at the hemiacetal or hemiketal carbon, also called the anomeric carbon. Put simply, the anomeric carbon is the carbonyl carbon (a carbon double-bonded to an oxygen), for example a ketone or aldehyde functional group, in a carbohydrate molecule. Anomerization is the process of conversion of one anomer to the other. Anomerization is the anomeric analogue of epimerization.
Two anomers are designated alpha (α) or beta (β), according to the configurational relationship between the anomeric centre and the anomeric reference atom, hence they are relative stereodescriptors.  The anomeric centre in hemiacetals is the anomeric carbon C-1, which is attached to the hemiacetal oxygen (in the ring) and in hemiketals it is carbon C-2, attached to the hemiketal oxygen. In aldohexoses and smaller carbohydrates the anomeric reference atom is the stereocenter that is farthest from anomeric carbon in the ring (the configurational atom, defining the sugar as D or L). In α-D-glucopyranose the reference atom is C-5.
If in the cyclic Fischer projection (see ) the exocyclic oxygen atom at the anomeric centre is cis (on the same side) to the exocyclic oxygen attached to the anomeric reference atom (in the OH group) the anomer is α. If the two oxygens are trans (on different sides) the anomer is β.  For cyclic compounds, however, use of the Fischer projection is complicated and not common.
Originally, the alpha and beta terminology was based on the relative positioning of the major ring constituents. For D-hexoses, viewed in Haworth projection, the C6 carbon is drawn above the ring plane. When the anomeric constituent was above the ring (nominally "the same"), the conformation was called beta. When the anomeric constituent was below the ring (nominally "different") the conformation was alpha. For L-hexoses, with C6 drawn below the ring, these definitions would then, necessarily, be reversed. Use of alpha and beta based on the old definition still finds its way into scientific literature and other references.
Anomerization is the process of conversion of one anomer to the other. For reducing sugars, anomerization occurs readily in solution. This reversible process typically leads to an anomeric mixture in which eventually an equilibrium is reached between the two single anomers. The ratio of the two single anomers is specific for the regarding sugar. For example, regardless of the configuration of the starting D-glucose, a solution will gradually move towards being a mixture of approximately 64% β-D-glucopyranoside and 36% of α-D-glucopyranose. If the ratio changes, also the optical rotation will change; this phenomenon is called mutarotation.
Mechanism of anomerization
Though the cyclic forms of sugars are usually heavily favoured, liquid monosaccharides (monosaccharides in aqueous solution) are always in equilibrium with their open-chain forms. In aldohexoses this equilibrium is established as the hemiacetal bond between C-1 (the only carbon bound to two oxygens) and C-5 is cleaved (forming the open-chain compound) and reformed (forming the cyclic compound). When the hemiacetal bond is reformed, the OH group on C-5 may attack either of the two stereochemically distinct sides of the aldehyde group on C-1. Which side it actually does attack on determines whether the α or β anomer is formed. This process is called mutarotation.
Physical properties and stability
Anomers are different in structure, and thus have different stabilizing and destabilizing effects from each other. The major contributors to the stability of a certain anomer are:
- The anomeric effect, which stabilizes the anomer that has an electron withdrawing group (typically an oxygen or nitrogen atom) in axial orientation on the ring. This effect is abolished in polar solvents such as water.
- 1,3-diaxial interactions, which usually destabilize the anomer that has the anomeric group in an axial orientation on the ring. This effect is especially noticeable in pyranoses and other six-membered ring compounds. This is a major factor in water.
- Hydrogen bonds between the anomeric group and other groups on the ring, leading to stabilization of the anomer.
- Dipolar repulsion between the anomeric group and other groups on the ring, leading to destabilization of the anomer.
For D-glucopyranoside, the β-anomer is the more stable anomer. The main effect in this case is the absence of 1,3-diaxial interactions. For D-mannopyranose, the α-anomer is the more stable anomer because this form avoids dipolar repulsion between the anomeric hydroxyl and the hydroxyl on the next carbon in the ring.
Because anomers are diastereomers of each other, they often differ in physical and chemical properties. One of the most important physical properties that is used to study anomers is the specific rotation, which can be monitored by polarimetry.
- Francis Carey (2000). Organic Chemistry, McGraw-Hill Higher Education press (4th ed.).
- IUPAC Gold Book α (alpha), β (beta)
- Nomenclature of Carbohydrates (Recommendations 1996) PDF
- Floyd H. Dean, Pyranose mutarotation. Journal of Colloid and Interface Science,1967,24(2) Pages 280-281 doi:10.1016/0021-9797(67)90235-4