BIOMOLECULES

CARBOHYDRATES

Monosaccharides • Disaccharides
 Introduction

Carbohydrates are primarily produced by plants and form a very large

group of naturally occurring organic compounds. Some common
examples of carbohydrates are cane sugar, glucose, starch, etc. Most
of them have a general formula, Cₓ(H₂O)ᵧ , and were considered as
hydrates of carbon from where the name carbohydrate was derived.
Chemically, the carbohydrates may be defined as optically active
polyhydroxy aldehydes or ketones or the compounds which produce
such units on hydrolysis. Carbohydrates are also called saccharides.
 Classification
(on the basis of behaviour on hydrolysis)

Monosaccharides Oligosaccharides Polysaccharides

A carbohydrate that cannot be Carbohydrates that yield two to ten Carbohydrates which yield a large
hydrolysed further to give simpler monosaccharide units on hydrolysis, number of monosaccharide units on
unit of polyhydroxy aldehyde or are called oligosaccharides. hydrolysis are called
ketone is called a monosaccharide. Examples- sucrose, maltose, etc. polysaccharides. Examples-
Examples- glucose, fructose, ribose, They can be di, tri, tetrasaccharide, cellulose, glycogen, gums, etc. They
etc. etc. are also called non-sugars.

The carbohydrates may also be classified as either reducing or non-reducing sugars. All those carbohydrates which reduce
Fehling’s solution and Tollens’ reagent are referred to as reducing sugars. All monosaccharides whether aldose or ketose are
reducing sugars.
MONOSACCHARIDES
Glucose • Fructose
 Classification

Carbon Atoms General Term Aldehyde (Aldose) Ketone (Ketose)

3 Triose Aldotriose Ketotriose

4 Tetrose Aldotetrose Ketotetrose

5 Pentose Aldopentose Ketopentose

6 Hexose Aldohexose Ketohexose

7 Heptose Aldoheptose Ketoheptose

 Glucose
Preparation

From sucrose (cane sugar) From starch

If sucrose is boiled with dilute HCl Commercially glucose is
or H₂SO₄ in alcoholic solution, obtained by hydrolysis of starch
glucose and fructose are by boiling it with dilute H₂SO₄ at
obtained in equal amounts. 393 K under pressure.₁
C₁₂H₂₂O₁₁ + H₂O (C₆H₁₀O₅)ₙ + nH₂O

H⁺ H⁺ 393K, 2-3 atm

C₆H₁₂O₆ + C₆H₁₂O₆ nC₆H₁₂O₆

Glucose Fructose Glucose
 Glucose
Structure

Glucose is an aldohexose and is also known

as dextrose. It is the monomer of many of the
CHO
larger carbohydrates, namely starch,
cellulose. It is probably the most abundant (CHOH)₄
organic compound on earth. It was assigned
this structure on the basis of the following
evidences: CH₂OH
Its molecular formula was found to be C₆H₁₂O₆.

On prolonged heating with HI, it forms n-hexane, suggesting that all the six carbon
atoms are linked in a straight chain.
CHO
(CHOH)₄ + HI CH₃-CH₂-CH₂-CH₂-CH₂-CH₃
CH₂OH

Glucose reacts with hydroxylamine to form an oxime and adds a molecule of

hydrogen cyanide to give cyanohydrin. These reactions confirm the presence of a
carbonyl group.
CN
CH CHO CH=N-OH
OH
HCN NH₂OH
(CHOH)₄ (CHOH)₄ (CHOH)₄
CH₂OH CH₂OH CH₂OH
Glucose gets oxidised to six carbon carboxylic acid (gluconic acid) on reaction with a
mild oxidising agent like bromine water. This indicates that the carbonyl group is
present as an aldehydic group.
CHO COOH
Br₂ water
(CHOH)₄ (CHOH)₄
CH₂OH CH₂OH

Acetylation of glucose with acetic anhydride gives glucose pentaacetate which

confirms the presence of five –OH groups. Since it exists as a stable compound, five –
OH groups should be attached to different carbon atoms.
CHO COOH
Acetic anhydride
(CHOH)₄ (CH-O-CO-CH₃)₄
CH₂OH CH₂-O-CO-CH₃
On oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic
acid, saccharic acid. This indicates the presence of a primary alcoholic (–OH) group in
glucose.
CHO COOH COOH
oxidation oxidation
(CHOH)₄ (CHOH)₄ (CHOH)₄
CH₂OH COOH CH₂OH
 Glucose
Spatial Arrangement

