CHAPTER 23
Carbohydrates
1
Introduction
• Carbohydrates are naturally occurring
compounds of carbon, hydrogen , and
oxygen.
• Carbohydrates have the empirical formula
CH2O.
• Carbohydrates have the general formula
CnH2nOn. eg. Glucose: C6H12O6
2
• Carbohydrates are defined as polyhydroxy
aldehydes or ketones or substances that
hydrolyze to yield polyhydroxy aldehydes
and ketones.
• Monosaccharides are carbohydrates that
cannot be hydrolyzed to simpler
carbohydrates = simple sugars.
• Disaccharides can be hydrolyzed to two
monosaccharides.
• Oligosaccharides yield 2 to 8
monosaccharides.
• 3 Polysacccharides yield >8 monosaccharides
Some important monosacharides
CHO
H
OH
H
C O
OH
H
HO
H
HO
H
OH
HO
H
H
OH
H
OH
H
OH
H
OH
HO
CH2OH
D-glucose
4
CH2OH
CHO
CH2OH
D- galactose
H
CH2OH
D-fructose
Glucose
• Blood sugar.
• Mammals
convert sucrose,
lactose, maltose,
and starch to
glucose
glucose which is
then used for
energy
fats
steroids
(cholesterole)
O
CH3C
protiens
acetyl group
in acetyl coenzyme A
CO2 + H2O + energy
5
Other monosaccharides
• Fructose Î sweetest
tasting sugar. It occurs in
fruit and honey.
• Galactose Î found
bonded to glucose in the
disaccharide lactose.
• Ribose and
deoxyribose Î form part
of the polymeric
backbone of nucleic
acids.
6
CHO
CHO
CH2
H
OH
H
OH
H
OH
H
OH
H
OH
CH2OH
D-ribose
C5H10O5
CH2OH
2-deoxy-D-ribose
deoxy = minus
an oxygen
Classification of Monosaccharides
• The suffix –ose is used to designate a
carbohydrate. Carbohydrate names
ending in –ose (eg. Glucose, galactose).
• Mononsacchrides that contain aldehyde
group Î Aldoses (aldehyde + ose) (
glucose and ribose).
• Monosacchrides with ketone group Î
Ketoses (ketone + ose) ( frctose)
7
• The number of carbon atoms are
designated by tri, tetr-, etc ( for example a
triose is three carbons)
8
• Ketones are often given the ending –ulose.
example. Fructose,
CH2OH
CHO
O
HO
H
H
H
OH
OH
9
OH
H
OH
H
OH
CH2OH
D-ribose
CH2OH
Ketohexose
or hexulose
H
Ketopentose
or pentulose
aldopentose
Configuration of monosaccharides
• Mpnpsaccharides with the same number
of carbons are structural isomers or
diastereomers.
• Diastereomers are nonenentiomeric
stereoisomers with two or more chiral
centers but differ in the projection of at
least one of them.
• If two diastereomers differ in the projection
of only one chiral center Î epimers
10
Examples
• D-glucose, Dgalactose, and Dfructose are all
structural isomers
( diastereomers).
• D-glucose and Dgalactose differ
only in the
projection at
carbon 4 Î
epimers
11
CHO
H
OH
CH2OH
CHO
H
C O
OH
H
HO
H
HO
H
OH
HO
H
H
OH
H
OH
H
OH
H
OH
HO
CH2OH
D-glucose
CH2OH
D- galactose
H
CH2OH
D-fructose
The D and L System
• If the OH on the last chiral carbon is
projected to the right Î D
• If the OH on the last chiral carbon is
projected to the left Î L
12
Examples
CHO
CHO
H
OH
CH2OH
HO
H
HO
H
H
D-glyceraldehyde
OH
CH2OH
D- lyxose
CHO
13
HO
H
HO
H
HO
H
CH2OH
CHO
H
HO
H
HO
OH
H
OH
H
CH2OH
L-aldohexose
L-ripose
The D Family of Aldoses
14
Cyclization of Monosaccharides
only sugars seem to make
stable hemiacetals
a hemiacetal
O
H
H
HO
H
H
1
1C
C
2
3
4
H
OH
HO
H
H
OH
5
6
CH2 OH
15
OH
H
glucose
..
O
..
H
H
2
3
4
OH
H
:O:
OH
5
6
CH2 OH
glucopyranose
FURANOSE AND PYRANOSE RINGS
H
4
: O:
5
O
O
2
3
OH
1
6
H
..
