Download CARBOHYDRATES

Carbohydrates are the most abundant organic
constituents of plants.
They serve as the major source of chemical energy
for living organisms (e.g. sugars and starch), as well as
important constituents of supporting tissues (e.g.
cellulose).
Carbohydrates are usually defined as:
“Polyhydroxy aldehydes or ketones, or
substances that hydrolyze to yield polyhydroxy
aldehydes or ketones”.
Simple carbohydrates are also known as “sugars or
saccharides” (latin: saccharum = sugar).
Monosaccharides
These consist of only one saccharide or sugar
unit and they are non-hydrolysable.
They are subclassified according to:
1-The number of carbon atoms present in their
molecule and,
2-The type of carbonyl group they contain.
 Thus, a monosaccharide containing three carbon atoms is
called a triose and that containing five is called a pentose
and so on.
 A monosaccharide containing an aldehyde group is called
an aldose and one containing a keto group is called a
ketose.
 These two classifications are frequently combined: e.g. a
five-carbon aldose, for example, is called an
aldopentose, a six-carbon ketose is called a ketohexose
Oligosaccharides
 These consist of 2 and up to 10 molecules of
simple sugars and are hydrolysable.
 They are sub classified into di-, tri- and
tetrasaccharides etc…, according to the number
of molecules of simple sugars they yield on
hydrolysis.
Polysaccharides
Polysaccharides are high molecular weight
polymers of monosaccharides of very complex
nature.
They are hydrolysable and yield a large number
of monosaccharides.
Different groups of polysaccarides are distinguished
according to the final products of hydrolysis:
1-The homosaccharides (or holosides)which yield
on hydrolysis similar simple sugar units (units of
the same monosaccharide). Those, which yield
pentose sugar units on hydrolysis, are called
pentosans and similarly those yielding hexose
units are called hexosans.
2-The heterosaccharides (or heterosides), which yield
upon hydrolysis dissimilar sugar units e.g. mixture of
hexoses and pentoses and thus called pentohexosans.(put example)
3-The derived carbohydrates, which yield on
hydrolysis monosaccharides in addition to other
components such as uronic acids, sulfate esters or
amino sugars.
Physical Characters of CHO
1-Condition: monosaccharides and most disaccharides are white,
crystalline in shape and with sharp melting points.
2-Taste: Most of the simple and low molecular weight sugars have a
sweet taste.
3-Solubility

Monosaccharides are soluble in cold water and hot alcohol.

Starch, pectin, mucilages and glycogen are difficulty soluble
in cold water, but more soluble in hot water and insoluble in
alcohol.
4-Optical activity
 Monosaccharides and water-soluble oligosaccharides are optically
active and determination of their specific rotation is useful for their
identification.
 A compound is optically active when, in solution, it is capable to rotate the
plane of polarized light either to right (dextrorotatory, + or d) or to the
left (levorotatory, - or l) and the two stereoisomers are called
enantiomers. This optical activity is due to presence of a chiral center in
the molecule.
Mutarotation
 When a sugar is dissolved in water, the specific rotation of
the solution gradually changes until it reaches to a
constant value, e.g. freshly prepared solution of glucose has
a specific rotation +112 o. When this solution is allowed to
stand (aged solution), the rotation falls to + 52.7 o, and remains
constant at this value.
 This change in the specific rotation is known as the
mutarotation phenomenon. All reducing sugars (except a few
ketoses) undergo mutarotation.(why does it happen?)
Reactions of carbohydrates:

Reactions similar to carbonyl compounds
Glycoside formation ( acetal formation )
Each carbonyl group (aldehydic or ketonic) reacts with two
molecules of alcohol to give an acetal.
O
ROH
OH
OR
Carbonyl compound
Hemiacetal
ROH
OR
OR
Acetal
+ H2O
 Oxidation
These reactions produce different products according
to the reagent used.
i-Bromine water; is a mild oxidizing reagent. It selectively
oxidizes the -CHO group into -COOH, and converts
aldoses to the corresponding aldonic acids
ii-Nitric acid; is a strong oxidizing agent. It oxidizes both
the -CHO and terminal -CH2OH of an aldose to -COOH
groups, and these dicarboxylic acids are known as
aldaric acids.
