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Sugars and Polysaccharides
March 24, 2015
Sugars and Polysaccharides
• Carbohydrates or Saccharides are the most
abundant class of biological molecules.
• General formula (C•H
O)
n
where n≥3.
• Monosaccharides are basic building blocks.
• Oligosaccharides : a few covalently linked
monosaccharides.
• Polysaccharides : many covalently linked
monosaccharides.!
Major Carbohydrate Classes
• Monosaccharides
– Single polyhydroxy aldehyde or ketone units
• Oligosaccharides
– Short chains of monosaccharides, <
– Most abundant are the disaccharides
– Most oligosaccharides of 3 or more units are joined
to nonsugar molecules
• Polysaccharides
– Contain more than 20 monosaccharide units
– Many contain hundreds or thousands of
monosaccharides
– May be in linear (cellulose) or branched (glycogen)
chains
Monosaccharides
• Consist of aldehydes or ketones with one or more
hydroxyl groups.
• Identified based on the nature of the carbonyl group:
- Aldehyde -> aldose
- Ketone -> ketose
• Further classified based on the number of carbon atoms.
- 3 -> trioses
- 4 -> tetroses
- 5 -> pentoses
- Etc…
• Often contain multiple chiral centers.
- D sugars have the same configuration as does D-glyceraldehyde
at the asymmetric center farthest from the carbonyl group.
[Fischer Convention]
n-
stereoisomers.!
Aldoses: Stereochemical Relationships
C
CHO
CH
2
OH
H OH
C
CHO
C
H OH
CH
2
OH
H OH
C
CHO
C
HO H
CH
2
OH
H OH
D-Glyceraldehyde
D-Erythrose D-Threose
C
C
C
HO H
CH
2
OH
H OH
C
C
C
HO H
CH
2
OH
H OH
C
C
C
H OH
CH
2
OH
H OH
C
C
C
H OH
CH
2
OH
H OH
CHO CHO
CHO CHO
H OH HO H
H OH HO H
D-Ribose (Rib) D-Arabinose (Ara) D-Xylose (Xyl) D-Lyxose (Lyx)
C
C
C
HO H
CH
2
OH
H OH
C
H OH
C
C
C
HO H
CH
2
OH
H OH
C
H OH
C
C
C
HO H
CH
2
OH
H OH
C
HO H
C
C
C
HO H
CH
2
OH
H OH
C
HO H
C
C
C
H OH
CH
2
OH
H OH
C
HO H
C
C
C
H OH
CH
2
OH
H OH
C
HO H
C
C
C
H OH
CH
2
OH
H OH
C
H OH
C
C
C
H OH
CH
2
OH
H OH
C
H OH
CHO CHO
H OH HO
CHO CHO CHO CHO CHO
CHO
H H OH HO H H OH HO H H OH HO H
D-Allose D-Altrose D-Mannose
(Man)
D-Gulose D-Idose D-Galactose
(Gal)
D-Glucose D-Talose
(Glc)
Aldopentoses
Aldotetroses
Aldotriose
Aldohexoses
Ketoses: Stereochemical
Relationships
• Ketoses have one less
chiral center than their
aldose counterparts.
• Ketoses have 2
n-
stereoisomers (n = number
of carbon atoms).
• Ketoses with the carbonyl
at the C2 position are most
prevalent.
• Nomenclature: insert - ul -
before the - ose ending in
the name of the
corresponding aldose.
CH 2
OH
C
CH 2
OH
O
Dihydroxyacetone
C
C
CH 2
OH
O
CH
2
OH
H OH
D-Erythrulose
C
C
CH 2
OH
O
C
H OH
CH 2
OH
H OH
C
C
CH
2
OH
O
C
HO H
CH 2
OH
H OH
D-Ribulose D-Xylulose
C
C
CH 2
OH
O
C
H OH
C
H OH
CH
2
OH
H OH
C
C
CH 2
OH
O
C
HO H
C
H OH
CH
2
OH
H OH
C
C
CH 2
OH
O
C
H OH
C
HO H
CH
2
OH
H OH
C
C
CH 2
OH
O
C
HO H
C
HO H
CH
2
OH
H OH
D-Sorbose
D-Tagatose D-Psicose D-Fructose
Configurations and Conformations
• Aldehydes and ketones
can react with alcohols
to form hemiacetals and
hemiketals respectively.
