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