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 selec ...

, a pair of anomers () is a pair of near-identical

stereoisomers In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms ...

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 ...

or ketone functional group in the sugar's open-chain 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 s ...

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

at the hemiacetal/hemiketal carbon in a cyclic

saccharide In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...

. 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 depen ...

s and specific rotations.

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 aldohexoses 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 configuration Absolute configuration refers to the spatial arrangement of atoms within a chiral molecular entity (or group) and its resultant stereochemical description. Absolute configuration is typically relevant in organic molecules, where carbon is bonde ...

s 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 sugar A reducing sugar is any sugar that is capable of acting as a reducing agent. In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. In such a reacti ...

s, anomerization is referred to as '' mutarotation'' 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 of the mixture changes; this phenomenon is called mutarotation.

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 hemi ...

s in aqueous solution are in equilibrium with their open-chain 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 In organic chemistry, the anomeric effect or Edward-Lemieux effect is a stereoelectronic effect that describes the tendency of heteroatomic substituents adjacent to a heteroatom within a cyclohexane ring to prefer the ''axial'' orientation instea ...

, 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 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 specific rotation, which can be monitored by

polarimetry Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves. Typically polarimetry is done on electromagnetic waves that have traveled through or ...

References

External links