• Aldehydes/ketones of
monosaccharides can
react with hydroxyl
groups intramolecularly
to form cyclic
hemiacetals/hemiketals.
• Haworth projection
formulas.!
CH
2
OH
C
C
C
H OH
H OH
HO H
H C OH
C
O H
OH
O
H
CH
2
OH
HO
H
H
OH
OH
H
1
2
3
4
5
6
6
1
2
3
4
5
O
H
HO
CH
2
OH
H
H
OH H
OH
OH
H
CH
2
OH
C
C
C
H OH
H OH
HO H
C O
CH
2
OH
2
3
4
5
6
1
OH
O
CH
2
HOCH OH
2
H OH
H
OH
HO
H
HOCH
2
H
OH
OH
H
O
CH
2
OH
D-Glucose
α-D-Glucopyranose
α-D-Fructofuranose
D-Fructose
Cyclic Sugars
• 6-membered rings:
pyranoses.
• 5-membered rings:
furanoses.
• Anomers differ in
configuration at the
hemiacetal or hemiketal
carbon.
• This carbon is called the
anomeric carbon.!
• Anomers: α and β
isomers.!
Pyran
Furan
O
O O
H
HO
CH 2
OH
H
H
OH
H
OH
OH
H
O
CH
2
HOCH OH
2
H OH
H
OH
HO
H
α-D-Glucopyranose
α-D-Fructoruranose
O
H
HO
CH
2
OH
H
H
OH
H
OH
OH
H
α-D-Glucopyranose
anomeric carbon
O
H
HO
CH 2
OH
H
H
OH H
OH
H
OH
β-D-Glucopyranose
Anomers
Conformational Variability
• Pyranoses can assume
boat and chair
conformations.
• Ring conformation
effects chemical
reactivity.
– Equatorial OH
groups esterify more
readily than axial
OH groups.
• Furanose rings have
similar conformational
variability.
• Substituents influence
conformational
preferences.!
O
H
HO
H
HO
H
H
OH
H
OH
OH
O
OH
H
OH
H
OH
OH
H
H
H
HO
a
e
a
e
a
a
e
a
e
e
a
e
Possible chair confomrations for β-D-glucopyranose
Boat Chair
Pyranoses
Furanoses
Glycosidic Bonds
• Glycosidic bonds are formed by the condensation of
the anomeric OH and another OH group (or in the
case of nucleosides, N).
• Formation of glycosidic bonds is acid catalyzed.
• Glycosidic bonds link sugar monomers in di- and
polysaccharides.!
O
H
HO
CH
2
OH
H
H
OH
H
OH
OH
H
α-D-Glucopyranose
O
H
HO
CH
2
OH
H
H
OH
H
OH
OCH
3
H
O
H
HO
CH
2
OH
H
H
OH
H
OH
H
OCH
3
+ CH
3
OH
H
Methyl−α-D-Glucoside Methyl−β-D-Glucoside
Oxidation and Reduction
• Aldehydes of aldoses can be
oxidized to carboxylic acids
under mild conditions (resulting
in aldonic acids).
• Saccharides bearing anomeric
carbons that are not involved in
glycosidic bonds are called
reducing sugars.
• Oxidation of the terminal
primary alcohol to the
carboxylic acid results in uronic
acids.
• Both aldoses and ketoses can
be reduced to their alcohols
resulting in sugar alcohols.!
CH 2
OH
HO H
H OH
H OH
HO H
H O
D-Mannose
CH 2
OH
HO H
H OH
H OH
HO H
HO O
D-Mannic acid
CH 2
OH
HO H
H OH
H OH
HO H
CH
2
OH
Mannitol
CO 2
H
HO H
H OH
H OH
HO H
H O
D-Mannuronic
Acid
Oxidation Oxidation
Reduction
Sugar Derivatives
• In deoxy sugars, an -OH
group has been replaced
with and H.
• In amino sugars, one or
more OH groups have
been replaced with amino
groups or acetylated
amino groups.
• Amino sugars are
common components in
polysaccharides.!
