Learn about anomers. Understand the definition of anomers, how they occur, their types, and explore examples of anomers in carbohydrates and see their structures.
Updated: 03/05/2022
What are anomers?
Anomers Definition
Two molecules are isomers of each other when they have the same chemical formula but different structures. How is this possible? They can either have different arrangements of their atoms or have different relative 3D structures. Molecules that have the same chemical formula but differ from each other in the arrangements of their atoms are known as constitutional isomers. An example of a pair of isomers is the molecules butane and isobutane. The chemical formula for both is C4H10, but the arrangement of their atoms is different. We can reflect this difference in their atomic arrangements by writing butane as CH3-CH2-CH2-CH3 and isobutane as HC(CH3)3.
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On the other hand, isomers that have the same chemical formula and arrangements of their atoms and only differ from each other on their relative 3D structures are called stereoisomers. Different relative 3D structures are possible when at least four unique molecular groups (atoms or groups of atoms) are bound to a single atom known as a stereocenter. For most molecules, we can calculate the number of possible stereoisomers by using the formula {eq}X=2^n {/eq}, where x is the number of stereoisomers and n is the number of stereocenters. This means that each stereocenter has two possible 3D arrangements of the groups attached to it. We can also use the number of stereocenters to define the relationships among its stereoisomers. A molecule with one stereocenter will have two stereoisomers that are mirror images of each other; these stereoisomers are known as enantiomers. A molecule with more than one stereocenter will have pairs of stereoisomers that are enantiomers; these molecules will differ in orientation from each other at every stereocenter. Any other pair of stereoisomers that differs in orientation at some (but not all) stereocenters do not mirror each other and are known as epimers or diastereomers. It is especially common to see epimer pairs in biological molecules given their size, complexity, and the ability of carbon to bind to 4 unique molecular groups.
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An anomer is a specific type of epimer that occurs in certain carbohydrate (sugar) molecules. Carbohydrates are cyclical carbon chains where each carbon is bound to a hydroxyl (OH) group. This means that every carbon atom in the carbohydrate ring is a stereocenter, and we typically refer to changes in stereochemistry between carbohydrate stereoisomers by numbering the carbon atoms (C1-C5 for a six-member ring and C1-C4 for a five-member ring). This adds a third criterion when naming a carbohydrate molecule: the size of its ring (glucose has a six-member ring while fructose has a five-member ring), its existence as a monomer or a polymer (glucose is a monomer while sucrose is a dimer), and distinction from other its epimers (glucose and galactose are epimers by differing in orientation at C5). Most carbohydrate epimers will have a different name, as is the case for glucose and galactose. But anomers (which differ in orientation at C1) are instead given alpha and beta forms of the same molecule (much like the R and S designations for enantiomers) instead of being given different names.
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How do Anomers Occur?
Why are anomers treated as being more like each other than non-anomeric epimers are? The answer lies in the biochemical nature of the C1 anomeric carbon. Anomeric carbons are found in reducing sugars, which refer to sugars that can be reduced to form an aldehyde or a ketone. The C1 carbon is an anomeric carbon if it is not attached to another anomeric carbon in another sugar molecule. This means that all monosaccharides are reducing sugars, as are polysaccharides that are connected to each other by one anomeric carbon. Polysaccharides that are connected to each other by all their anomeric carbons (such as sucrose) are not reducing sugars.
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The conversion of stereochemistry at a stereocenter usually requires catalyzation by an enzyme known as an isomerase. But reducing sugars will spontaneously convert between the cyclical carbonyl and the aldehyde/ketone molecule (and back again) in a solution containing acids and bases. This flexibility between these two forms is known as mutarotation. Another way to visualize this conversion is to consider that the anomeric carbon is described as being in a hemiacetal or a hemiketal bond with the oxygen atom in the carbon chain. The prefix 'hemi' means partial, meaning that an anomeric carbon will (as an average of the states it exists in over time) be in a bond that lies somewhere between a single and a double bond with the oxygen atom.
Examples of Anomers
While anomers will regularly convert between each other, their different orientations at either end of the mutarotation are given a different designation as alpha or beta. Some examples of how a sugar molecule will have its anomers named are alpha-D-glucopyranose/ beta-D-glucopyranose and alpha-D-fructofuranose/ beta-D-fructofuranose. Note that the alpha/beta designation refers to anomers, while the D/L designation refers to the stereochemistry at the non-anomeric carbons.
While we consider anomers to be flexibly converting between each other in solution, this does not mean that each anomer is favored equally in a solution. As an example, a sample of D-glucose will eventually resolve into an equilibrium consisting of approximately 64% beta-D-glucopyranoside and 36% alpha-D-glucopyranoside. The relative amounts of each anomer will depend on the biochemistry of the sugar molecule and the environmental conditions.
Types of Anomers
Anomers are given either an alpha or a beta designation depending on the orientation of the anomeric carbon. The alpha designation is given when the hydroxyl group on the anomeric carbon points up in a chair projection (or left in a Fischer projection), and down if the hydroxyl group points down or right.
Lesson Summary
In summary, isomers are molecules that have the same chemical formula but different structures. They are divided into constitutional isomers (which have the same chemical formula but different connections among their atoms) and stereoisomers (which have the same chemical formula and connections among their atoms but have different relative 3D orientations). Stereoisomers have at least one stereocenter, which is an atom that is bound to at least four unique molecular groups (atoms or groups of atoms). Stereoisomers that are different from each other at all of their stereocenters are mirror images of each other and known as enantiomers, while stereoisomers that differ at some (but not all) of their stereocenters are not mirror images of each other and are known as epimers or diastereomers.
Anomers are a specific kind of epimer that only differ in orientation at their anomeric carbons. Anomeric carbons are present in reducing sugars where a carbon atom can convert between a cyclical hydroxyl bond and an aldehyde or a ketone bond. Anomers will regularly convert back and forth between the cyclical carbonyl and the linear acetone/ketone forms in a solution containing acids and bases, and this flexibility is known as mutarotation. Anomers are given an alpha or beta designation depending on the orientation shown in a chair or Fischer projection.
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Frequently Asked Questions
What is an anomer?
An anomer is a specific type of epimer that occurs in carbohydrates (sugars). They only differ in 3D orientation between each other at the anomeric carbon (usually C1 in chair and Fischer projections).
What are the two types of anomers?
The two types of anomers are alpha and beta. These are determined by seeing the orientation of the anomeric carbon.
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BackAnomers Structure & Examples | What are Anomers?
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