3. Objectives
• B y the end of this lesson students should be able to:
1. Classify a monosaccharide according to its functional group
and number of carbon atoms.
2. Designate a monosaccharide as a D-Sugar or L-sugar.
3. Describe formation of cyclic hemiacetals and hemiketals from
open-chain monosaccharides.
4. Explain the reason why monosaccharides and some
disaccharides are said to be reducing sugars.
5. Distinguish a-gycosidic linkages of di- and polysaccharides
from b-gycosidic linkages.
6. Describe the bonding in some important polysaccharides such
as starch, gylcogen, and cellulose.
3
4. SYNTHESIS OF CARBOHYDRATES:
PHOTOSYNTHESIS
• Carbohydrates are synthesized in plants by
photosynthesis
– Light from the sun is absorbed by chlorophyll and this is
converted to the energy necessary to biosynthesize
carbohydrates
• Carbohydrates act as a repository of solar energy
– The energy is released when animals or plants metabolize
carbohydrates
4
5. CLASSIFICATION OF CARBOHYDRATES
• Carbodydrares: These are polyhydroxy
aldehydes and ketones or substances that
hydrolyze to yield polyhydroxy aldehydes and
ketones.
• Monosaccharides: These are simple
carbohydrates that cannot be hydrolyzed into
smaller simpler carbohydrates.
5
6. CLASSIFICATION OF CARBOHYDRATES
• Disaccharides: On a molecular basis, these are
carbohydrates that undergo hydrolysis to
produce only two monosaccharide molecules .
• Trisaccharides: Those carbohydrates that yield
three monosaccharide molecules.
• Polysaccharide: Carbohydrates that yield a
large number of monosaccharide molecules
(﹥10).
6
7. CLASSIFICATION OF MONOSACCHARIDES
• Monosaccharides are classified according to:
1. The number of carbon atoms present in the molecule.
2. Whether they contain an aldehyde or keto group.
Three carbon atoms
Four carbon atoms
Five carbon atoms
Six carbon atoms
triose
tetrose
pentose
hexose
7
8. CLASSIFICATION OF MONOSACCHARIDES
• These two classification are frequently combined. For
example:
C4 aldose aldotetrose
C5 ketose ketopentose
O
CH
CHOH
CHOH
CH2OH
O
CH
(CHOH)n
CH2OH
CH2OH
C
(CHOH)n
O
CH2OH
CH2OH
C O
CHOH
CHOH
CH2OH
An aldose A ketose aldotetrose ketopentose
8
9. D AND L DESIGNATIONS OF MONOSACCHARIDES
• The simplest carbohydrates are glyceraldehyde, which is
chiral, and dihydroxyacetone, which is achiral.
• Glyceraldehyde exists in two enantiomeric forms which
have the following absolute configurations:
9
O
C
C
CH2OH
OH
H
H
O
C
C
CH2OH
H
HO
H
(+)-Glyceraldehyde (-)-Glyceraldehyde
10. D AND L DESIGNATIONS OF MONOSACCHARIDES
• (+)-glyceraldehyde is designated (D) and (-)-glyceraldehyde is
designated (L)
• A monosaccharide whose highest numbered stereogenic center
has the same configuration as D-(+)-glyceraldehyde is a D sugar
• A monosaccharide whose highest numbered stereogenic center
has the same configuration as L-(-)-glyceraldehyde is an L sugar
10
CHO
CHOH
CHOH
C
CH2OH
C
CHOH
CHOH
C
CH2OH
OH
H H
HO
*
* *
*
O
CH2OH
Highest number stereogenic center
1
2 3
4
5
1
2
3
4
D-aldopentose L-ketohexose
11. STRUCTURAL FORMULAS FOR MONOSACCHARIDES
• Fisher projection formula: horizontal lines project out
towards the reader and vertical lines project behind
the plane of the page.
CHO
H OH
HO H
H OH
H OH
CH2OH
CHO
C
H OH
C
C
HO H
C
H OH
H OH
CH2OH
CHO
H
HO
OH
H
OH
H
H OH
CH2OH
Fisher projection
formula
Circle-and-line
formula
Wedge-line-dashed
wedge formula
1 2 3 11
12. STRUCTURAL FORMULAS FOR MONOSACCHARIDES
• Open-chain structures shown on the previous slide (1,
2, or 3) exists in equilibrium with two cyclic forms 4
and 5 or 6 and 7.
