Anomer

In carbohydrate chemistry, an anomer is a special type of epimer. It is a stereoisomer (diastereomer, more exactly) of a cyclic saccharide that differs only in its configuration at the hemiacetal or hemiketal carbon, also called the anomeric carbon.^[1]

Nomenclature

α-D-glucopyranose	β-D-glucopyranose
α-L-arabinopyranose	β-L-arabinopyranose

Anomers are identified as "α" or "β" based on the relation between the stereochemistry of the exocyclic oxygen atom at the anomeric carbon and the oxygen attached to the configurational atom (defining the sugar as D or L), which is often the furthest chiral centre in the ring.^[2] The α anomer is the one in which these two positions have the same configuration; they are opposite in the β anomer.^[3] Thus the structure of α-D-glucopyranose has the same stereochemistry at both C1 and C5 whereas β-D-glucopyranose has opposite stereochemistry at C1 compared to C5. This relationship is strictly based on the absolute stereochemistry, not cis/trans relationships or axial/equatorial positioning in the ring conformation. In α-L-arabinopyranose, the absolute configurations are still the same even though the position of the anomeric oxygen is different than in α-D-glucopyranose.

Anomerization

Anomerization is the process of conversion of a structure from one anomer to the other. For reducing sugars, anomerization occurs readily in solution. This reversible process typically leads to a sample of either single anomer compound becoming an anomeric mixture of both. Eventually an equilibrium is reached with a certain mixture of the two (see mutarotation). For example, regardless of the starting configuration of D-glucose, a solution will gradually move towards being a mixture of approximately 64% β-D-glucopyranoside and 36% of α-D-glucopyranose.

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 optical rotation, which can me monitored by polarimetry.

See also

• stereochemistry
• Monosaccharide nomenclature

External links

• IUPAC rules for carbohydrates nomenclature [www.iupac.org/publications/pac/1996/pdf/6810x1919.pdf]
• http://www.cem.msu.edu/~reusch/VirtualText/carbhyd.htm

References