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Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

Biopolymers 229

culture medium for bacteria. Even stronger gels can be ob- carbon 2, is postively charged and can associate with neg-
tained in the presence of locust bean galactomannan (Sec- atively charged ions. It is therefore used as a flocculent for
tion II.B.3). waste-water treatment. Chitosan and derivatives such as
Not all seaweed polysaccharides contain disaccharide N-carboxymethyl chitosan are good chelating agents and
repeats. Alginic acid, the major skeletal material of brown can be used to remove transition metals in water reclama-
algae, may well be the marine equivalent of the pectins. It tion projects.
is a (1 → 4)-linked polymer of β-D-mannuronic acid and
α-L-guluronic acid (which differs from β-D-mannuronic
5. Microbial Polysaccharides
acid in having a mirror-image arrangement of substituents
at carbon 5). The monomer residues are believed to occur Dextrans are extracellular polysaccharides synthesized
in blocks (i.e., a stretch of chain containing one monomer from sucrose by bacteria of species Leuconostoc, Lacto-
only, followed by a stretch containing the other) as well bacillus, and Streptococcus. The molecules are branched,
as in alternating sequences. The guluronic acid blocks are usually with main chains of α-(1 → 6)-linked glucopyra-
thought to give a regular egg-box structure in the presence nose units and branches attached through α-(1 → 2), α-
of common metal ions such as calcium, while the man- (1 → 3), or α-(1 → 4) bonds. Many different dextrans are
nuronic acid blocks are unable to do this (Fig. 14). The found, the structure depending on the strain of bacteria
regular structures form the cross-links, or so-called junc- from which they are obtained. Some dextrans are water
tion zones, of stable gels; again this gel-forming ability soluble and again give highly viscous solutions. Strepto-
makes the alginates important in the food industry. coccus oral bacteria produce dextran which becomes a
Chitin, while being common in insects and fungi, is ex- major component of dental plaque. The polysaccharide
tracted primarily from the shells of crustaceans such as holds the bacteria near the tooth surface and traps nutri-
crabs and lobsters. It is a linear polymer of (1 → 4)-linked ents required for bacterial metabolism. Sugar cane and
2-acetamido-2-deoxy-β-D-glucopyranose (N-acetyl glu- sugar beet can become infected by dextran-synthesizing
cosamine, Fig. 8e) residues and is insoluble in most sol- bacteria, and the resultant polysaccharides can interfere
vents. It is extracted with dilute acid followed by hot, with sugar refining by blocking pipelines. On a commer-
dilute alkali, and the process brings about deacetylation cial scale, Leuconostoc dextrans have been used as blood
to give a polysaccharide of low acetyl content known as plasma extenders, and cross-linked derivatives are use-
chitosan. Chitosan, because of the free amino groups on ful in modern laboratories as molecular sieves—materials
which separate molecules on the basis of size. As such,
they play an important part in the laboratory purification
of other biopolymers, particularly proteins.
Xanthan is a widely used polysaccharide, produced ex-
tracellularly by Xanthomonas campestris. The molecules
have a cellulose-like backbone of β-(1 → 4)-linked glu-
cose residues, but side chains are attached at carbon 3 of
every second glucose residue. The side chains themselves
have the structure
β-D-Man p-(1 → 4)-β-D-Glc pA-(1 → 2)-

α-D-Man p-(1 →
The second mannose is usually acetylated at carbon
6 while a proportion of the first mannose units carry a
pyruvic acid residue linked to carbons 4 and 6. The
polysaccharide is therefore negatively charged, except at
low pH, and gives solutions of high viscosity, stable over
a wide range of temperature. In addition, the viscosity de-
creases markedly with high shear, and the polysaccharide
FIGURE 14 “Egg box” model for calcium ion-induced dimeriza-
tion of α-L-guluronan regions of alginic acid chains. Oxygen atoms is of special importance in drilling muds. Xanthan gum
involved in cation chelation are represented by filled circles. [Modi- has wide use in the food, pharmaceutical, and cosmetic
fied with permission from Thom, D., Grant, G. T., Morris, E. R., and industries as a thickener and stabilizer of suspensions and
Rees, D. A. (1982). Carbohydr. Res. 100, 29–42. Copyright 1980
by Elsevier, Amsterdam. From Aspirall, G. O. “Polysaccharides” emulsions. It forms strong gels with the galactomannans,
in the Encyclopedia of Physical Science and Technology, Vol. 11, guar and locust bean gum, and is used in this way in the
p. 180. Copyright 1987 by Academic Press, Inc., New York.] food industry.