POLYSACCHARIDES By SANGH PARTAP Department of Pharmacy
Teerthanker Mahaveer University, Moradabad, U.P., INDIA
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Definition Polysaccharides are high molecular weight polymers
build up by repeated condensation of polyhydroxy aldehydes/
polyhydroxy ketones which are joined together by glycosidic
linkages, and can be hydrolysed to a large no. of
monosaccharides.
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EXAMPLES ARE CELLULOSE INULIN STARCH CHITIN HEPARIN HYALURONIC
ACID
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Types of Polysaccharides Homopolysaccharides :-also called as
homoglycans. On hydrolysis yield only one type of monosaccharide
unit. Example- cellulose, inulin, starch etc. Heteropolysaccharides
:- also called as heteroglycans. On hydrolysis yield more than one
type of monosaccharide unit. Example- - heparin, hyaluronic acid
etc.
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Cellulose Cellulose is the chief constituent of the cell wall
Of plant and most widely distributed carbohydrate. Cellulose is an
organic compound with the formula (C 6 H 10 O 5 ) n, a
polysaccharide consisting of a linear chain of several hundred to
over ten thousand (14) linked D- glucose units.
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Types of cellulose - Cellulose :- is very much less degraded
and does not dissolve in aqueous NaOH. - Cellulose:- is soluble in
aqueous NaOH but insoluble in dill. Acid. - Cellulose:- is
insoluble in both.
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Cellulose Structure and properties Cellulose has no taste, is
odorless, is hydrophilic, is insoluble in water and most organic
solvents. 1.Mol.formula of Cellulose has been found to be (C 6 H 10
O 5 ) n from analytical data. 2.When Cellulose is hydrolysed with
fuming HCL it give D- glucose in 95-96% yield.This reaction reveal
that Cellulose is made up of glucose unit.It means that the
structure of Cellulose is based on the D- glucose unit.
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Methylation,acetylation and nitration of Cellulose produce
trisubstitution product as a max. substituted product, it means
that each glucose unit present possesses 3 OH group in an
uncombined state.
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Fully methylated cellulose, when subjected to hydrolysis yield
2,3,6 tri- O-methyl D- glucose (90%) as main product and also
2,3,4,6 tetra-O- methyl D- glucose 0.6% as minor product.
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Chemical method Determination of the proportion of end group
and comparing with the total no. of unit in molecule. Haworth
methylation method:- Cellulose is completely methylated in inert
atmosphere then methylated cellulose is hydrolysed by dill. Acid to
cleave the glycosidic linkage.the nonreducing end will yield
2,3,4,6 tetra-O-methyl D- glucose whereas all other will undergo
hydrolysis to yield 2,3, tri-O-methyl D- glucose. The two
hydrolytic product are seprated by chromatographic tech. Hence by
knowing the % of tetra methyl derivative or ratio of tetra methyl-
tri methyl derivative. This method is known as the end-group
assay.
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Periodic oxidation method When cellulose is treated with sodium
periodate or periodic acid,two mole of formic acids are obtained
from reducing end whereas one mole of formic acid from the
non-reducing end. the amount of formic acid is estimated by the
titration method. This estimation gives the value of chain
length.
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Starch Starch is a carbohydrate consisting of a large number of
glucose units joined together by glycosidic bonds. This
polysaccharide is produced by all green plants as an energy
store.carbohydrateglucoseglycosidic bondspolysaccharideplants It is
the most important carbohydrate in the human diet and is contained
in such staple foods as potatoes, wheat, maize (corn), rice, and
cassava.staple foodspotatoes wheatmaizericecassava
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Pure starch is a white, tasteless and odorless powder that is
insoluble in cold water or alcohol. It is produced in the green
plant by the reaction b/w water and oxygen under catalytic effect
of chlorophyll and sunlight.
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It consists of two types of molecules: the linear and helical
amylose:-Amylose is a linear polymer made up of D-glucose
units.helicalamylose polymerglucose the branched amylopectin:
Branching takes place with (16) bonds occurring every 24 to 30
glucose units.amylopectin Depending on the plant, starch generally
contains 20 to 25% amylose and 75 to 80% amylopectin.
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Constitutions of amylose It is empirical formula is C 6 H 10 O
5. On complete hydrolysis amylose give D-glucose units.this
indicate that amylose is composed of only D-glucose
units.glucose
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Enzymatic hydrolysis amylose give maltose. Maltose is 4-o-
(-glucopyranosyl) D-glucopyranose, all the glucose unit in starch
are linked through C1 and C4. hence amylose posesses the foolowing
str. Which explain the hydrolysis product. The amylose is confirmed
by hydrolysis of its fully methylated derivative to 2,3,6
tri-o-methyl-D-glucopyranose and 0.32% of 2,3,4,6
tetra-o-methyl-D-glucopyranose
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Constitution of amylopectin Its empirical formula is C 6 H 10 O
5 On complete hydrolysis amylopectin give D-glucose units.this
indicate that amylose is composed of only D- glucose units.glucose
Fully methylated amylopectin on hydrolysis gives 2,3,6
tri-o-methyl-d-glucose and 0.32% of 2,3,4,6 tetra-o-
methyl-d-glucose.
