Biomolecules & Polymers

LECTURE-1

Carbohydrates

Introduction

Carbohydrates (hydrates of carbon) are naturally occurring compounds having general

formula CX(H2O)y, which are constantly produced in nature and participate in many
important bio-chemical reactions.

Ex.- Glucose C6H12O6 C6(H2O)6

Fructose C6H12O6 C6(H2O)6

Cellulose and Starch (C6H10O5)n (C6(H2O)5)n

• Sucrose (Cane sugar) – C12H22O11, and

• Maltose (Malt Sugar) C12(H2O)11

But some compound which have formula according to CX(H2O)y are not known as
carbohydrate

Ex. CH2O Formaldehyde

C2(H2O)2 Acetic acid

C3(H2O)3 lactic acid

There are many compounds, which shows chemical behaviour of carbohydrate but not
confirm the general formula CX(H2O)y such as – C5H10O4 (2-deoxyribose), C6H12O5
(Rahmnose)

C7H14O6 (Rahmnohexose)

Modern Concept :

Carbohydrates are defined as polyhydroxy aldehydes or polyhydroxy ketones or

substances which give these on hydrolysis and contain at least one chiral carbon atom.
It may be noted here that aldehydic and ketonic groups in carbohydrates are not
present as such but usually exist in combination with one of the hydroxyl group of the
molecule in the form of hemiacetals and hemiketals respectively.

H2 O/H+
• Carbohydrtes → Polyhydroxy aldehyde or ketone

• In plants carbohydrates are synthesised by photosynthesis

(iii) Almost all of these compounds are chiral and optically active. An exception of this
is 1,3-dihydroxypropanone.

(iv) All natural carbohydrates have D-configuration.

Carbohydrates are often referred to as Saccharides (Latin, Saccharum = sugar) because

of the sweet taste of the simpler members of the class, the sugars.

Classification of Carbohydrates

(A) Classification of the basis of number of hydrolysed products

(i) Monosaccharide : A carbohydrate that cannot be hydrolyzed to simpler

compounds is called monosaccharide.

Monosachharide which have six carbon are either aldohexoses or ketohexoses

With a few exceptions, they have general formula, CnH2nOn. Glucose is the most
important member in their class. These are crystalline in nature, readily dissolve in
water and are sweet in taste (sugars).
+
C6H12O6 + H2O ⎯⎯→
H
No reaction.
Glucos e or fructose

(ii) Oligosaccharides : The oligosaccharides are carbohydrates which yield a definite

number (2–10) of monosaccharides molecules on hydrolysis.
 The oligosaccharides containing two monosaccharide units are called
disaccharides, and those containing three, four or five units are termed as
trisaccharides, tetrasaccharides or pentasaccharides respectively, e.g., Sucrose and
Maltose, both disaccharides, yield two molecules of monosaccharides on
hydrolysis.
+
C12H22O11 + H2O ⎯⎯→
H
C6H12O6 + C6H12O6
Sucrose Glucos e Fructose

+
C12H22O11 + H2O ⎯⎯→
H
2C6H12O6
Maltose Glucos e

Rafinose, a trisaccharide, with molecular formula C18H32O16, yields three

monosaccharide units.
+
C18H32O16 2H2O ⎯⎯→
H
C6H12O6 + C6H12O6 + C6H12O6
Raffinose Glucos e Fructose Galactose

Majority of oligosaccharides are colourless crystalline solids, soluble in water and

sweet in taste.

(iii) Polysaccharides: The polysaccharides are carbohydrates of high molecular weight

which yield many monosaccharide molecules on hydrolysis. Examples are starch
and cellulose, both of which have molecular formula, (C6H10O5)n.

(C H
6
O
10 5 n) + nH O ⎯⎯→nC H
2
H+
6 12 6
O
starch glucos e

Polysaccharides are colourless, amorphous solids having no taste and insoluble in

cold water. There are also called non-sugars.

(B) Classification on the basic of functional groups

S.No. Carbohydrate No. of functional group Examples

1 Aldose CH = O Aldehyde Glyceraldehyde,
| Erythrose, Threose,
(CHOH)n Ribose &
| 2-Deoxyribose
CH2 OH
Glucose, mannose,
Allose.
 2 Ketose CH2 OH
| n = 0; Ketotriose,
C=O Ketone n = 1; Ketotetrose,
|
n = 2; Ketopentose,
(CHOH)n
n = 3; Ketohexose,
|
CH2 OH

The carbohydrates may also be classified as either reducing or non-reducing sugars.

All those carbohydrates which have the ability to reduce Fehling’s solution and Tollen’s
reagent are referred to as reducing sugars, while other s are non-reducing sugars. All
monosaccharides and disaccharides other than sucrose are reducing sugars.

