Anomeric Carbon: The alpha-beta Anomerization

Anomeric carbon is the carbonyl carbon (aldehydic or ketonic functional group) of open chain carbohydrates which becomes the stereogenic center upon cyclization. Anomers are the monosaccharide structures, epimers to each other. In other words, anomeric carbon is the ring carbon derived from the carbonyl group, the one containing two oxygen atoms, directly attached to it.

Anomeric carbon

Anomeric center, The epimeric carbon

The anomeric center is also termed as the stereogenic center in stereoisomers. In carbohydrates, these stereoisomers are created by intermolecular cyclization through acetal or ketal groups. There exist two possibilities of the orientation of the hydroxyl group present on such carbon centers. Based on these possibilities, that specific carbon is called the anomeric carbon, and also the stereogenic center.

During the ring formation of carbohydrates (Aldose and Ketose sugars), at C-1 of aldoses and C-2 of ketoses, an anomeric carbon center is formed. The resultant structures are designated as alpha(α) or beta(β) carbohydrates. These cyclic anomers of sugars are in equilibrium with each other in solution form and spontaneously interconvert by the process of mutarotation.

For example, α-D glucose and β-D glucose are anomers to each other. They are found as an equilibrium mixture upon dissolution.

Anomeric carbon in alpha and beta D glucose

Identification of Alpha and Beta Sugars

Enzymes, due to their specificity, are best suited for the identification of alpha and beta sugars or to differ between between the two. This specificity leads to specific products in certain conditions. For example, glycogen, a polysaccharide of glucose in animals is synthesized from α-D glucopyranose. On the other hand, cellulose, a polysaccharide in plants, is synthesized from β-D glucopyranose.

Anomeric carbon and Reducing sugars

The anomeric carbon, or to be very specific, the aldehydic carbon in aldoses and ketonic carbon in ketoses, has a hydroxyl group, which can be termed as anomeric hydroxyl. If that specific hydroxyl is not attached to any other structure, that sugar is a reducing sugar. Reducing sugars can therefore react with oxidizing agents, like Benedict’s solution. Except for the anomeric hydroxyl, all other such hydroxyls are not involved in determining whether a sugar is reducing one or not.

It can be concluded here that, it is the oxygen atom present on anomeric carbon, which determines whether a sugar structure is a reducing or not. If that oxygen is free, the sugar turns out to be a reducing one, and if it is bound to any further structure, it will be a non-reducing sugar. In other words, the free hydroxyl forms (as in hemiacetals) make reducing sugars, while occupied hydroxyls (as in acetals) make non-reducing sugars.

Read more about Acetals and Hemiacetals here.

  • Examples of reducing sugars are; glucose, fructose, galactose, xylose, arabinose, mannose, maltose, lactose, etc.
  • Examples of non-reducing sugars are; sucrose, trehalose, raffinose, stachyose, verbascose, glycosides, etc.

Anomerization

Anomerization is the process of conversion of one anomer to another. It is a spontaneous process that may sometimes involve acidic or basic catalysts. For reducing sugars, this anomerization is in fact mutarotation. It is a reversible reaction mechanism that typically leads to an equilibrium mixture of two anomeric forms of a carbohydrate, whichever may be the starting one.

The anomerization equilibrium shifts to one side if there is extra stability seen in one of the anomers. For example, due to more stable equatorial positions of hydroxyl groups and less strained chair conformation, β-D glucose is more stable than α-D glucose. This leads the equilibrium mixture of α-D glucose and β-D glucose to be in a ratio of 9:16 with a minute quantity of acyclic aldehyde form, as shown above.

Nomenclature of anomers

Anomers are generally named by prefixes of alpha (α) or beta (β). Alpha indicates the position of hydroxyl group (attached to anomeric carbon) to be in the same or axial direction as that of hydroxyl fo C-4. Some literature suggests that it should be regarded as the same side as CH2OH. On the other hand, the beta position indicates the opposite or equatorial position of anomeric and C-4 hydroxyl groups. It can also be understood as the opposite side to that of CH2OH.

An easy way to remember this nomenclature is that alpha(α) goes for axial, (a→a), while beta(β) goes for equatorial, (b→e).

Anomerization and nomenclature of anomers

Anomeric effect

The anomeric effect or Edward Lemieux effect is the stereoelectronic effect describing the preference of axial orientation on the anomeric carbon, while the equatorial one is supposed to be the preferred one being less hindered.  The magnitude of this anomeric effect is quantitated to be about 1.4 kcal/mol for OCO stabilization systems.

The anomeric effect is synthetically important as it is being an important part of in Koenigs-Knorr glycosidation, the oldest glycosylation reaction.

