The key difference between alpha and beta anomers is that in alpha anomer the hydroxyl group at the anomeric carbon is cis to the exocyclic oxygen at the anomeric centre, whereas in beta anomer the hydroxyl group is trans to the exocyclic oxygen.
An anomer is a geometric variation among carbohydrates. It is a type of epimer in which we can see an anomeric carbon atom. An anomeric carbon atom is derived from the carbonyl carbon of the open-chain structure of the carbohydrate. Therefore, the formation of the cyclic structure from the open-chain structure is called anomerization.
CONTENTS
1. Overview and Key Difference
2. What are Alpha Anomers
3. What are Beta Anomers
4. Side by Side Comparison – Alpha vs Beta Anomers in Tabular Form
5. Summary
What are Alpha Anomers?
Alpha anomer is the configuration of a carbohydrate in which the hydroxyl group is cis to the exocyclic oxygen at the anomeric centre. That means, the hydroxyl group and the exocyclic oxygen atom are on the same side of the molecular projection. When we draw a Haworth formula, the hydroxyl group is in the downward direction if it is the alpha anomer. The following example shows the alpha anomer of D-glucopyranose.
Figure 01: The hydroxyl group in the downward direction is shown in green colour in the above image. The downward direction resembles the alpha anomer.
Since anomers are different in chemical structures, they are different in their properties as well. The anomers are diastereomers of each other.
What are Beta Anomers?
Beta anomer is the configuration of a carbohydrate in which the hydroxyl group is trans to the exocyclic oxygen at the anomeric centre. That means, the hydroxyl group and the exocyclic oxygen atom are on the opposite sides of the molecular projection.
Figure 02: The hydroxyl group in the upward direction is shown in red colour in the above image. The upward direction resembles the alpha anomer.
When we draw a Haworth formula, the hydroxyl group is in the upward direction if it is the beta anomer. The above example shows the beta anomer of D-glucopyranose.
What is the Difference Between Alpha and Beta Anomers?
The key difference between alpha and beta anomers is that in alpha anomer, the hydroxyl group at the anomeric carbon is cis to the exocyclic oxygen at the anomeric centre, whereas in beta anomer, the hydroxyl group is trans to the exocyclic oxygen. In the Haworth projection of a sugar molecule, we can see the hydroxyl group at the anomeric carbon atom in downward direction if it is an alpha anomer. But, in the beta anomer, the hydroxyl group at the anomeric carbon atom is in the upward direction. For example, alpha D-glucopyranose is the alpha anomer of glucopyranose, while beta D-glucopyranose is the beta anomer of glucopyranose molecule.
The following table summarizes the difference between alpha and beta anomers.
Summary – Alpha vs Beta Anomers
An anomer is a geometric variation among carbohydrates. The key difference between alpha and beta anomers is that in alpha anomer, the hydroxyl group at the anomeric carbon is cis to the exocyclic oxygen at the anomeric centre, whereas in beta anomer, the hydroxyl group is trans to the exocyclic oxygen. In the Haworth projection of a sugar molecule, we can see the hydroxyl group at the anomeric carbon atom in downward direction if it is an alpha anomer. But, in the beta anomer, the hydroxyl group at the anomeric carbon atom is in the upward direction.
Reference:
1. “Alpha Anomer.” OChemPal, Available here.
Image Courtesy:
1. “Alpha-D-glucopyranose-2D-skeletal” (Public Domain) via Commons Wikimedia
2. “Beta-D-glucopyranose-2D-skeletal” (Public Domain) via Commons Wikimedia