The exact spatial arrangement of different -OH groups was given by Fischer
1
after studying many other properties.
CHO
Glucose is correctly named as D(+) - glucose. ‘D’ before the name of glucose
2
H OH represents the configuration whereas ‘(+)’ represents dextrorotatory nature of
3 the molecule. It should be remembered that ‘D’ and ‘L’ have no relation with the
OH H optical activity of the compound. They are also not related to letter ‘d’ and ‘l’.
4
H OH The letters ‘D’ or ‘L’ before the name of any compound indicate the relative
5
configuration of a particular stereoisomer of a compound with respect to
H OH configuration of some other compound, configuration of which is known. In the
case of carbohydrates, this refers to their relation with a particular isomer of
CH₂OH glyceraldehyde. Glyceraldehyde contains one asymmetric carbon atom and
6
exists in two enantiomeric forms as shown ahead.
 (+) Isomer of glyceraldehyde has ‘D’ configuration. It means that when its structural formula is
written on paper, the -OH group lies on right hand side in the structure. All those compounds which
can be chemically correlated to D (+) isomer of glyceraldehyde are said to have D configuration
whereas those which can be correlated to L (-) isomer of glyceraldehyde are said to have L
configuration.

In L (-) isomer, -OH group is on left hand side. For assigning the configuration of monosaccharides, it
is the lowest asymmetric carbon atom which is compared. As in (+) - glucose, -OH on the lowest
asymmetric carbon is on the right side, it is assigned D-configuration. Other asymmetric carbon
atoms of glucose are not considered for this comparison. Also, the structure of glucose and
glyceraldehyde is written in a way that most oxidised carbon (in this case -CHO) is at the top.

CHO CHO
H OH OH H
CH₂OH CH₂OH
(+) - Glyceraldehyde (-) - Glyceraldehyde
 Glucose
Cyclic Structure

The previous structure of glucose explained most of its properties but the following
reactions and facts could not be explained by this structure:

1. Despite having the aldehyde group, glucose does not give Schiff’s test and it does not
form the hydrogensulphite addition product with NaHSO₃.
2. The pentaacetate of glucose does not react with hydroxylamine indicating the
absence of free -CHO group.
3. Glucose is found to exist in two different crystalline forms which are named as α and
β. The α-form of glucose is obtained by crystallisation from concentrated solution of
glucose at 303 K while the β-form is obtained by crystallisation from hot and
saturated aqueous solution at 371 K.
 This behaviour could not be explained by the open chain structure for glucose. It was proposed
that one of the -OH groups may add to the -CHO group and form a cyclic hemiacetal structure. It
was found that glucose forms a six-membered ring in which -OH at C5 is involved in ring formation.
This explains the absence of -CHO group and also existence of glucose in two forms as shown
below. These two cyclic forms exist in equilibrium with open chain structure.

1 1
H C OH HO C H
2 2
H OH H OH
3
O 3
O
OH H OH H
4 4
H OH H OH
5 5
H H
CH₂OH CH₂OH
6 6

α - D - (+) - Glucose β - D - (+) - Glucose

The six membered cyclic structure of glucose is called pyranose structure (α- or β-), in analogy
with pyran. The cyclic structure of glucose is more correctly represented by Haworth structure
as given below.
A pair of anomers is a pair of near-identical stereoisomers that differ at only the anomeric
carbon, the carbon atom that bears the aldehyde or ketone functional group in the sugar's
open-chain form.
The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group
at C1, hence they are anomers.
6 6

CH₂OH CH₂OH

H
5
O H H
5
O OH
4
H 1 4
H 1
OH H OH H
HO 3 2 OH HO 3 2 H

H OH H OH
α - D - (+) - Glucopyranose β - D - (+) - Glucopyranose
 Fructose
Introduction and Strucure