Reducing and non Reducing properties of Sugars:

(I) Reducing sugars (II) Non Reducing Sugars

1. Reduces Tollen’s reagent, Fehling’s Don’t reduce Tollen’s, Fehling’s &
solution and Benedicts’s solution Benedict’s solution
2. Should have atleast one hemiacetal or Should have acetal linkage.
hemiketal functional group.
Ex. All Mono and Oligosaccharides except Ex. All Polysaccharides and few
Sucrose Oligosaccharides (Ex. Sucrose)

Monosaccharide :

The monosaccharides are the basis of carbohydrate chemistry, since all carbohydrates
are either monosaccharides or are converted into monosaccharides on hydrolysis. The
monosaccharides are either polyhydroxy aldehydes or ketones.

These are, therefore, classified into two main groups, viz., Aldoses (
O O
|| ||
containing − C − H group ), and Ketoses ( containing − C − group ).

The aldoses and ketoses are further divided into sub-groups, on the basis of the
number of carbon atoms in their molecules as trioses, tetroses, etc.

To classify a monosaccharide completely, it is necessary to specify both, the type of

the carbonyl group and the number of carbon atom present in the molecule. Thus
monosaccharides are generally referred to as aldotrioses, ketotrioses, aldotetroses,
ketotetroses, etc.

The aldoses and ketoses may be represented by the following formulas.

Except ketotriose (dihydroxy acetone), all aldoses and ketoses contain asymmetric
carbon atoms and are optically active.

Some facts:

1. Number of carbons in monosaccharides are generally 3 to 7.

2. Simplest aldose in Glyceraldehyde and simplest Ketose is Dihydroxyacetone.

3. Most naturally occurring monosaccharides are :

(a) Pentoses E.g. D-Ribose (present in RNA) and 2-Deoxyribose (present in DNA)

(b) Hexoses E.g. D-Glucose, D-Mannose, D-Allose, D-Galactose and D-Fructose

On the basis of C-atom monosaccharides can be farther classified as, Trioses, Tetroses,
Pentoses, Hexoses.
 Table

Aldoses Ketoses
3C Tropose or Triose Aldotriose Ketotriose
4C Tetrose Aldotetrose Ketotetrose
5C Pentose Aldopentose Ketopentose
5C Including-CHO Aldopentose (Ribose)

5C Including − C − Ketopentose
||
O

6C Hexose Aldohexose Ketohexose

6C Including–CHO Aldohexose (Glucose)

6C Including − C − Ketohexose (Fructose)

||
O
 LECTURE-2

Stereochemistry of Carbohydrates :

D & L-Sugars : The series of aldoses or ketoses in which the configuration of the
penultimate C-atom (C-next to CH2-OH group) is described as D-sugars if –OH is
towards RHS & L-sugars if it is towards LHS.

Smallest carbohydrate * Aldotriose CHO

|
* Glyceraldehyde
CH − OH
|
CH2OH

Classification of Aldotetros : (i) Erythrose

(ii) Threoese

D-Aldopentose :

No. of C* = 3 (in Aldopentose)

No. of optical isomers 23 = 8

No. of D Sugars 4

No. of L Sugars 4

All Isomeric D-sugars are diastereomers.

Epimers: Diastereomers with more than one stereocentre that differ in the
configuration about only one stereocentre (other than anomeric carbon) are called
epimers.

i. D-Erythrose and L-threose are epimers.

ii. D-glucose and D-galactose are C-4 epimers and

iii. D-idose and D-talose are C-3 epimers.

iv. D-glucose and D-mannose are C-2 epimers.

v. Epimerisation of glucose at C-2 gives mannose.

vi. Epimerisation of glucose at C-3 gives allose.

vii. Epimerisation of glucose at C-4 gives galactose.

Another example with C2 epimeric carbon is

Epimerisation [The conversion of one epimer into another epimer]

The process of formation of epimer is called epimerisation

e.g. Glucose → Mannose

Method of ascending of the sugar series : An aldose may be converted into it’s next
higher aldose eg. An aldopentose into an aldohexose.

Anomers :

Anomers are diastereomers that differ in the configuration at the acetal or hemiacetal
C atom of a sugar in its cyclic form (Anomeric carbon: A carbon bonded with two ‘O’
atoms). For example,  D(+) and -D(+) glucose are anomers. -D(–) and -D(–)
fructose are anomers.

The two sugars that differs in configuration only on the carbon that was the carbonyl
carbon in the open chain form is called as anomers  glucose and  glucose are known
as anomers their equilibrium mixture contains 36% -D-glucose, 63.8% -D-glucose
and 0.2% open chain form.

C1 Carbon is known as anomeric carbon.

Haworth suggested to write  glucose and  glucose in pyran structure

Anomers are epimers but epimers may not be anomers.

 Chair conformation of D-glucose

Chair forms of (conformation)  and  D-Glucose :

-D-Glucose (most stable glucose form) all groups are equatorial.

-D-Glucose –OH group at anomeric carbon is axial.