4
H
O
O:
3
H
2
H
1
O
5
O
16
a pyranose
ring
H
furan
O
pyran
OH
two anomers
are possible
in each case
a furanose
ring
for clarity no
hydroxyl groups
are shown on the
chains or rings
ANOMERS
β-D-(+)-Glucose
H
: O:
β = OH up
O
O
OH
H
H
anomeric
carbon
H
: O:
H
O
for clarity
hydroxyl groups
on the chain are
not shown
17
(hemiacetal)
O
H
OH
α-D-(+)-Glucose
α = OH down
anomers differ in configuration
at the anomeric carbon # 1
Glucose
OH
OH
H2C
HO
O
hemiacetals
H
HO
H2C
HO
OH
OH
HO
OH
α-D-(+)-Glucose
O
OH
34%
H
β-D-(+)-Glucose
[α] = + 18.7°
[α] = + 112.2°
66%
O
H
H
HO
H
H
1
OH
C
2
3
4
OH
H
HO
OH
5
6
CH2 OH
18
H2C
..
O
..
H
Equilibrium mixture:
: O:
[α] = + 52.7°
O
HO
OH
H
< 0.001%
H
open chain
HAWORTH PROJECTIONS
It is convenient to view the cyclic sugars (glucopyranoses)
as a “Haworth Projection”, where the ring is flattened.
Standard Position
HAWORTH
PROJECTION
CH2OH
O H
H
H
OH H
OH
HO
H
OH
α-D-(+)-glucopyranose
19
upper-right
O back
This orientation is
always used for a
Haworth Projection
HAWORTH PROJECTIONS
HERE ARE SOME CONVENTIONS YOU MUST LEARN
O
1) The ring is always oriented with the oxygen
in the upper right-hand back corner.
CH2OH
2) The -CH2OH group is placed
UP for a D-sugar and
DOWN for an L-sugar.
O
D
O
L
CH2OH
3)
α-Sugars have
the anomeric hydroxyl group down.
CH2OH
O
α
OH
4)
20
β-Sugars have
the anomeric hydroxyl group up
CH2OH
O
OH
β
SOME HAWORTH PROJECTIONS
cis
-CH2OH
up = D
=β
CH2OH
O OH
H
H
OH H
H
HO
-CH2OH
up = D
H
OH
CH2OH
O H
H
H
OH H
OH
HO
H
OH
21
D-SUGARS
β-D
trans
=α
BOTH OF THESE ARE D-GLUCOSE
ANOMERS
α-D
CONVERTING
FISCHER PROJECTIONS
TO HAWORTH PROJECTIONS
22
CONVERTING TO HAWORTH PROJECTIONS
D-(+)-glucose
U
P
1
2
HO
3
4
5
6
CHO
OH
D
O
W
N
OH
OH
CH2OH
FISCHER
PROJECTION
cis
=β
6 CH2OH
H
on right
=D
23
-CH2OH
up = D
4
HO
H5
OH
3
H
O OH
1
H
2
H
BOTH
ANOMERS OF
A D-SUGAR
(D-glucose)
OH
trans
=α
CH2OH
O H
H
H
OH H
OH
HAWORTH
HO
PROJECTIONS
H
OH
CONVERTING TO ACTUAL CONFORMATIONS
-CH2OH
up = D
cis
β-D-(+)-glucopyranose
=β
CH2OH
O OH
H
H
OH H
H
HO
H
H
HO
OH
CH2 H
OH
HO
H
OH
trans
O
H
O
OH
H
OH
=α
CH2OH
H
CH2 H
O H
H
O
H
HO
OH H
H
HO
OH
HO
OH
H
H
OH
OH
H
24
HAWORTH
α-D-(+)-glucopyranose
O
CONFORMATION
HAWORTH PROJECTIONS OF L-SUGARS
L-(+)-glucose
U
P
CHO
HO
OH
HO
HO
CH2OH
on left
=L
FISCHER
PROJECTION
25
D
O
W
N
-CH2OH
down=L
trans
H
O OH
CH2OH
H HO
H
H
OH H
cis
H
O H
HO
CH2OH
H HO
OH
H
HO
OH
H
BOTH
ANOMERS OF
A L-SUGAR
(L-glucose)
HAWORTH
PROJECTIONS
CONVERTING FISCHER TO HAWORTH PROJECTIONS
Genral rules
LEFT = UP
RIGHT = DOWN
β = up
α = down
26
These rules
are the same
for both
D- and Lsugars
The only difference when
converting D- and L- sugars
is :
D-sugars -CH2OH = UP
L-sugars -CH2OH = DOWN
FRUCTOFURANOSES
27
standard position
FRUCTOSE
O
cis = β
up = D
1
2
HO
CH2OH
..