CHO
H
HO
CHO
COOH
OH
H
H
Br2/H2O
HO
OH
H
H
HO
COOH
OH
H
H
HNO3
OH
HO
H
H
OH
H
OH
H
OH
H
OH
H
OH
H
OH
H
OH
H
OH
CH2OH
Glucose
CH2OH
Gluconic acid
CH2OH
Glucose
COOH
Glucaric acid
iii-Controlled oxidation: this is carried out by first protecting the -CHO
group, followed by oxidation of the -CH2OH group, or in one step by the
aid of a specific enzyme to give alduronic acid, e.g. oxidation of
glucose into glucuronic acid.
 Reduction
Aldoses and ketoses can be reduced with sodium
borohydride into the corresponding alditols.
Examples: glucose gives glucitol (-CHO is converted to –
OH) or sorbitol, mannose gives mannitol, galactose gives
dulcitol.
Specific chemical reactions:
 Reaction with phenylhydrazine (Osazone formation):
The carbonyl group of an aldose reacts with phenylhydrazine
to give a crystalline phenyl osazone.
 Osazones are used in identification of mono- and
disaccharides as they differ in physical characters (e.g. m.p.
and shape of crystals).
 Monosaccharides give the same crystalline osazone on hot,
while reducing disaccharides (lactose and maltose) give
different crystalline shapes and on cold.
Maltosazone
Lactosazone
Glucosazone
 Action of mineral acids
Treatment with hot concentrated mineral acid
(HCl or H2SO4) leads to dehydration of sugars.
Pentoses and methylpentoses give furfural and
methylfurfural, respectively by the action of hot
hydrochloric acid, while ketoses and aldoses give
hydroxymethylfurfural.
The formation of furfural derivatives is the basis of the
color reactions used for qualitative and quantitative
determination of monosaccharides by coupling with
phenolic compounds or amines.
Examples:
 In Molisch’s test the phenol used is -naphthol.
 In Seliwanoff’s test (rapid furfural) the phenol is resorcinol.
 Reaction with oxidising cations
 All monosaccharides and reducing disaccharides are
readily oxidized by metal ions such as Cu+2 (Fehling’s
reagent).
 These reactions are used for identification and
quantification of reducing sugars.
Quantitative analysis of carbohydrates:
Phenol-Sulfuric Acid Method
Carbohydrates are destroyed by heat and acid. They
are particularly sensitive to strong acids and high
temperatures. Under these conditions, a series of complex
reactions take place, beginning with a simple dehydration
reaction.
Phenol-Sulfuric Acid Method
 Continued heating in the presence of acid produces various furan
derivatives. They will condense with various phenolic compounds, such
as phenol, resorcinol, orcinol, to produce colored compounds that are
useful for carbohydrate analysis.
 This method is simple, rapid, sensitive, accurate, specific for
carbohydrates, and widely applied. The reagents are inexpensive, readily
available, and stable. A stable color is produced, and concentration could
be obtained from a standard calibration curve. The results are
reproducible.
Specific Analysis of Mono and Oligosaccarides
 High Performance liquid Chromatography is the method of
choice for analysis of mono- and oligosaccharides and can be
used for analysis of polysaccharides after hydrolysis.
 HPLC gives both qualitative analysis(identification of the
carbohydrate) and also allows for quantitative analysis.
HPLC analysis is rapid, can tolerate a wide range of sample
concentrations, and provides a high degree of precision and
accuracy.
Monosaccharides
The common monosaccharides belong to three groups:
 The pentoses,
 The hexsoses, and
 The deoxysugars (as deoxyribose in DNA).
Pentoses
 These are simple sugars containing five carbon atoms.
 They give furfural when warmed with dilute acids.
They are rarely present in the free state but generally occur as
pentosans or form the sugar moiety of glycosides.
Examples of pentoses:
-D-Ribose
found in all plant and animal cells as the carbohydrate
part of nucleic acids e.g. ribonucleic acid (RNA).
-D-Xylose (or wood sugar)
prepared from corncobs (thick cylindrical part on which
the grains grow), or any woody material.
-L-Arabinose (or pectin sugar ): found in gums, and forms
the sugar part of several glycosides.
Hexoses
Properties
These contain six carbon atoms.
They occur free, combined as oligo- and
polysaccharides, or forming the sugar part of
glycosides.
Examples
Two groups of commonly occurring hexoses are
distinguished:
The aldohexoses such as glucose, mannose and
galactose.
The ketohexoses such as fructose.