Anomeric effect :

-D-glucose is more stable then -D glucose because there is more room for a
substituent in the equatorial position. However when glucose reacts with an alcohol to
form a glucoside, the major product is the -glucoside. The preference for the axial
position by certain substituents bonded to the anomeric carbon is called anomeric
effect.

What is responsible for the anomeric effect? One clue is that all the substituents that
prefer the axial position have lone pair electrons on the atom bonded to the ring. The
lone pair electrons of the anomeric substituent have repulsive interaction with the lone
pair electron of the ring oxygen if the anomeric substituent is the -position, but not
if it the -position.

Apparently attractive interaction of the hydrogen of the anomeric OH group of D-

glucose with the lone pair electron of the ring oxygen decrease the importance of the
anomeric effect making -D glucose more stable than -D glucose. However, when
the hydrogen is replaced by an alkyl group, the anomeric effect decreases the stability
of the -position so, -glycosides are more stable than -glycosides.

Mutarotation

Specific rotation of alpha glucose + 112° Specific rotation of beta glucose

+19°
 Equilibrium mixture
[]D = 52.5degree mLg-1dm–1

When pure -D glucose is dissolved in water its specific rotation is found to be +112°
with time, however the specific rotation of the solution decreases ultimately reaches
stable value of +52.5°. When  D-glucose is dissolved in water, it has a specific rotation
of 19°. The specific rotation of this solution increases with time also to +52.5°.

This change of optical rotation with time is called mutarotation. It is caused by the
conversion of  and  glucopyranose anomers into an equilibrium mixture of both.
Mutarotation is catalysed by both acid and base, but also occurs is even in pure water.
Mutarotation is characteristic of the cyclic hemiacetal form of glucose.

Mutarotation occurs first by opening of the pyranose ring to the free aldehyde form.

LECTURE-3

Structure of aldohexoses : All form of Aldose or ketose may exist in open chain
form as well as in cyclic pyranose or furanose form.
Open chain structure of mono saccharides

Carbohydrate Structure Functional Group Typical nature

D-Glucose aldehyde 3rd (L)

D-Allose aldehyde No (L)

D-Mannose aldehyde 2,3 (L)

D-Galactose aldehyde 3,4 (L)

D-Fructose Ketone 3 (L)

(ii) cyclic structure of monosaccharides

Carbohydrate Cyclic structure

Glucose

Allose

Mannose

Fructose

MONOSACCHARIDES

Glucose : Glucose is the most common monosaccharide. It is known as Dextrose

because it occurs in nature principally as the optically active dextrorotatory isomers. It
is act as a reducing agent (reduces both Fehling’s solution and ammonical silver nitrate
solution). When heated with sodium hydroxide, an aqueous solution of glucose turns
brown. It is known as dextrose and found as grapes, honey, cane sugar, starch and
cellulose.

Preparation of Glucose :

(i) By acid hydrolysis of canes sugar (a disaccharide ) :

If sucrose is boiled with dil. HCl or H2SO4 in alcoholic solution. Glucose & fructose are
obtained in equal amount.

H O/H+
C12H22O11 ⎯⎯⎯⎯
2
→ C6H12O6 + C6H12O6
Disaccharides −glucos e − fructose
Sucrose

-Glucose being much less soluble in alcohol than fructose separate out by
crystallization on cooling.

(ii) By enzymatic action over starch : Glucose is obtained by hydrolysis of starch by

boiling it with dil. H2SO4 at 393 K under pressure.

Starch ⎯⎯⎯⎯
Diastase
→Maltose ⎯⎯⎯⎯
Maltase
→ Glucose
Polysaccharides Disaccharides Monosaccharides
(C6H10O5 )
n
(C12H22O11 ) (C6H12O6 )

(b) Properties :

(i) It is white crystalline solids having melting point 146°C. It is readily soluble in water.

(ii) Glucose is sweet in taste and also optically active (dextro rotatory).

(iii) Glucose shows mutarotation.

(iv) It is sparingly soluble in alcohol but insoluble in ether.

(v) It shows characteristic of hydroxyl and aldehydic group.

Chemical Reactions :
(1) Reaction due to OH group :

Note :

(A) Acylation with acid halide or acetic anhydride gives pentaacetates which confirms
the presence of five –OH groups.

(B) After Hydrolysis product of pentamethyl derivatives, aldehyde group and hydroxy
of C5 regenerated hence hydroxy of C5 is involved in the hemiacetal formation.

(C) (i) Sugars in the form of acetals are called glycosides. (glucose → glucoside,
mannose → mannoside, ribose → riboside, fructose → fructoside etc.)

(ii) In the formation of glycosides only one mole of alcohol is required so

monosaccharides are already present in the hemiacetal form with one of the
hydroxyl group and carbonyl group.

(iii) Glycosides are non-reducing and will not show mutarotation because in neutral
and basic condition glycosides are stable (cyclic form cannot open to the free
carbonyl compound).

(iv) After acidic hydrolysis of glycosides, product form will have reducing property
and also show mutarotation.

(2) Reaction due to aldehyde :