Properties and Consequences of anomers

Having the same molecular formula, many properties of alpha and beta sugars are the same. For example, alpha and beta glucose have the same solubilities on water, the heat of combustion, and reducing properties. Some different properties also exist such as, they have different specific rotations for plane-polarized light, melting points, etc.

Examples of anomers (Anomeric carbon)

Some examples of anomers are as follows:

  1. α-D glucose and β-D glucose, as pyranose forms i.e. [α-D glucopyranose and β-D glucopyranose]
  2. α-D fructofuranose and β-D fructofuranose
  3. Methyl α-D glucopyranoside and Methyl β-D glucopyranoside

Key Takeaway(s)

  • Anomers are special case epimers with differences in orientation of branched atoms at stereogenic centers.
  • Aldehyde functional group from aldoses and ketone functional group from ketoses, bring this phenomenon of anomers into existence.
  • Anomers are interconvertible i.e. If you dissolve only one of the anomers in water, polarimetry will show the presence of both anomers according to their relative percentage existence.
  • As in anomers of glucose, alpha(α) glucose is less stable than beta(β) glucose, and they exist as 36:64 in a dissolved state.
  • All anomers are epimers while all epimers are not anomers.

Concepts Berg

How to identify an anomeric carbon?

The anomeric carbon can be identified as the carbon center which is an aldehydic or ketonic functional group carbon, in open-chain form.

What are anomeric carbon carbohydrates?

Anomeric carbon carbohydrates are saccharides that have differences in their epimeric forms. These differences must be present on stereogenic carbon for that epimers to be anomers.

What is the importance of anomeric carbon?

Anomeric carbons are important because;

  • They are the ring opening sites.
  • A difference in specific rotation can be observed to enhance the functions of carbohydrates.
  • Reactions such as the Milliard reaction may occur, etc.

What is an example of anomeric effect?

An example of the anomeric effect and its usage is Koenigs-Knorr glycosidation.

What are anomers and anomeric carbon?

Anomers are special epimers with differences in orientation of C-1 species and anomeric carbon is the site, this all occurs.

What are epimers How do you identify them?

Epimers are isomers with multiple stereogenic centers but differ from each other on only one of those. They can be identified as special diastereoisomers (or enantiomers if there is only one chiral center), that are same in all chiral centers, except one.

Why is anomeric carbon different from epimeric carbon?

Among carbohydrates, anomeric carbon is only different from epimeric carbon when the stereogenic center of epimers is not the one with two oxygen atoms directly linked to it.

What are anomers, enantiomers, and epimers?

  • Enantiomers are non-superimposable mirror images of each other.
  • Epimers are special molecules, either enantiomers or diastereoisomers. If the molecule has one chiral center, those epimers are enantiomers, but if there are more chiral centers, they are diastereoisomers.
  • Anomers are special epimers with epimeric carbon to be the same as the functional group carbon in open-chain form.

How do anomers and epimers differ?

In short, anomers are special epimers. Two epimers are anomers if the point of difference among them is aldehydic or ketonic carbon in open-chain form.

Why is glucose called a reducing sugar?

The presence of a free carbonyl group makes glucose a reducing sugar. This reducing power may get eliminated with the free carbonyl group gone.

What is the cause of mutarotation of glucose?

A free hydroxyl group attached to the anomeric carbon is the reason behind the mutarotation capability of glucose.

Why does D glucose show the phenomenon of mutarotation?

D-glucose shows the phenomenon of mutarotation because of the anomeric carbon (C-1).

What is the difference between epimers and enantiomers?

Enantiomers are non-superimposable mirror images of each other, whereas epimers are a special class of diastereoisomers with differences in one of the chiral centers. So, if there is only one chiral center, epimers become enantiomers.

Where is the anomeric carbon on a Fischer projection?

In Fischer or Haworth’s projections of glucose or other aldoses and ketoses, the anomeric carbon is the one in the aldehyde or ketone functional groups.

Reference books

  • The Anomeric effectby Dr. Eusebio Juaristi y Cosio, and Dr. Gabriel Eduardo Cuevas González Bravo (Centro de Investigación Y De Estudios Avanzados del Instituto Politécnico Nacional, Mexico)
  • Biochemistry, by Pamela C. Champe (UMD, New Jersey), Richard A. Harvey (UMD, New Jersey), Denise R. Ferrier (Drexel UC of Medicine, Philadelphia, Pennsylvania)
  • Essentials of Glycobiology, edited by Ajit Varki (University of California, San Diego), Maarten J. Chrispeels (Kansas State University)

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