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Chapter 25: Carbohydrates
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What do the α- and β-
forms look like?
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example: α- and β-D-glucose
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Summary
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The cyclic forms of carbohydrates can exist in two forms, α-
and β- based on the position of the substituent
at the anomeric center.
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The two forms are sometimes described as "anomers" since they are
isomers
at
the anomeric center.
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To assign the cyclic form of a carbohydrate as the α-
or β- form look at the relative positions of
the -CH2OH group and -OH (or -OR) group at the anomeric center.
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In the α- form, the exocyclic O
group at the anomeric center is on the opposite face to the
-CH2OH group, and
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In the β- form, the exocyclic O
group at the anomeric center is on the same face as the -CH2OH
group.
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If a mixture of the α- and β-
anomers are present, then this is often represented by using a "wavy" line
to represent the bond:
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In general the two forms are stable solids, but in solution they rapidly
equilibrate (see mutarotation).
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The following figures shows several representations of the α-
and β- anomers of D-glucose.
- You should Manipulate the 3D JSMOL images until you can confirm the important relationship, the relative position of the anomeric -OR group and the -CH2OH.