CHO
H
HO
OH
CH 2OH
O
OH
OH
H
H
OH
H
OH
O
HO
HO
-D-glucose
CH 2OH
O
CH 2OH
HO
H
H
H
OH
H
OH
CH 2OH
O
H
H
OH
H
OH
HO
HO
H
CH 2OH
OH
OH
-D-galactose
OH
O
HO
HO
O
CH 2OH
CHO
OH OH
OH
-D-mannose
HO
OH
CH 2OH
H
OH
OH
H
HO
OH
CH 2OH
O
CHO
HO
H
O
-D-glucose
H
OH
HO
OH
HO
HO
CH 2OH
O
CHO
OH
O
HO
HO
OH
OH
-L-galactose
OH
-D-Glucose (dextrose, grape sugar, blood sugar)
Source
D-glucose is found free in sugar juices or forms the sugar part of many
glycosides.
Glucose can also be obtained from honey, which contains a mixture of
glucose, fructose and sucrose, or by inversion of sucrose.
Properties
 D-Glucose is white crystalline, sweet, readily soluble in water.
 It reduces Fehling's and Barfoed's solutions.
 It gives an osazone that crystallizes on hot. Glucosazone occurs in the
form of golden yellow, needle -shaped crystals.
Uses
It has a great pharmaceutical importance as
ingredient in i.v. injections, as nutrient, diuretic and
sweetening agent. It is used in ice-cream and candy
industries.
Ketohexoses
β-D-Fructose (levulose or fruit sugar)
Source
It is found free in honey and in fruits juices, or as constituent of
polysaccharide e.g. inulin.
Properties
 Fructose occurs as crystals with intense sweet taste.
 It reduces Fehling's, and Barfoed's solutions
 It forms an osazone similar to that of glucose
 It gives a positive Seliwanoff’s test for ketoses (rapid furfural).
Uses
Fructose is used as food for diabetics and in infant
feeding formulae (more easily digested than
glucose).
 Diabetes is a disorder affecting the way the body produces and uses insulin
and how it handles blood glucose. Insulin is essential for aiding glucose
transport into cells. People with type I diabetes do not produce insulin,
whereas those with type II diabetes either do not produce enough insulin or
cannot efficiently use the insulin their bodies make. Factors such as
overweight and obesity, lack of physical activity, and genetic predisposition
all increase the risk for type II diabetes.
People with diabetes must pay attention to the amount of all
carbohydrates—sugars and starches—they consume.
 Because fructose does not require insulin, individuals with diabetes can
often tolerate it better than other sugars.
Disaccharides
According to the position of the linkage between the sugar
units, disaccharides are classified into non-reducing such as
sucrose and reducing such as maltose and lactose.
Non-reducing disaccharides
Sucrose (saccharose, cane sugar or beet sugar)
Source
Sucrose is the most widely occurring disaccharide, it is found in many fruit
juices, seeds, leaves, roots and honey.
Properties
 It has a sweetening power more than glucose and less than fructose.
 On heating from 200 to 250oC, sucrose changes into an amorphous brown
substance known as caramel (a decomposition product widely used as
flavoring and coloring matter).
 It gives positive results with cobalt nitrate test (violet).
 It does not reduce Fehling’s solution. It does not form an osazone.

The enzymatic or dilute acid hydrolysis of sucrose is called “inversion”
due to the fact that: sucrose ([α]25D= +66.5o) is hydrolyzed to an
equimolecular mixture of D-(+)-glucose ([α]25D= +52.7o) and D-(-)fructose ([α]25D = -92o), because of the high negative rotatory power of
fructose, the final solution has [α]25D= - 20.4o .

The sign of rotation being changed from (+) in the original solution of
sucrose into (-) in the hydrolyzed solution, the process is called inversion.
Uses of sucrose
In Pharmaceutical industries, sucrose is used in
syrup preparation and tablet manufacture. It is
used as nutrient and demulcent.
Reducing disaccharide
Maltose, or malt sugar, is a disaccharide formed from two units of
glucose joined with an α(1→4) linkage.
 It is the second member of an important biochemical series of glucose
chains. The addition of another glucose unit yields maltotriose; further
additions will produce dextrins (also called maltodextrins) and eventually
starch (glucose polymer).
Lactose (or milk sugar)
Source
Lactose is the principal sugar of mammalian milk and
is not present in higher plants.
Structure
It consists of galactose and glucose, linked by a
β(1— 4) linkage .
Preparation
Lactose is obtained from whey (a by-product from
cheese manufacture) after concentration, upon which
deposits of lactose crystallize out.
Uses
Lactose is used as nutrient in infant foods, since it is
less sweet than sucrose and more easily digested. It
is also used as inert diluent for other drugs.
