O
H
OH
OH
NH 2
OH
O
H
O
H
OH
OH
OH
OH
O
O
1
2
3
4
5
6
Glucosamine, an amino sugar
1
2
3
4
5
6
Glucuronic acid, a sugar acid
6.3.2 Stereochemistry of sugars
With monosaccharides, the configuration of the highest numbered chiral
carbon is compared with that of D- or L-glyceraldehyde (the simplest
aldose); e.g., D-sugar has the same configuration as D-glyceraldehyde and
L-sugar has the same configuration as L-glyceraldehyde. It can be noted that
D- and L-notations have no relation to the direction in which a given sugar
rotates the plane-polarized light i.e. (þ) or (À).
OH
H
CH 2 OH
O
H
H
O
H
CH 2 OH
O
H
*
(+)-D-Glyceraldehyde
The hydroxyl group
on the chiral carbon
is on the right hand side
*
(−)-L-Glyceraldehyde
The hydroxyl group
on the chiral carbon
is on the left hand side
Glucose, fructose, and many other natural monosaccharides have the
same configuration as D-glyceraldehyde at the chiral centre farthest from
the carbonyl group. In Fischer projections, most natural sugars have the
hydroxyl group at the highest numbered chiral carbon pointing to the right.
All these sugars are referred to as D-sugars, e.g. D-glucose.
OH
H
O
H
H
O
H
OH
H
OH
H
CH 2 OH
1
2
3
4
5
6
*
*
*
*
D-Glucose
Hydroxyl group at the highest numbered
chiral carbon (C-5) is pointing to the right,
i.e similar to D-glyceraldehyde
OH
H
O
H
H
O
H
OH
H
H
O
H
CH 2 OH
1
2
3
4
5
6
*
*
*
*
L-Glucose
Hydroxyl group at the highest numbered
chiral carbon (C-5) is pointing to the left,
i.e similar to L-glyceraldehyde
All L-sugars have the configuration as L-glyceraldehyde at the chiral centre
farthest from the carbonyl group, e.g. L-glucose. In Fischer projections,
L-sugars have the hydroxyl group at the highest numbered chiral carbon
pointing to the left. Thus, an L-sugar is the mirror image (enantiomer) of the
corresponding D-sugar.
6.3.3 Cyclic structures of monosaccharides
Monosaccharides not only exist as open chain molecules (acyclic), but
also as cyclic compounds. Cyclization leading to the formation of a cyclic
6.3 CARBOHYDRATES
305
H
OH
OH
NH 2
OH
O
H
O
H
OH
OH
OH
OH
O
O
1
2
3
4
5
6
Glucosamine, an amino sugar
1
2
3
4
5
6
Glucuronic acid, a sugar acid
6.3.2 Stereochemistry of sugars
With monosaccharides, the configuration of the highest numbered chiral
carbon is compared with that of D- or L-glyceraldehyde (the simplest
aldose); e.g., D-sugar has the same configuration as D-glyceraldehyde and
L-sugar has the same configuration as L-glyceraldehyde. It can be noted that
D- and L-notations have no relation to the direction in which a given sugar
rotates the plane-polarized light i.e. (þ) or (À).
OH
H
CH 2 OH
O
H
H
O
H
CH 2 OH
O
H
*
(+)-D-Glyceraldehyde
The hydroxyl group
on the chiral carbon
is on the right hand side
*
(−)-L-Glyceraldehyde
The hydroxyl group
on the chiral carbon
is on the left hand side
Glucose, fructose, and many other natural monosaccharides have the
same configuration as D-glyceraldehyde at the chiral centre farthest from
the carbonyl group. In Fischer projections, most natural sugars have the
hydroxyl group at the highest numbered chiral carbon pointing to the right.
All these sugars are referred to as D-sugars, e.g. D-glucose.
OH
H
O
H
H
O
H
OH
H
OH
H
CH 2 OH
1
2
3
4
5
6
*
*
*
*
D-Glucose
Hydroxyl group at the highest numbered
chiral carbon (C-5) is pointing to the right,
i.e similar to D-glyceraldehyde
OH
H
O
H
H
O
H
OH
H
H
O
H
CH 2 OH
1
2
3
4
5
6
*
*
*
*
L-Glucose
Hydroxyl group at the highest numbered
chiral carbon (C-5) is pointing to the left,
i.e similar to L-glyceraldehyde
All L-sugars have the configuration as L-glyceraldehyde at the chiral centre
farthest from the carbonyl group, e.g. L-glucose. In Fischer projections,
L-sugars have the hydroxyl group at the highest numbered chiral carbon
pointing to the left. Thus, an L-sugar is the mirror image (enantiomer) of the
corresponding D-sugar.
6.3.3 Cyclic structures of monosaccharides
Monosaccharides not only exist as open chain molecules (acyclic), but
also as cyclic compounds. Cyclization leading to the formation of a cyclic
6.3 CARBOHYDRATES
305