1
Fructose is an important ketohexose. It is obtained along with glucose
CH₂OH
by the hydrolysis of sucrose. It is a natural monosaccharide found in
2 C=O fruits, honey and vegetables. In its pure form it is used as a sweetener.
3
OH H Fructose also has the molecular formula C₆H₁₂O₆ and on the basis of its
4 reactions it was found to contain a ketonic functional group at carbon
H OH number 2 and six carbons in straight chain as in the case of glucose. It
5
H OH belongs to D-series and is a laevorotatory compound. It is
appropriately written as D - (-) - Fructose. Its open chain structure is
CH₂OH shown on the left.
6
It also exists in two cyclic forms which are obtained by the addition of -OH at C5 to the
carbonyl group. The ring, thus formed is a five membered ring and is named as
furanose with analogy to the compound furan. Furan is a five membered cyclic
compound with one oxygen and four carbon atoms.

1 2 2 1

HOH₂C C OH HO C CH₂OH
3 3
OH H O OH H O
4 4
H OH H OH
5 5
H H
CH₂OH
6
CH₂OH
6

α - D - (-) - Fructofuranose β - D - (-) - Fructofuranose

The cyclic structures of two anomers of fructose are represented by Haworth
structures as shown below.

6 1 6
HOH₂C O CH₂OH HOH₂C O OH
5 2 5 2

H HO H HO
H 4 3
OH H 4 3
CH₂OH
1

OH H OH H
α - D - (-) - Fructofuranose β - D - (-) - Fructofuranose
DISACCHARIDES
Sucrose • Maltose • Lactose
 Introduction
Disaccharides on hydrolysis with dilute acids or enzymes yield two molecules of
either the same or different monosaccharides. The two monosaccharides are
joined together by an oxide linkage formed by the loss of a water molecule. Such a
linkage between two monosaccharide units through oxygen atom is called
glycosidic linkage.

In disaccharides, if the reducing groups of monosaccharides (aldehydic or ketonic

groups) are bonded, these are non-reducing sugars, like sucrose. On the other
hand, sugars in which these functional groups are free, are called reducing
sugars, like maltose and lactose.
 Sucrose
It is one of the most common disaccharides.
It’s made up of two monosaccharides, namely glucose and fructose. There is a glycosidic linkage
between C1 of α-D-glucose and C2 of β-D-fructose. Since the reducing groups of glucose and
fructose are involved in glycosidic bond formation, sucrose is a non-reducing sugar.
6

CH₂OH
1

H
5
O H HOH₂C H
O
4
H 1 2 5
OH H H HO
O
HO 3 2
3 4
CH₂OH
6

H OH
OH H
α - D - Glucose β - D - Fructose
On hydrolysis, it gives an equimolar mixture of D-(+)-glucose and D-(-)-fructose.
C₁₂H₂₂O₁₁ + H₂O C₆H₁₂O₆ + C₆H₁₂O₆

Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and

laevorotatory fructose. Since the laevorotation of fructose is more than
dextrorotation of glucose, the mixture is laevorotatory. Thus, hydrolysis of
sucrose brings about a change in the sign of rotation, from dextro (+) to laevo (–)
and the product is named as invert sugar.
 Maltose
Maltose is composed of two α - D - Glucose units in which C1 of one
glucose (I) is linked to C4 of another glucose unit (II). The free aldehyde
group can be produced at C1 of second glucose in solution and it shows
reducing properties so it is a reducing sugar.
6 6

CH₂OH CH₂OH

H
5
O H H
5
O H
4
H 1 4
H 1
OH H OH H
O
HO 3 2 3 2
OH

H OH H OH
α - D - Glucose α - D - Glucose
 Lactose
It is more commonly known as milk sugar since this is found in milk. It is
composed of β - D -galactose and β - D - glucose. The linkage is between
C1 of galactose and C4 of glucose. Free aldehyde group may be
produced at C1 of glucose unit, hence it is also a reducing sugar.
6 6

CH₂OH CH₂OH

HO
5
O H
5
O OH
4
H 1 O 4
H 1
OH H OH H
H 3 2
H 3 2
H

H OH H OH
β - D - Galactose β - D - Glucose
THANK YOU!
Agrim Garg
Mahi Sheoran
Saumya Singh
Tanisha Dabas
Tara Hazra