carbohydrate chemistry Carbohydrate chemistry is a subdiscipline of chemistry primarily concerned with the detection, synthesis, structure, and function of carbohydrates. Due to the general structure of carbohydrates, their synthesis is often preoccupied with the select ...

, a pair of anomers () is a pair of near-identical stereoisomers that differ at only the anomeric carbon, the carbon that bears the

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...

functional group in the sugar's

open-chain In chemistry, an open-chain compound (also spelled as open chain compound) or acyclic compound (Greek prefix "α", ''without'' and "κύκλος", ''cycle'') is a compound with a linear structure, rather than a cyclic one. An open-chain compound h ...

form. However, in order for anomers to exist, the sugar must be in its

cyclic Cycle, cycles, or cyclic may refer to: Anthropology and social sciences * Cyclic history, a theory of history * Cyclical theory, a theory of American political history associated with Arthur Schlesinger, Sr. * Social cycle, various cycles in soc ...

form, since in open-chain form, the anomeric carbon is planar and thus

achiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from i ...

. More formally stated, then, an anomer is an

epimer In stereochemistry, an epimer is one of a pair of diastereomers. The two epimers have opposite configuration at only one stereogenic center out of at least two. All other stereogenic centers in the molecules are the same in each. Epimerization is ...

at the hemiacetal/hemiketal carbon in a cyclic saccharide. Anomerization is the process of conversion of one anomer to the other. As is typical for stereoisomeric compounds, different anomers have different physical properties,

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...

s and

specific rotation In chemistry, specific rotation ( �'') is a property of a chiral chemical compound. It is defined as the change in orientation of monochromatic plane-polarized light, per unit distance–concentration product, as the light passes through a sampl ...

Nomenclature

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; in hemiketals, it is the carbon derived from the carbonyl of the ketone (e.g. C-2 in D-fructose). In

aldohexose In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms. The chemical formula for all hexoses is C6H12O6, and their molecular weight is 180.156 g/mol. Hexoses exist in two forms, open-chain or cyclic, that easily convert ...

s 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). For example, in α-D-glucopyranose the reference atom is C-5. If in the cyclic Fischer projection the exocyclic oxygen atom at the anomeric centre is

cis Cis or cis- may refer to: Places * Cis, Trentino, in Italy * In Poland: ** Cis, Świętokrzyskie Voivodeship, south-central ** Cis, Warmian-Masurian Voivodeship, north Math, science and biology * cis (mathematics) (cis(''θ'')), a trigonome ...

(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. Thus, the absolute configurations of the anomeric carbon and the reference atom are the same (both ''R'' or both ''S'') in the α anomer and opposite (one ''R'' and the other ''S'') in the β anomer.

Anomerization

Anomerization is the process of conversion of one anomer to the other. For reducing sugars, anomerization is referred to as ''

mutarotation Mutarotation is the change in the ''optical rotation'' because of the change in the equilibrium between two anomers, when the corresponding stereocenters interconvert. Cyclic sugars show mutarotation as α and β anomeric forms interconvert. The op ...

'' and occurs readily in solution and is catalyzed by acid and base. This reversible process typically leads to an anomeric mixture in which eventually an equilibrium is reached between the two single anomers. The

ratio In mathematics, a ratio shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...

of the two 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. As the ratio changes, the

optical rotation Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...

of the mixture changes; this phenomenon is called

Mechanism of anomerization

Though the cyclic forms of sugars are usually heavily favoured,

hemiacetal A hemiacetal or a hemiketal has the general formula R1R2C(OH)OR, where R1 or R2 is hydrogen or an organic substituent. They generally result from the addition of an alcohol to an aldehyde or a ketone, although the latter are sometimes called hemike ...

s in aqueous solution are in equilibrium with their

forms. In aldohexoses this equilibrium is established as the

bond between C-1 (the carbon bound to two oxygens) and C-5 oxygen is cleaved (forming the open-chain compound) and reformed (forming the cyclic compound). When the

group is reformed, the OH group on C-5 may attack either of the two stereochemically distinct sides of the

group on C-1. Which side it attacks on determines whether the α- or β-anomer is formed. Anomerization of glycosides typically occurs under acidic conditions. Typically, anomerization occurs through protonation of the exocyclic acetal oxygen, ionization to form an oxocarbenium ion with release of an alcohol, and nucleophilic attack by an alcohol on the reverse face of the oxocarbenium ion, followed by deprotonation.

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 Axial may refer to: * one of the anatomical directions describing relationships in an animal body * In geometry: :* a geometric term of location :* an axis of rotation * In chemistry, referring to an axial bond * a type of modal frame, in music * ...

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

pyranose Pyranose is a collective term for saccharides that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom. There may be other carbons external to the ring. The name derives from its similarity ...

s and other six-membered ring compounds. This is a major factor in water. *

Hydrogen bonds In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

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 in water. For D-mannopyranose, the α-anomer is the more stable anomer. 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

, which can be monitored by polarimetry.

References

External links