O
HOCH OH
2
H H
H
OH
H
OH
β-Ribose
O
HOCH OH
2
H H
H
OH
H
H
β- 2 - Deoxyribose
O
H
HO
CH 2
OH
H
H
OH
H
OH
OH
H
α-D-Glucose
O
H
HO
CH 2
OH
H
H
OH
H
NH
2
OH
H
α-D-Glucosamine
O
H
HO
CH 2
OH
H
H
OH
H
HN
OH
H
N - Acetyl-α-D-Glucosamine
O
CH
3
Disaccharides!
• Disaccharides are two
monosaccharides joined
by an O -glycosidic
bond.
• The reaction generally
involves the anomeric
carbon.
• They end with a free
anomeric carbon is the
reducing end.
• Few tri- or higher
oligosaccharides.
O
HOCH H
2
CH
2
OH
H
OH
HO
H
Glucose Fructose
O
H
HO
CH
2
OH
H
H
OH
H
OH
H
O
α β
1
2 3
1
2
3
Sucrose
O
HO
H
CH
2
OH
H
H
OH
H
OH
H
O
H
CH
2
OH
H
H
OH
H
OH
OH
H
O
Galactose Glucose
β
1 4
Lactose
O
H
HO
CH
2
OH
H
H
OH
H
OH
H
O
H
CH
2
OH
H
H
OH
H
OH
OH
H
Glucose
α
1 4
Glucose
O
Maltose
Naming of Disaccharides
• Anomeric configuration of left
monosaccharide is given first.
• Nonreducing residue is named,
including furanosyl or pyranosyl.
• The two carbons in the glycosidic
bond are identified: (1! 4), (1-
>6) etc.
• The second residue is named.
- Sucrose : O-α-D-glucopyranosyl-(1-
2)-β-D-fructofuranoside [note -ide
ending].
- Lactose : O-β-D-galactopyranosyl-(1-
4)-D-glucopyranose.
O
HOCH H
2
CH
2
OH
H
OH
HO
H
Glucose Fructose
O
H
HO
CH
2
OH
H
H
OH
H
OH
H
O
α β
1
2 3
1
2
3
Sucrose
O
HO
H
CH
2
OH
H
H
OH
H
OH
H
O
H
CH
2
OH
H
H
OH
H
OH
OH
H
O
Galactose Glucose
β
1 4
Lactose
O
H
HO
CH
2
OH
H
H
OH
H
OH
H
O
H
CH
2
OH
H
H
OH
H
OH
OH
H
Glucose
α
1 4
Glucose
O
Maltose
Polysaccharides!
• Structural Polysaccharides:
Cellulose and Chitin.
• Cellulose is the primary
component of plant cell walls.
• Linear polymer of D-glucose.
• Linkages, β(1->4).
• Up to 15,000 saccharide
units.
• Extensive interstrand
hydrogen bonding.
• Accounts for almost half the
carbon in the biosphere.!
O
H
CH
2
OH
H
H
OH H
OH
H
O
H
CH
2
OH
H
H
OH H
OH
H
β
1 4
Glucose
O
n
Glucose
Cellulose
O
H
CH
2
OH
H
H
OH H
HN
H
O
H
CH
2
OH
H
H
OH H
HN
H
β
1 4
O
O
n
N - Acetylglucosamine
Chitin
O
CH
3
O
CH
3
N - Acetylglucosamine
Structural Polysaccharides!
• Structural Polysaccharides:
Cellulose and Chitin.
• Chitin is the primary
component of invertebrate
exoskeletons.
• Almost as abundant as
cellulose.
• Linear homopolymer of N -
acetyl-D-glucosamine.
• Linkages, β(1->4).
• Structure is similar to that of
cellulose.
O
H
CH
2
OH
H
H
OH H
OH
H
O
H
CH
2
OH
H
H
OH H
OH
H
β
1 4
Glucose
O
n
Glucose
Cellulose
O
H
CH
2
OH
H
H
OH H
HN
H
O
H
CH
2
OH
H
H
OH H
HN
H
β
1 4
O
O
n
N - Acetylglucosamine
Chitin
O
CH
3
O
CH
3
N - Acetylglucosamine