Homosaccharides (Homopolysaccharides or
holosides)
Examples
The most common holosaccharides are:
The glucans (glucosans) such as starch, dextrin ,
glycogen and cellulose.
The fructosans such as inulin
Starch
Source
 Starch is the most abundant and widely distributed
plant substance next to cellulose and hemicellulose.
 It occurs as microscopic granules in the seeds, fruits,
tubers and roots of plants.
 The most common commercial sources are the
graminaceous fruits (e.g. rice and wheat ), maize and
potato tubers.
Structure
 It is an α-glucan ( or glucosan ) polysaccharide.
 The final product of hydrolysis is glucose.
 On heating starch with water, the starch granules
swell and produce a colloidal suspension from which
two major components can be isolated: amylose
and amylopectin.
Uses
Starch has a wide variety of applications in food and
drug industries it is used as:
 Antidote for iodine poisoning.
 Diluent in powders and tablets manufacture.
 Nutrient, demulcent, protective and adsorbent.
 Starting material in the manufacture of glucose, liquid
glucose, maltose, and dextrins.
The Mucopolysaccharides
 Mucopolysaccharides are polysaccharides which on
hydrolysis yields amino sugar units.
 Amino sugars are derived from monosaccharides by
replacement of a hydroxyl group by an amino group.
ex. Heparin
Source
Heparin is the powerful blood anticoagulant.
Uses
Heparin and heparin analogues are used as
anticoagulants.
They have multiplicity of biological activities such as,
anti-inflammatory, anti- cancer, antiviral, as well
as in treatment of Alzheimer’s disease.
Biologically Active Carbohydrates
 According to recent researches carbohydrates promise to
be a major source of drug discovery. The diversity and
complexity of carbohydrates explain their wide range of
biological functions.
 There are several established carbohydrates-based
products with “biopharmaceutical” application, as well as,
other new products with potential application in medicine,
e.g., development of specific cancer vaccines, new nonsteroidal anti-inflammatory drugs, and many other
examples.
I-Antibiotics
 This class of useful and potential therapeutics has gained
attention with the appearance of complex nucleoside
antibiotics, which exhibit a variety of biological activities. Of
these are natural the thiosugars, aminoglycosides and
macrolide antibiotics produced by various species of bacteria
and fungi.
II-Antitumour agents
 Only some non-toxic antitumour polysaccharides
derived from bacteria, fungi, and algae have
demonstrated good antitumour activity.
 Example (1-3)- β-D-glucans, such as lentinan
isolated from the fruiting body of Lentinus edodes.
 Schizophyllan (Sonifilan®) is an extracellular of product of
the fungus Schizophyllum commune. It is an
immunostimmulant related to lentinan.
 It is useful in combination with other antineoplastic
treatments in the management of carcinomas of the
lungs, stomach, uterus and breasts.
III-Laxatives:
Lactulose is used in treatment of chronic
constipation.
IV-Sucralfate is an aluminum hydroxide complex
of sucrose sulfate that is used in the therapy of
duodenal ulcers.
V-Sweeteners
 Carbohydrates play an important factor in increasing the incidence of
diabetes, obesity and dental caries.
 There is increasing need for other alternatives to sucrose as
sweetening agents for medical purposes, especially in case of
diabetes, and for diet improvement.
 These agents should have high solubility in water, good stability, and a
relative sweetness close or equivalent to that of sucrose.
 They should be safe, low caloric, and non-carcinogenic.
Sweeteners are either of “natural” or “artificial” origin:
Natural sweeteners
Natural sweeteners are classified into two broad classes:
 The bulk sweeteners, having sweetening effect nearly similar to
sucrose.
 The intense sweeteners having sweetening effect many times as that
of sucrose.
Bulk sweeteners
 These are the traditional sweeteners such as sucrose,
glucose, fructose and the polyols or sugar alcohols e.g.
sorbitol, mannitol, xylitol and lactitol.
Intense sweeteners
 These are either synthetic e.g. saccharin, aspartame,
cyclamate or natural e.g. steviol glycosides, glycyrrhizin.
Steviol glycosides
 These are a group of diterpene glycosides obtained from
Stevia rebaudiana, Family Asteraceae.
 The most important of which is stevioside. They are 100-
300 times sweeter than sucrose.
Glycyrrhizin
 This is a triterpene glycoside isolated from Glycyrrhiza
glabra, Family Leguminosae.
 It is an example of intense sweetener, sweetness 50 times
as sucrose.