O
H
OH
H
OH
H
OH
H
OH
CH2OH
H
O
H
OH
OH
H
H
OH
H
OH
CH2OH
H
4 5
6 7
Haworth formulas
+
+
O
HO
HO
H2C
OH
OH
OH
O
HO
HO
H2C
OH
OH
OH
D-(+)-Glucopyranose D-(+)-Glucopyranose
Chair Conformations
12
13. STRUCTURAL FORMULAS FOR MONOSACCHARIDES
• The cyclic forms of D-(+)-Glucose are hemiacetals
formed by an intramolecular reaction of the –OH
group at C-5 with the aldehyde group.
• An aldehyde can react with an alcohol to form a
hemiacetal
13
14. STRUCTURAL FORMULAS FOR MONOSACCHARIDES
14
• Cyclic structures of
monosaccharides are either 5 -
membered or 6 - membered
rings.
• A 5-membered ring
monosaccharide is similar to the
structure of Furan, hence it is
called a Furanose.
• A 6-memebered ring
monosaccharide is similar to the
structure of Pyran, hence it is
called a Pyranose.
15. STRUCTURAL FORMULAS FOR MONOSACCHARIDES
• Cyclization of open-chain D-Glucose
Carbon 1
Carbon 1
15
16. STRUCTURAL FORMULAS FOR MONOSACCHARIDES
• The two cyclic forms of D-Glucose are diastereomers
that differ only in the configuration of C-1.
• In carbohydrate chemistry diastereomers of this type
are called anomers, and the hemiacetal carbon atom
is called the anomeric Carbon atom
• In the orientation shown, the α anomer has the –OH
down and the β anomer has the –OH up.
Cyclization of open-chain D-Glucose
16
17. STRUCTURAL FORMULAS FOR MONOSACCHARIDES
• A ketone can react with an alcohol to form a
hemiketal:
17
19. Selected Monosaccharides: Glucose
• aka blood sugar & grape
sugar.
• aldohexose = sugar
containing aldehyde group
and 6 carbons
• source of energy for cells
– 5 to 6 grams in blood stream
supply energy for about 15
minutes 19
20. Fructose
• aka fruit sugar
• ketohexose = sugar
containing ketone group
and 6 carbons
• sweetest known natural
sugar
O
H
OH
OH
H
OH
CH2
OH
H
HOH2
C
20
21. Galactose
• occurs in brain and
nervous system
• only difference between
glucose and galactose is
spatial orientation of
groups on C4
Glucose
21
22. Glycoside Formation
• Glycosides are acetals at the anomeric carbon of
carbohydrates.
When glucose reacts with methanol in the presence of
catalytic acid, the methyl glycoside is obtained.
A glycoside made from glucose is called a glucoside.
22
23. Oxidation Reactions of Monosaccharides:
Reducing Sugars
Benedict’s or Tollens’ Reagents
• Aldoses and ketoses give positive tests when treated with
Tollens’ solution or Benedict’s reagent
• Tollens’ reagent [Ag(NH3)2OH] gives a silver mirror when Ag+ is
reduced to Ag0
• Benedict’s reagent (an alkaline solution of cupric citrate
complex) gives a brick red precipitate of Cu2O
23
24. Oxidation Reactions of Monosaccharides: Reducing Sugars
• Carbohydrates with hemiacetal linkages are
reducing sugars because they react with Tollens’
and Benedict’s reagents
The hemiacetal form is in equilibrium with a small amount of the
aldehyde or ketone form, which can react with Tollens’ and Benedict’s
reagents
• Carbohydrates with only acetal groups (glycosidic
linkages) do not react with these reagents and are
called non-reducing sugars (i.e. Some disaccharides
and Polysaccharides).
