![]() ![]() Dipolar repulsion between the anomeric group and other groups on the ring, leading to destabilization of the anomer.įor D-glucopyranoside, the β-anomer is the more stable anomer in water.Hydrogen bonds between the anomeric group and other groups on the ring, leading to stabilization of the anomer.This effect is especially noticeable in pyranoses and other six-membered ring compounds. 1,3-diaxial interactions, which usually destabilize the anomer that has the anomeric group in an axial orientation on the ring.This effect is abolished in polar solvents such as water. 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.The major contributors to the stability of a certain anomer are: Physical properties and stability Īnomers are different in structure, and thus have different stabilizing and destabilizing effects from each other. ![]() 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. Which side it attacks on determines whether the α- or β-anomer is formed.Īnomerization of glycosides typically occurs under acidic conditions. When the hemiacetal group is reformed, the OH group on C-5 may attack either of the two stereochemically distinct sides of the aldehyde group on C-1. In aldohexoses this equilibrium is established as the hemiacetal 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). Though the cyclic forms of sugars are usually heavily favoured, hemiacetals in aqueous solution are in equilibrium with their open-chain forms. As the ratio changes, the optical rotation of the mixture changes this phenomenon is called mutarotation. 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. The ratio of the two anomers is specific for the regarding sugar. This reversible process typically leads to an anomeric mixture in which eventually an equilibrium is reached between the two single anomers. For reducing sugars, anomerization is referred to as mutarotation and occurs readily in solution and is catalyzed by acid and base. Anomerization Īnomerization is the process of conversion of one anomer to the other. 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. If the two oxygens are trans (on different sides) the anomer is β. If in the cyclic Fischer projection 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. For example, in α- D-glucopyranose the reference atom is C-5. 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). 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. 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. 1 = Fischer projection with C-1 at the top the anomeric centre. As is typical for stereoisomeric compounds, different anomers have different physical properties, melting points and specific rotations.ĭifferent projections of α-D-glucopyranose. Anomerization is the process of conversion of one anomer to the other. More formally stated, then, an anomer is an epimer at the hemiacetal/hemiketal carbon in a cyclic saccharide. However, in order for anomers to exist, the sugar must be in its cyclic form, since in open-chain form, the anomeric carbon is planar and thus achiral. In carbohydrate chemistry, a pair of anomers (from Greek ἄνω 'up, above', and μέρος 'part') is a pair of near-identical stereoisomers that differ at only the anomeric carbon, the carbon that bears the aldehyde or ketone functional group in the sugar's open-chain form. ( Learn how and when to remove this template message) ( May 2011) ( Learn how and when to remove this template message) Please help improve it to make it understandable to non-experts, without removing the technical details. This section may be too technical for most readers to understand. ![]()
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