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Differences between starch & cellulose STARCH 1. It is
reserve food material of plant and is found mainly in seeds, roots
and tubers of the plant. Wheat, maize, patatoes and rice are its
main commercial source. 2. It can be separated in two components,
amylose and amylopectin. CELLULOSE 1. It is very widely
distribution in nature as the chief component of wood and plant
fibres. Cotton, wood and juite are its main source. 2. It is single
compound which can not be separated into or more components.
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STARCH CELLULOSE 3. Both amylose and amylopectin consists of
D-(+) glucose unit linked through - glucoside linkage. 4. It gives
blue colour with iodine. 3. It is also made up of D-(+) glucose
unbrached chain but they are interlinked by - glucoside linkage. 4.
It does not give blue colour with iodine.
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Linear homopolysaccharide composed of N-acetyl glucosamine
residues in linkage The only chemical difference from cellulose is
the replacement of the hydroxyl group at C-2 with an acetylated
amino group. Chitin forms extended fibers similar to those of
cellulose, and like cellulose cannot be digested by vertebrates.
Chitin is the principal component of the hard exoskeletons of
nearly a million species of arthropodsinsects, lobsters, and crabs,
for example and is probably the second most abundant
polysaccharide, next to cellulose, in nature. CHITIN
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Chitin
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Hetropolysaccharides Hyaluronic acid Heparin Agarose Hyaluronic
acid Hyaluronic acid is an important GAG (Glycosamonoglycan) found
in the ground substance of synovial fluid of joints and vitreous
humor eye. serves as a lubricant and shock absorbant in joints.
composed of alternate unit of D-gluconic acidN-acetyl D-
glucosamine.
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Heparin Heparin is an anticoagulant( prevant blood clothing)
produced by basophils and mast cells that occur in blood, lung,
liver, kidney, spleen etc.basophilsmast cells Heparin is composed
of alternate unit of N-sulfo D- glucosamine 6-sulfate and glu
2-sulfate.
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Agarose The repeating unit consists of D-galactose ( 1
4)-linked to 3,6-anhydro-L-galactose (in which an ether ring
connects C-3 and C-6). These units are joined by ((1 3)-)
glycosidic links to form a polymer 600 to 700 residues long. A
small fraction of the 3,6-anhydrogalactose residues have a sulfate
ester at C-2.
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Uses Agarose gels are used as inert supports for the electro
pho- retic separation of nucleic acids, an essential part of the
DNA sequencing process. Agar is also used to form a surface for the
growth of bacterial colonies. Another commercial use of agar is for
the capsules in which some vitamins and drugs are packaged; the
dried agar material dissolves readily in the stomach and is
metaboli- - cally inert.
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Polysaccharides (glycans) serve as stored fuel and as
structural components of cell walls and extracellular matrix. The
homopolysaccharides starch and glycogen are stored fuels in plant,
animal, and bacterial cells. They consist of D- glucose with
linkages, and all three contain some branches. The
homopolysaccharides cellulose, chitin, and dextran serve structural
roles. Cellulose, composed of (1n4)-linked D- glucose residues,
lends strength and rigidity to plant cell walls. Chitin, a polymer
of (1n4)-linked N-acetylglucosamine, strengthens the exoskeletons
of arthropods. Dextran forms an adhesive coat around certain
bacteria. SUMMARY
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Homopolysaccharides fold in three dimensions. The chair form of
the pyranose ring is essentially rigid, so the conformation of the
polymers is determined by rotation about the bonds to the oxygen on
the anomeric carbon. Starch and glycogen form helical structures
with intrachain hydrogen bonding; cellulose and chitin form long,
straight strands that interact with neighboring strands. Bacterial
and algal cell walls are strengthened by hetero-
polysaccharidespeptidoglycan in bacteria, agar in red algae. The
repeating disaccharide in peptidoglycan is GlcNAc(1n4)Mur2Ac; in
agarose, it is D-Gal(1n4)3,6- anhydro-L-Gal.
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Glycosaminoglycans are extracellular heteropolysacch- arides in
which one of the two monosaccharide units is a uronic acid and the
other an N-acetylated amino sugar. Sulfate esterson some of the
hydroxyl groups give these polymers a high density of negative
charge, forcing them to assume extended conformations. These
polymers (hyaluronate, chondroitin sulfate, dermatan sulfate,
keratan sulfate, and heparin) provide viscosity, adhesiveness, and
tensile strength to the extracellular matrix.