Page 103 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Carbohydrates 411

natural or synthetic polymers having OH groups to form
cross-linked polymers. Cross linking with cellulose af-
fords products possessing enhanced wet strength that are
used in the paper industry to make paper towels. Cross
linkage with cotton affords shrink- and crease-resistant
fabrics for the textile industry. It is also possible to re-
act the free radicals produced by oxidation of starch with
ceric ions with monomers to form graft polymers (see
Scheme 29). For example, copolymerization of oxidized
starch with polyacrylonitrile yields excellent absorbents
used in the manufacture of disposable diapers capable of FIGURE 18 Bush structure of glycogen. Note that the molecule
absorbing 5000 times their weight in water. is more symmetric than that of amylopectin.
Cross-Linked Starches: Cross-linked starches are ob-
tained by forming bridges between the hydroxyl groups
between glycogen and starch is in the source (starch is
of the starch molecule and the hydroxyl groups of an-
found in plants) and the shape of its aggregates and their
other molecule. The resulting compounds are extensively
size. Instead of starch granules, glycogen is found in the
used by the food industry as thickeners or to impart added 5
form of spheres called β-partcles, having a DP of 10
mechanical stability to prevent the food from loosing its
and aggregates of spheres called α-particles having a DP
shape during cooking. In theory any molecule possessing of 10 . The shape of the glycogen macromolecules resem-
7
two groups capable of reacting with hydroxyl groups can bles a bush or a tree (see Fig. 18). However, its agregates
be used as a bridge between starch molecules. An exam- are much more symmetric than those of amylopectin (see
ple of such bridge-forming molecules is epichlorhydrin,
Fig. 17). This is why the aggregates of starch are called
which forms distarch glycerol depicted below.
granules and those of glycogen, spheres.
Starch-OH + CH 2 -CH-CH 2 -Cl
❙
O 3. Dextrans and Glucans
Epicholorohydrin Dextrans are α-D-glucopuranose polymers linked for the
−→ Starch-O-CH 2 -CH-CH 2 -O-Starch major part through (1 → 6) acetal bonds as well as by
some (1 → 3) and (1 → 4) linkages. Dextrans are highly
OH 6
branched polymers having a DP in excess of 10 . Al-
Distarch glycerol though dextrans are obtained from a multitude of bac-
teria, most of the industrially produced products are ob-
Starch Ethers: Starch ethers are produced by reacting
tained from Leuconostoc mesenteroides and Leuconostoc
starch with ethylene or propylene oxide to the desired DS.
dextranicum.
For example, starch hydroxypropyl alcohol is produced
D-glucans are obtained from fungi and may either be
by reacting starch with propylene oxide. The nucleophilic
α-D-plucopyranose or β-D-glucopyranose polymers; they
substitution is carried out in basic media and the result-
contain mainly (1 → 3) and (1 → 4) glycosidic linkages
ing starch ether, depicted below, is widely used in paper,
and rarely (1 → 6) bonds.
textile, and food industries.
Starch-OH + CH 2 -CH-CH 3 −→ Starch-O-CH 2 -CH-CH 3 4. Cellulose
❙
O OH
Cellulose is the most abundant organic compound on
Propylene oxide Starch ether earth; its amount exceeds by far that of any other or-
ganic polymer or monomer. Cellulose is found in a nearly
pure form in cotton and flax and in much larger amounts,
2. Glycogen
but less pure form in wood, from which it can be pro-
Glycogen is a homopolysaccharide found in the liver and duced by dissolving its contaminants, namely hemicel-
muscles of animals, where it is used to store energy. Chem- lulose and lignin, with alkali. Cellulose-based industries
ically, glycogen is related to starch and closely resembles include the pulp and rayon industries where it is pro-
amylopectin. It is composed of linear chains of disubsti- duced and purified, respectively, and the paper and the
tuted(1→4)-linkedα-D-glucopyranosylresiduesattached textile industries, where it is processed. Like amylose,
to (1→6) linked branches originating from 1,4,6-tri-O- cellulose is a linear polymer made up of (1 → 4)-linked
substituted α-D-glucopyranosyl residues. The difference D-glucopyranosyl residues, which differ from those of