Acetals are not in equilibrium with the aldehyde or ketone and so
cannot react with these reagents
24
25. DISACCHARIDES: SUCROSE
• also known as table sugar, cane sugar, beet
sugar
• glucose + fructose = sucrose
a - 1:2-linkage involves aldehyde group from
glucose and ketone group from fructose
· glycosidic link
• nonreducing
25
27. DISACCHARIDES: MALTOSE
• Maltose is the disaccharide of D- glucose having an a- 1:4
glycosidic linkage
• Maltose results from hydrolysis of starch by the enzyme
diastase
– Maltose has a hemiacetal group in one glucose unit; it is a
reducing sugar
– The two glucose units of maltose are joined by an a-glucosidic
linkage
a-Glycosidic linkage
C
O
H
OH
H
OH
H
OH
CH2OH
H
1
2
3
4
5
6
H
O
C
O
H
H
OH
H
OH
CH2OH
H
1
2
3
4
5
6
HO
H
27
28. DISACCHARIDES: CELLOBIOSE
• Cellobiose is the disaccharide of D-glucose having a
b-linkage
• Cellobiose results from partial hydrolysis of cellulose
– Cellobiose has a hemiacetal group in one glucose unit; it is a
reducing sugar
– The two glucose units are connected by a b-glycosidic linkage
C
O
H
OH
H
OH
H
OH
CH2OH
H
1
2
3
4
5
6
O
H
O
H
H
OH
H
OH
CH2OH
H
2
3
4
5
6
1
OH
H
b-Glycosidic linkage
28
29. POLYSACCHARIDES
• Polysaccharides contain hundreds or
thousands of carbohydrate units.
• Polysaccharides are not reducing sugars, since
the anomeric carbons are connected through
glycosidic linkages.
• We will consider three examples of
polysaccharides, all of which are polymers of
glucose: starch, glycogen, and cellulose.
29
30. STARCH
• Starch is a polymer consisting of D-glucose
units.
• Starches (and other glucose polymers) are
usually insoluble in water because of the high
molecular weight, but they can form thick
colloidal suspensions with water.
• There are two forms of starch: amylose and
amylopectin.
30
31. STARCH — AMYLOSE
• Amylose consists of long, unbranched chains
of glucose (from 1000 to 2000 molecules)
connected by α(1→4) glycosidic linkages.
• 10%-20% of the starch in plants is in this form.
31
C
O
H
H
OH
H
OH
CH2OH
H
H
O
CH
O
H
H
OH
H
OH
CH2OH
H
OH
HO
n
n > 1000 a1:4-glycosidic linkages
32. STARCH — AMYLOSE
• Amylose forms helices (coils) which can trap
molecules of iodine, forming a characteristic
deep blue-purple color. (Iodine is often used
as a test for the presence of starch.)
32
33. STARCH — AMYLOPECTIN
• Amylopectin consists of long chains of glucose (up to 105
molecules) connected by α(1→4) glycosidic linkages,
with α(1→6) branches every 24 to 30 glucose units
along the chain.
• 80%-90% of the starch in plants is in this form.
33
C
O
H
O
H
OH
H
OH
CH2OH
H
H
O
C
O
H
H
OH
H
OH
CH2OH
H
OH
O
C
O
H
O
H
OH
H
OH
CH2OH
H
H
O
C
O
H
H
OH
H
OH
CH2OH
H
H
C
O
H
O
H
OH
H
OH
H2C
H
H
O
C
O
H
H
OH
H
OH
CH2OH
H
OH
O
Branch
Main chain
1:6 branch point
a
...
...
...
34. GLYCOGEN
• Glycogen, also known as animal starch, is structurally
similar to amylopectin, containing both α(1→4)
glycosidic linkages and α(1→6) branching points.
• Glycogen is even more highly branched, however,
with branches occurring every 8 to 12 glucose units.
• Glycogen is abundant in the liver and muscles; on
hydrolysis it forms glucose, which maintains normal
blood sugar level and provides energy.
34
35. CELLULOSE
• Cellulose is a polymer consisting of long, unbranched
chains of D-glucose connected by β(1→4) glycosidic
linkages.
• It may contain from 300 to 3000 glucose units in one
molecule.
35
C
O
H
H
OH
H
OH
CH2OH
H
O
H
O
H
H
OH
H
OH
CH2OH
H O
H
n
b
The glycosidic linkages are , 1: 4
36. CELLULOSE
• Because of the β-linkages, cellulose has a different
overall shape from amylose, forming extended
straight chains which hydrogen bond to each other,
resulting in a very rigid structure.
• Cellulose is an important structural polysaccharide,
and is the single most abundant organic compound
on earth.
• It is the material in plant cell walls that provides
strength and rigidity; wood is 50% cellulose.
36
