Chapter 18:
Carbohydrates
18.3 - Types of Carbohydrates
18.4 and 18.6 - D and L Notations from Fischer Projections
18.8 - Classification of Monosaccharides
18.9 - Structures of Some Important Monosaccharides
18.10 and 18.11 - Cyclic Structures of Monosaccharides
18.12 - Chemical Properties of Monosaccharides
18.13 - Disaccharides
18.14 and 18.16 - Polysaccharides
Carbohydrates

Most abundant organic compounds in nature

Produced by photosynthesis in plants

Composed of the elements C, H and O

Also called saccharides, which means “sugars”

A major source of energy from our diet

A source of C for synthesis of other biomolecules

Can be linked to cell membranes or proteins
Types of Carbohydrates
 Monosaccharides are the simplest carbohydrates
 Empirical formula = CH2O
 Oligosaccharides contain a few monosaccharides (2-10)
 Disaccharides consist of two monosaccharides
 Polysaccharides contain many monosaccharides
Chiral Molecules

Chiral molecules




Achiral molecules



Have “handedness”
Nonsuperimposable on mirror images
Ex: hand, shoe
Do not have handedness
Ex: glass, spoon
Chiral molecules contain carbons
with 4 different groups

Called chiral carbons or chiral center
CHO
Br
C
CH3
OH
Fischer Projections

Used to represent carbohydrates

Places the most oxidized group at the top

All chiral carbons are represented as intersection of lines


C is not shown
Implied 3D arrangement of atoms

Horizontal lines for bonds that come forward

Vertical lines for bonds that go back
D and L Notations


By convention, the letter L is assigned to the
structure with the —OH on the left
The letter D is assigned to the structure with
—OH on the right
D and L Monosaccharides

The —OH on the chiral atom farthest from the
carbonyl group is used to assign the D or L
configuration
O
C
CHO
H
H
OH
H
OH
HO
H
OH
CH 2OH
D
D-Ribose
O
H
OH
C OH
HO
H
H
H
OH
H
OH
H
OH
H
OH
HO
CH2OH
D
D-Glucose
H
CH2OH
LL-Galactose
Learning Check

Indicate whether each is the D or L isomer:
CH2OH
CHO
HO
CHO
H
HO
H
HO
H
CH2OH
H
HO
OH
H
CH2OH
O
HO
H
H
OH
H
OH
CH2OH
Ribose
Threose
Fructose
Classification of
Monosaccharides


Monosaccharide = unbranched chain of 3-8 C
atoms; one is carbonyl, others attached to -OH
Aldose


monosaccharide with an aldehyde group (1st carbon)
Ketose

monosaccharide with a ketone group (2nd carbon)
CH2OH
O
CHO
CHO
HO
H
CH2OH
Aldose
HO
H
H
HO
H
OH
H
OH
CH2OH
H
OH
Aldose
CH2OH
OH
Ketose
Monosaccharides


Monosaccharides are also classified according to
the number of carbon atoms
A triose has three carbons; a tetrose has four
carbons; a pentose has five carbons; and a hexose
has six carbons.
CH2OH
O
CHO
HO
CHO
HO
H
H
CH2OH
triose
aldotriose
H
HO
H
OH
H
OH
CH2OH
H
OH
tetrose
aldotetrose
CH2OH
OH
hexose
ketohexose
Learning Check
Identify each as tetrose, pentose or hexose, and
as aldose or ketose:
CH2OH
CHO
O
HO
H
HO
HO
H
H
OH
HO
H
H
OH
CH2OH
A
H
CH2OH
B
D-Glucose

Most common hexose

Found in fruits, corn
syrup, and honey

An aldohexose with
the formula C6H12O6

Known as blood sugar
in the body

Building block for
many disaccharides
and polysaccharides
D-Galactose

Aldohexose

Differ from D-glucose at C4




C1 is at the top
Not found in the free form
in nature
Obtained from lactose, a
disaccharide (milk
products)
Important in cellular
membranes in CNS (brain
sugar)
CHO
H C OH
HO C H
HO C H
H C OH
CH2OH
D-Galactose
D-Fructose

Ketohexose C6H12O6

Differ from glucose at C1 and C2
(location of carbonyl)

The sweetest carbohydrate (2x
sucrose)
CH2OH
C O
HO C H

Found in fruit juices and honey

Formed from hydrolysis of sucrose
H C OH

Converts to glucose in the body
H C OH
CH2OH
D-Fructose
Learning Check
Draw the structure of D-fructose:
Hemiacetal Review


What is a hemiacetal?
How is a hemiacetal formed?
OH
O
+
H
CH3
C
+
H
CH3
CH3OH
C
H
OCH3

What if the alcohol and carbonyl are attached?
H
O
H
H
H
HOCH2
H
HO
H
O
O
O
=
C
H
O
C
HOCH2
CH2OH
O
H+
C
=
C
HO
H
O
O
C
H+
O
O
C
H
H
Hexose hemiacetals

Favor formation of 5- or 6-membered rings

Hydroxyl group on C5 reacts with the aldehyde or ketone

The Haworth structure can be written from the Fischer projection

The cyclic structure of a D-isomer has the last CH2OH group located
above the ring

