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In carbohydrate chemistry, a pair of anomers (from Greek ἄνω 'up, above' and μέρος 'part') is a pair of near-identical stereoisomers or diastereomers that differ at only the anomeric carbon, the carbon atom that bears the aldehyde 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 form, since in open-chain form, the anomeric carbon atom is planar and thus achiral. More formally stated, then, an anomer is an epimer at the hemiacetal/hemiketal carbon atom in a cyclic saccharide.^[1] 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 points and specific rotations.

Nomenclature

Every 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.^[2] 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 the 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^[3] 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.^[4]

Anomerization

Anomerization is the process of conversion of one anomer to the other. For reducing sugars, 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 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

Open-chain form as an intermediate product between α and β anomer

Though the cyclic forms of sugars are usually heavily favoured, hemiacetals in aqueous solution are in equilibrium with their open-chain forms. 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). 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. 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 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.

References

^ Francis Carey (2000). Organic Chemistry (4th ed.). McGraw-Hill Higher Education.
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "α (alpha), β (beta)". doi:10.1351/goldbook.A00003
^ "Chemistry - Queen Mary University of London".
^ Nomenclature of Carbohydrates (Recommendations 1996) Archived 2010-10-27 at the Wayback Machine PDF

External links

Media related to Anomer at Wikimedia Commons

[1] Francis Carey (2000). Organic Chemistry (4th ed.). McGraw-Hill Higher Education.

[2] IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "α (alpha), β (beta)". doi:10.1351/goldbook.A00003

[3] "Chemistry - Queen Mary University of London".

[4] Nomenclature of Carbohydrates (Recommendations 1996) Archived 2010-10-27 at the Wayback Machine PDF

