He Anomeric carbon Is the carbon derived from the carbonyl carbon (ketone or aldehyde functional group) of the open chain form of the carbohydrate molecule. Anomerization is the process of converting one anomer to another.
Sometimes carbohydrates (compounds of carbon and hydrogen) exist in a linear or acyclic form, but given the stability, it is common that they are ciclen forming rings.
Fig. 1. Formation of the anomeric carbon. Image taken from http://www.chem.ucla.edu/~harding/IGOC/A/anomeric_carbon01.png
Less than one percent of each of the monosaccharides (Champe, Harvey, & Ferrier, 2005) with five or more carbons exists in the form of a chain, that is, they are linear or acyclic. Therefore, most are in cyclic form, or also referred to as ring, wherein the functional group either aldehyde or ketone, has reacted with an alcohol of the same sugar.
Usually occurs in sugars, which are simply simple carbon hydrated compounds, where the most common are: glucose, fructose, lactose and galactose.
During cyclization, the carbonyl moiety that formed part of that linear structure is transformed into a new stereocenter. By"stereocenter", carbon is understood to have four different substituents, also called"chiral carbon".
When a stereocenter is formed in the cyclization, two new"diastereoisomers"also occur, which are a class of stereoisomers, which are mirror images of each other, but not superimposable, that is to say, they are not"enantiomers".
The diastereoisomers formed differ in the position of a group, in the new stereocenter formed. The new stereocenter is then called anomeric carbon.
Position of anomeric carbon inside the ring
Once the cycle has been formed, to locate the anomeric carbon (Chang, n.d.), first locate the oxygen inside the ring, and then look at the carbon on both sides of the ring. One will be attached to the CH group 2 OH, and the other carbon that is not bound to that group will be the anomeric carbon.
Thus, in a cyclic carbohydrate, the anomeric carbon will be (UCLA, 2015) which was carbonyl (CH 2 O) in the acyclic or linear form.
In the cyclic form, the anomeric carbon can be found next to the oxygen atom in the pyranose and furanose rings, but at the opposite site of the carbon bearing the linear group CH 2 OR.
Being a stereocenter the anomeric carbon, an important characteristic, is the direction that acquires the group alcohol (-OH) attached to that carbon, since depending on its direction is denominated a or b.
This will be called a when it is down in the equatorial, or axial, form; And b, when it is upward in both the axial and equatorial positions.
Fig. 2. On the right, alcohol group down, is alpha, and the molecule is known as alpha-D-glucopyranose, in the figure on the right, the OH group is up, and is known as beta- D-glucopyranose.
The two sugars shown in Fig. 2 are both glucose, but anomers with respect to each other. The importance of this is that there are enzymes that are able to distinguish between both structures and have preference for one of them. Thus, for example, glycogen is synthesized by alpha-D-glucanopyranose, while cellulose is synthesized by beta-D-glucopyranose.
It is common for both cyclic forms (alpha and beta) to be in equilibrium in aqueous solution, and to convert spontaneously into one another in a process called 'mutarotation'.
Acetal and hemiacetal functional groups
When two monosaccharides are bound, they do so through an acetal group (C-O-C-O). Once this happens, the anomeric carbon is fixed either in the alpha or beta position.
However, when hydrogen remains bound and oxygen is not bound to the COC form, it is called the hemiacetal group because the OH group can close and open to any shape in a mixture of these molecules, so there are some alpha and Other beta.
The formation of rings of the linear sugars which are less stable, is through the union of an aldehyde (-C = OH) and an alcohol group (-OH), giving rise to the hemiacetal bond.
Properties of anomeric carbons
Anomeric carbons possess the property known as anomeric effect, which is the preference of the electronegative element to be in the axial position as opposed to the equatorial.
Anomeric carbon has a property known as the anomeric effect, which is the preference for an electronegative to be in an axial orientation as opposed to the equatorial orientation.
This also happens in carbohydrates, acyclic systems and saturated heterocycles. It is known, therefore, that the electronegative element which is in the anomeric position will be located in the axial location.
The best examples of anomeric carbons are found in monosaccharides such as glucose, in which a rotation occurs. In most cases, the anomeric carbons can be identified as seen above, when a carbon bonded to two single bond carbon atoms is located.
The result also, of this rotation, are the two configurations previously mentioned like alpha and beta. Many carbohydrates are able to change spontaneously between the two configurations by a rotation.
If the oxygen of the anomeric carbon - initially carbonyl group - of a sugar is not bound to any other structure, sugar is a reducing sugar, that is, it can donate electrons to another molecule.
Reducing sugars can react with chemical reagents, giving the so-called Reaction Benedict, which is a test to check the presence of monosaccharides and disaccharide sugars in food.
It can also be used to check the presence of glucose in the urine. If it were given the urine would be called"glucosuria"and may be indicative of diabetes mellitus, although it does not serve to diagnose the disease. This is due to the false positives that can be given by other reducing substances, such as ascorbic acid during the ingestion of vitamin complexes, some drugs, etc.
With regard to reducing sugars, it is also possible to speak of the Maillard Reaction, which is what they give when cooking food, in which they acquire a look, taste and aroma of a certain caramelization, but which may become carcinogenic .
Irrespective of the uses of the reducing sugars, once the reaction is carried out and is capable of reducing other reagents, the anomeric carbon becomes oxidizing. Only the oxygen oxidation state of the anomeric carbon determines whether the sugar is reducing or non-reducing, the other hydroxyl group (OH) of the molecule does not intervene in the process.
References
- Champe, P.C., Harvey, R.A., & Ferrier, D. R. (2005). Illustrated Reviews: Biochemistry (3 to ). Baltimore.
- Chang, S. (n.d.). A Guide to the Anomeric Carbon . Retrieved from web.chem.ucla.edu.
- UCLA. (2015). Anomeric Carbon. Retrieved January 1, 2017, from chem.ucla.edu.
- The Maillard Reaction. (N.d.). Retrieved from chm.bris.ac.uk.