Page 317 - Carrahers_Polymer_Chemistry,_Eighth_Edition

280 Carraher’s Polymer Chemistry

Meat Conalbumin
Bacon Ovomucoid
Mucins
Globins

9.1 POLYSACCHARIDES
Carbohydrates are the most abundant organic compounds, constituting three-fourths of the dry
weight of the plant world. They represent a great storehouse of energy as a food for humans and
animals. About 400 billion tons of carbohydrates are produced annually through photosynthesis,

dwarfing the production of other natural polymers, with the exception of lignin. Much of this syn-
thesis occurs in the oceans, pointing to the importance of harnessing this untapped food, energy,
and renewable feedstocks storehouse.
The potential complexity of even the simple aldohexose monosaccharides is indicated by the
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presence of five different chiral centers, giving rise to 2 or 32 possible steroisomeric forms of the

basic structure, two of which are glucose and mannose. While these sugars differ in specifi c biolog-
ical activity, their gross chemical reactivities are almost identical, permitting one to often employ
mixtures within chemical reactions without regard to actual structure. Their physical properties are
also almost the same, again allowing for the mixing of these structures with little loss in physical
behavior.
Carbohydrates are diverse with respect to occurrence and size. Familiar mono and disaccharides
include glucose, fructose, sucrose (table sugar), cellobiose, and mannose. Familiar polysaccharides
are listed in Table 9.1 along with their source, purity, and molecular weight range.
The most important polysaccharides are cellulose and starch. These may be hydrolyzed to lower
molecular weight carbohydrates (oligosaccharides) and finally to d-glucose. Glucose is the build-

ing block for many carbohydrate polymers. It is called a monosaccharide since it cannot be further
hydrolyzed while retaining the ring. Three major types of representations are used to refl ect sac-
charide structures. These are given in Figure 9.1. Here, we will mainly use the Boeseken-Haworth
planer hexagonal rings to represent polysaccharide structures.
Simple sugars exist in both cyclic and linear forms. Intramolecular nucleophillic reactions occur
creating equilibrium combinations of these linear and cyclic forms. Monosaccharides are classifi ed
according to certain characteristics. Here we will deal only with the cyclic designations since the
polysaccharides are cyclic in nature. The only exception is the nature of the end groups that may
be cyclic or linear. For our purposes these characteristics are the stereo placement of the alcohol
groups on the ring; the size of the ring; and the placement of the ether linkage. Figure 9.2 contains
cyclic representations of glucose, which contains six atoms in the ring so it is a hexose. With the
exception of carbons 1 and 6, the carbons are stereocenters or stereogenic, making d-glucose one
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of 2 = 16 possible stereoisomers.
Of particular importance is the placement of the hydroxyl group at carbon 1 since many of the
most important complex or polysaccharides are linked through ether formation at this hydroxyl
group. Assuming the bond between carbons 5 and 6 is coming toward you (out of the page), the
linkage at carbon 1 is called the α form since it is going away from us or into the page; that is, it
is on the opposite side of the bond between carbons 5 and 6; while β is the other form where the
hydroxyl at carbon 1 is on the same side as the bond between carbons 5 and 6. In some ways, con-
sidering only the placement of carbon 6 relative to the hydroxyl on carbon 1, the α form is trans
while the β form is cis.
Kobayashi and others have pioneered in the synthesis of synthetic polysaccharides. Polysaccharide

synthesis is particularly difficult because of the need to control the stereochemistry of the anomeric
carbon and regioselectivity of the many hydroxyl groups with similar reactivities. This problem was
initially solved employing enzymatic polymerizations with the synthesis of cellulose in 1991 when the

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