O:
5
6
5
OH
..
OH
..
H
4
OH
OH
2
anomeric
carbon
CH2OH
3
H
1
CH2OH
D-(-)-Fructose
28
O
H HO
6
3
4
HOCH2
β-D-(-)-Fructofuranose
Anomeric Effect
β-anomer is the
most stable
anomer
CHO
H
OH
H
OH
H
OH
CH2OH
O OH
O
+
HO
OH OH
HO
β-D-ribopyranose
(56%)
HOCH2
OH
O
α-D-ribopyranose
(20%)
HOCH2
+
D-ribose
29
OH OH
β-D-ribofuranose
(18%)
OH
OH OH
O
OH
OH OH
α-D-ribofuranose
(6%)
MUTAROTATION
30
Mutarotation
of
Glucose
OH
H2C
HO
OH
O
hemiacetals
H
HO
H2C
HO
OH
OH
36%
H
β-D-(+)-Glucose
[α] = + 18.7°
[α] = + 112.2°
OH
H2C
HO
OH
HO
OH
α-D-(+)-Glucose
O
H
Equilibrium mixture:
: O:
[α] = + 52.7°
O
HO
OH
< 0.001%
31
64%
H
open chain
Glycoside Formation
• Glycosides are acetals at the anomeric carbon
of carbohydrates
OH
OR''
+
Remember
H
RCHOR' + R''OH
RCHOR' + H2O
An acetal
a hemiacetal
CH2OH
CHO
H
HO
OH
O
O
OH
H
H
OH
H
OH
CH2OH
32
CH2OH
OH
CH3OH
H+
OH
OH
β –D-glucopyranose
OCH3
OH
OH
OH
Methyl β –Dglucopyranoside
Glycosides
Glycoside has two or groups attached to the
anomeric carbon
two OR
O
groups
OR''
two OR
groups
OCH3
RCHOR'
An acetal
A glycoside
Glycosides can be hydrolyzed in aqueous acid
CH2OH
CH2OH
O
OH
OCH3
+
O
+
H2O
OH
H
OH
OH
+
CH3OH
OH
OH
Methyl β –D-glucopyranoside
OH
D-glucopyranose
aglycone
Glycoside
formation
Hydrolysis of
glycosides
34
Oxidation of Monosaccharides
• Aldoses and ketoses are easily oxidized when
treated with Tollens’ reagent
CH2OH
CH2OH
OH
O
OH
OH
OH
H2O
O
CH
OH
OH
OH
OH
a reducing sugar
Ag(NH3)2+
OH-
CH2OH
OH
CO2-
OH
35
OH
OH
+
Ag
Reducing Sugars
• Carbohydrates with hemiacetal linkages
are reducing sugars because they react
with Tollens’ reagent
36
Ketoses are also reducing
sugars
CO2-
CH2OH
CHOH
C O
HO
H
H
OH
H
OH
CH2OH
D-fructose
37
HO
Ag(NH3)2+
OH-
H
H
OH
H
OH
CH2OH
+
Ag
Fructose is oxidized readily because it is in
equlibrium with aldehydes through an
endiol intermediate.
A ketone
38
An aldose
Glycosides are nonreducing
sugars
• Glycosides are not in equilibrium with aldehydes
Î nonreducing sugars
O
OCH3
Ag(NH3)2+
OH-
A glycoside
39
no reaction
Aldonic Acids
• Bromine in water selectively oxidizes the
aldehyde group of an aldose to the
corresponding carboxylic acid
40
Example
CO2H
CHO
H
HO
H
OH
H
H
OH
H
OH
CH2OH
D- glucose
Br2 , H2O
pH 5-6
HO
OH
H
H
OH
H
OH
CH2OH
D-gluconic acid
An aldonic acid
41
Aldaric Acids
Dilute nitric acid oxidizes both the aldehyde
and primary hydroxyl groups of an aldose to
an aldaric acid
42
Example
43
Uronic Acids
In biological systems, the terminal CH2OH group
can be oxidized without oxidation of the
aldehyde group to uronic acid
CH2OH
CO2H
O
OH
OH
OH
O
enzymes
OH
OH
OH
OH
D-glucose
44
O
OH
D-glucoronic acid
a uronic acid
Periodic Acid Oxidation:
Oxidative Cleavage of Polyhydroxy
Compounds
Compounds with hydroxyl groups on adjacent carbons
undergo cleavage of carbon-carbon bonds between the
hydroxyl groups
The products are aldehydes, ketones or carboxylic
acids
45
• The carbonyl group is oxidized to a
carboxyl group, while the hydroxyl
group is oxidized to an aldehyde or
ketone.