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+{{short description|Pair of stereoisomers that differ only at the carbon atom bearing an aldehyde or ketone group}}
 {{Multiple issues|
+{{refimprove|date=August 2024}}
 {{lead rewrite|date=July 2014}}
 {{Technical|section|reason=The first paragraph uses technical terms like "stereoisomer" and "hemiacetal" without explanation.|date=May 2011}}
 }}
-An '''anomer''' is a type of geometric variation found at certain atoms in [[carbohydrate]] molecules. An [[epimer]] is a [[stereoisomer]] that differs in configuration at any single [[stereogenic center]]. An anomer is an epimer at the [[hemiacetal|hemiacetal/hemiketal]] carbon in a cyclic [[saccharide]], an atom called the '''anomeric carbon'''.<ref>{{cite book |author=Francis Carey|title=Organic Chemistry, McGraw-Hill Higher Education press|edition=4th|year=2000}}</ref> The anomeric carbon is the carbon derived from the carbonyl carbon compound  (the [[ketone]] or [[aldehyde]] functional group) of the open-chain form of the carbohydrate molecule. '''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]]s and [[specific rotation]]s.
+In [[carbohydrate chemistry]], a pair of '''anomers''' ({{ety|el|ἄνω|up, above||μέρος|part}}) is a pair of near-identical [[stereoisomers]] or diastereomers that differ at only the '''anomeric carbon''', the carbon atom that bears the [[aldehyde]] 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 compound|cyclic]] form, since in open-chain form, the anomeric carbon atom is planar and thus [[Chirality (chemistry)|achiral]]. More formally stated, then, an anomer is an [[epimer]] at the [[hemiacetal|hemiacetal/hemiketal]] carbon atom in a cyclic [[saccharide]].<ref>{{cite book |author=Francis Carey |title=Organic Chemistry |publisher=McGraw-Hill Higher Education |edition=4th |year=2000}}</ref> '''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]]s and [[specific rotation]]s.
-The word "anomer" is derived from the Greek word ἄνω, meaning "up, above", and the Greek word μέρος (“part”), as in "isomer".
 ==Nomenclature==
-[[File:Alpha glucose views.svg|thumb|upright=2|Different [[Structural formula|projections]] of  α-D-glucopyranose. '''1''' = Fischer projection with C-1 at the top the anomeric centre. C-5 is the anomeric reference atom. '''2''', '''3''' = Haworth projections. '''4''' = Mills projection.]]
+[[File:Alpha glucose views.svg|thumb|upright=2|Different [[Structural formula|projections]] of  α-D-glucopyranose. '''1''' = Fischer projection with C-1 at the top of the anomeric centre. C-5 is the anomeric reference atom. '''2''', '''3''' = Haworth projections. '''4''' = Mills projection.]]
-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 [[Cahn–Ingold–Prelog priority rules|stereodescriptors]].
+Every 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 [[Cahn–Ingold–Prelog priority rules|stereodescriptors]].<ref>{{GoldBookRef|file=A00003|title=α (alpha), β (beta)}}</ref> The anomeric centre in [[Hemiacetal#Cyclic hemiacetals and hemiketals|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 <small>D</small>-fructose). In [[aldohexose]]s the anomeric reference atom is the stereocenter that is farthest from the anomeric carbon in the ring (the configurational atom, defining the sugar as <small>D</small> or <small>L</small>). For example, in α-<small>D</small>-glucopyranose the reference atom is C-5.
-<ref>{{GoldBookRef|file=A00003|title=α (alpha), β (beta)}}</ref>
-The anomeric centre in [[Hemiacetal#Cyclic hemiacetals and hemiketals|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 <small>D</small>-fructose). In [[aldohexose]]s the anomeric reference atom is the stereocenter that is farthest from anomeric carbon in the ring (the configurational atom, defining the sugar as <small>D</small> or <small>L</small>). For example, in α-<small>D</small>-glucopyranose the reference atom is C-5.
-If in the cyclic Fischer projection<ref>{{Cite web | url=http://www.chem.qmul.ac.uk/iupac/2carb/05.html |title = Chemistry - Queen Mary University of London}}</ref> the exocyclic oxygen atom at the anomeric centre is [[Cis–trans isomerism|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.<ref>[http://www.chem.qmul.ac.uk/iupac/2carb/06n07.html Nomenclature of Carbohydrates (Recommendations 1996)]  {{webarchive|url=https://web.archive.org/web/20101027084526/http://www.chem.qmul.ac.uk/iupac/2carb/06n07.html |date=2010-10-27 }}&nbsp; [http://www.iupac.org/publications/pac/1996/pdf/6810x1919.pdf PDF]</ref> Thus, the [[absolute configuration]]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.<ref>{{Cite book | chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK1955/ |isbn = 9780879697709|title = Essentials of Glycobiology|chapter = Structural Basis of Glycan Diversity|publisher = Cold Spring Harbor Laboratory Press|year = 2009}}</ref>
+If in the cyclic Fischer projection<ref>{{Cite web |url=http://www.chem.qmul.ac.uk/iupac/2carb/05.html |title=Chemistry - Queen Mary University of London}}</ref> the exocyclic oxygen atom at the anomeric centre is [[Cis–trans isomerism|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.<ref>[http://www.chem.qmul.ac.uk/iupac/2carb/06n07.html Nomenclature of Carbohydrates (Recommendations 1996)]  {{webarchive|url=https://web.archive.org/web/20101027084526/http://www.chem.qmul.ac.uk/iupac/2carb/06n07.html |date=2010-10-27 }}&nbsp; [http://www.iupac.org/publications/pac/1996/pdf/6810x1919.pdf PDF]</ref>
 ==Anomerization==
-Anomerization is the process of conversion of one anomer to the other. For [[reducing sugar]]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.
+Anomerization is the process of conversion of one anomer to the other. For [[reducing sugar]]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]] of the two anomers is specific for the regarding sugar. For example, regardless of the configuration of the starting <small>D</small>-glucose, a solution will gradually move towards being a mixture of approximately 64% β-<small>D</small>-glucopyranoside and 36% of α-<small>D</small>-glucopyranose. As the ratio changes, the [[optical rotation]] of the mixture changes; this phenomenon is called [[mutarotation]].
@@ Line 26: / Line 24: @@
 [[File:D-galactose.png|thumb|left|upright=0.5|Open-chain form of <small>D</small>-galactose]]
-Though the cyclic forms of sugars are usually heavily favoured, [[hemiacetal]]s in aqueous solution are in equilibrium with their [[open-chain]] forms. 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). 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.  Which side it attacks on determines whether the α- or β-anomer is formed.
+Though the cyclic forms of sugars are usually heavily favoured, [[hemiacetal]]s in aqueous solution are in equilibrium with their [[open-chain]] forms. 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). 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. 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.