The –OH group on the left (C3) is drawn up

The –OH groups on the right (C2, C4) are drawn down
 and  Anomers for D-Glucose


Anomers are isomers which differ in placement of
hydroxyl on C1
The –OH is drawn down for the -anomer, and up
for the -anomer
CH2OH
O
O
OH
OH
OH
OH
OH
OH
-D-Glucose


CH2OH

OH
OH
-D-Glucose
Mashed potatoes or mashed paper?
Mutarotation

In solution, -D-glucose is in equilibrium with β-D-glucose

Mutarotation involves the conversion of the cyclic anomers
into the open chain

At any time, there is only a small amount of open chain
aldehyde
Cyclic Structure of Fructose



As a ketohexose, fructose forms a cyclic
structure when the —OH on C5 reacts with
the ketone on C2
Result is 5-atom ring
Anomeric carbon is C2
CH2OH
C O
HO C H
CH2OH
O
H C OH
H C OH
CH2OH
D-Fructose
CH2OH
CH2OH
O
OH
-D-Fructose
OH
CH2OH
OH
OH
OH
OH
-D-Fructose
Learning Check
Write the cyclic form of -D-galactose:
H
CH2OH
O
C
H C OH
HO C H
o
OH
OH
HO C H
OH
H C OH
CH2OH
OH
-D-galactose
Review:
Reactions of aldehydes
1.
Oxidation to form carboxylic acids
2.
Reduction to form alcohols
3.
Formation of hemiacetal
4.
Hemiacetal + alcohol → acetal

Now we’ll see all of these with
monosaccharides…
1. Oxidation of
Monosaccharides


Monosaccharides are reducing sugars if their carbonyl
groups oxidize to give carboxylic acids.
In the Benedict’s text, D-glucose is oxidized to D-gluconic
acid. Glucose is a reducing sugar.
O
O
H
C
C
H C OH
HO C H
H C OH
OH
H C OH
+ Cu2+
HO C H
H C OH
H C OH
H C OH
CH2OH
CH2OH
D-Glucose
D-Gluconic acid
+ Cu2O(s)
2. Reduction of
Monosaccharides

The reduction of
the carbonyl
group produces
sugar alcohols, or
alditols

D-Glucose is
reduced to Dglucitol (also
called sorbitol)
3. Monosaccharides + alcohol

Formation of hemiacetal (cyclic structures)
4. Monosaccharide
hemiacetals + alcohol

When a cyclic monosaccharide reacts with an
alcohol:

A glycoside is produced (acetal)

The bond is a glycosidic bond or glycosidic linkage
CH2OH
CH2OH
O
O
OH
+ HOCH3
OH
glycosidic bond
O CH3
OH
+ H2O
OH
OH
OH
OH
-D-Glucose
Methanol
Methyl--D-glucoside
Learning Check
Write the products of the oxidation and
reduction of D-mannose.
O
C H
HO
H
HO
H
H
OH
H
OH
CH2OH
D-Mannose
Disaccharides

A disaccharide consists of two monosaccharides
Disaccharide
Monosaccharides
H+

Maltose + H2O
Glucose + Glucose

Lactose + H2O
Glucose + Galactose

Sucrose + H2O
Glucose + Fructose
Maltose

Malt sugar

A disaccharide in
which two D-glucose
molecules are joined
by an -1,4glycosidic bond
Obtained from starch
Used in cereals,
candies, and brewing
A reducing sugar
(has a hemiacetal)



Lactose

Milk sugar

Composed of
galactose and
glucose linked by
a -1,4-glycosidic
bond

Lactose
intolerance

A reducing sugar
Sucrose

Table sugar

Is composed of
glucose and fructose
molecules joined by
,-1,2-glycosidic
bond
Has no isomers
because mutarotation
is blocked


Not a reducing sugar
(no hemiacetal)
Sweetness of Sweeteners

Sugars and
artificial
sweeteners differ
in sweetness

Each sweetener is
compared to
sucrose (table
sugar), which is
assigned a value
of 100
Learning Check
Identify the monosaccharides in lactose, maltose,
and sucrose as glucose, fructose, and/or galactose:
A. Lactose
B. Maltose
C. Sucrose
Polysaccharides

Polysaccharides are
polymers of
monosaccharides

“Complex” carbohydrates

Important polysaccharides
of D-glucose are:

Starch (Amylose and
Amylopectin)

Glycogen

Cellulose
Starch and Glycogen

Storage polysaccharides



Form monosaccharides used for energy
Starch

Plants

Amylose is a continuous chain of glucose molecules
linked by -1,4 glycosidic bonds.

Amylopectin is a branched chain of glucose molecules
linked by -1,4- and -1,6-glycosidic bonds.
Glycogen

Humans, animals

Similar to amylopectin, but more highly branched.
Structures of
Amylose and Amylopectin
Cellulose

Structural polysaccharide


Plant cell walls (cellulose) and animal exoskeletons (chitin)
Cellulose is a polymer of glucose molecules linked by -1,4glycosidic bonds

Enzymes in saliva can hydrolyze -1,4-glycosidic bonds in starch,
but not -1,4-glycosidic bonds in cellulose
Learning Check
Identify the types of glycosidic bonds in:
1) Amylose
2) Glycogen
3) Cellulose
End of Chapter 18!