• With three or more contiguous hydroxyl
groups, the internal carbons become
formic acid
46
Cleavage also takes place when a hydroxyl group is
adjacent to an aldehyde or ketone group
–An aldehyde is oxidized to formic acid; a ketone is
oxidized to carbon dioxide
47
48
ÎNo cleavage results if there are
intervening carbons that do not
bear hydroxyl or carbonyl groups
49
Reduction of Monosaccharides
• Aldoses and ketoses can be reduced to
alditols
50
Example
51
Reaction at the
Hydroxyl Groups
52
Acetate Formation
Carbohydrates react with acetic
anhydride in the presence of weak base
to convert all hydroxyl groups to
acetate esters
53
Ether Formation
Reaction of monosaccharides with excess
dimethylsulfate and NaOH produces multi methyl
ether.
O
O
RO-
+
CH3
OSOCH3
ROCH3
+ -OSOCH3
O
O
OCH3
OH
CH2
HO
HO
(CH3O)2SO2
O
OH
OH
OH
-
CH2
O
CH3O
CH3O
O
CH3
54
OCH3
Cyclic Acetal Formation
Carbohydrates form cyclic acetals with
benzaldehyde selectively between 1,3-diol.
OH
C6H5CH
H2C
HO
HO
H2C
C6H5CHO
OH
OH
H+
O
O
HO
OH
OH
Cyclic acetal
55
Cyclic Ketal Formation
Carbohydrates form cyclic ketals with
acetone selectively between cis-vicinal
hydroxyl groups.
56
Disaccharides
A disaccharide is a carbohydrate
compound of two units of
monosaccharides joined together by
glycoside link from carbon 1 of one
unit to an OH of the other unit
57
H
O
H
H
b
c
OH H
OH
HO
H
Maltose
CH2OH
CH2OH
enzyme
mediated
a
H
OH H
OH
O
.. :
H
H
OH
O
Contain two
units of Dglucopyranose
OH
α-D-(+)-Glucose
1,4’-αGlycosidic Linkage
CH2OH
H
O
b
CH2OH
H
H
c OH H
OH H
a
H
OH
H
Maltose
OH
O
HO
58
O
H
OH
Humans can
digest
α-1,4
Cellobiose
H
Two
glucose
units
CH2OH
CH2OH
H
O b OH
OH H
c
H
HO
H
H
OH
β-D-(+)-Glucose
CH2OH
59
β-1,4
c
H
OH
H
OH
O a OH
OH H
H
H
H
HO
OH H
O
.. :
H
b O
OH H
Humans can’t
digest
O
OH
CH2OH
H
H
a
enzyme
mediated
1,4’-β Glycosidic Linkage
If continued, you
get cellulose.
O
OH
Cellobiose
Sucrose (Table sugar)
Sucrose is a disaccharide formed from Dglucose and D-fructose
–The glycosidic linkage is between C1 of
glucose and C2 of fructose (both anomeric
carbon atoms).
–Sucrose is a nonreducing sugar because of
its acetal linkage
60
β-D-fructofuranosyl α-D-glucopyranoside
1,2`link
61
Polysaccharides
• A polysaccharide is a compound in which
the molecule contain many units of
monosaccharide joined together by
glycoside link.
• Two important polysaccharides are starch
and cellulose
62
Starch
•The two forms of starch are amylose and amylopectin
•Amylose consists typically of more than 1000 Dglucopyranoside units connected by α linkages between
C1 of one unit and C4 of the next
1,4`α link
63
Amylopectin
•Amylopectin is similar to amylose but has branching
points every 20-25 glucose units
–Branches occur between C1 of one glucose unit and C6 of another
64