Page 19 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: FPP 2nd Revised Pages
Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

Biopolymers 223

Within a sample of a complex polysaccharide there can be structures can exclude water from the neighborhood of
molecules differing in length and distribution of branches, the sugar hydroxyl groups and render the polysaccharides
and in distribution and number of each kind of monomer insoluble.
residue.Becauseofthisheterogeneityithasbeenmoredif- When structures are not so regular, however, polysac-
ficult to study the three-dimensional structure of polysac- charides may be able to form gels, characterized by an
charides, for conventional investigations require crystals opennetworkthatisstabilizedbyso-calledjunctionzones,
made of large numbers of identical molecules. and where the open spaces are filled by solvent molecules.
The kinds of glycosidic bonds and linkage positions Such structures can be fairly rigid while containing as little
as well as the nature of the monosaccharide residues can as 1% polysaccharide. The junction zones are formed by
have a profound effect on the shape of the polysaccharide close association of polysaccharide chain segments hav-
molecules of which they are part. Polysaccharide chains ing a regular repeating structure, but are interrupted by
fold,forthemostpart,byrotationabouttheC OandO C segments of irregular structure. If a single polysaccha-
bonds of the interunit linkage. Free rotation is restricted, ride molecule possesses more than one stretch of chain
however, by the need for any large groups attached to with structural regularity, each regular section can partici-
the monosaccharide rings to keep as far apart as possi- pate in a junction zone involving different molecules, thus
ble. In polysaccharides where monosaccharide residues generating a network where the junction zones are sep-
give a regular repeating structure, only a small number of arated by segments of nonassociating chains of irregular
chain conformations are likely, and some polysaccharide structure.
chains are most stable with a secondary structure of ex- Chain associations may be of the types indicated in
tended ribbons, while others can coil up to give helices. A Figure 10, i.e., extended chains held together by hydro-
chain of β-(1 → 4)-linked D-glucopyranose residues, for gen bonding (Fig. 10a), chains wound round each other
example, readily takes up an extended ribbon-like shape in multiple helices (Fig. 10b), or chains containing ion-
while a chain of α-(1 → 4)-linked D-glucopyranoses can ized acid groups may associate by electrostatic attrac-
form a helix. The extended chains and helices can be tion of divalent cations such as Ca 2+ to the negatively
stabilized by intrachain hydrogen bonding involving OH charged polysaccharide (Fig. 10c). The latter is called the
groups of the sugar residues. Bundles of extended chains “egg box” model for formation of pectate and alginate
can be held together by interchain hydrogen bonding, gels.
and helices can consist of one or more chains and also Junction zones may be interrupted by insertion of a dif-
pack together in a regular way (Fig. 10). These organized ferent monosaccharide unit in the main chain, causing a
kink in the structure, or alteration to an existing sugar
residue so that the ring conformation changes, or changes
to substituents on an existing monosaccharide, also caus-
ing a ring conformational change.

2. Classification
Whereas it is relatively easy to describe different proteins
in terms of their functions, this becomes more compli-
cated for polysaccharides. Some polysaccharides seem to
fulfill both structural and reserve roles, for example. It is
more convenient, therefore, to consider polysaccharides in
groups based on structure; based, in fact, on the nature of
the monosaccharide residues making up the main polymer
chain.
Polysaccharides can then be classified according to
structure and systematic names are given, depending
FIGURE 10 Stabilization of individual chain conformational pref-
on monosaccharide composition. In some cases, how-
erences through interchain associations. (a) Extended ribbons in
parallel or antiparallel alignment; (b) multiple helices as in amylose ever, a trivial name is used, particularly for very com-
double helices or triple helices elsewhere; and (c) “egg box” model plex polysaccharides, where the name preceded knowl-
with anionic carboxyl groups cross-linked by divalent cations (•) edge of structure. Homoglycans are composed of one
with further coordination from hydroxyl groups (see Fig. 14). [From
Aspinall, G. O. “Polysaccharides” in the Encyclopedia of Physical kind of monosaccharide only; thus glucans consist of
Science and Technology, Vol. 11, p. 176. Copyright 1987 by Aca- chains of glucose residues, while xylans contain xylose
demic Press, Inc., New York.] only. They may be linear, with one or more linkage type.