2009_Book_FoodChemistry

4.4 Polysaccharides 299
sequence interferences result in conformational
disorders. This will be explained in more detail
with ι-carrageenan, mentioned above, since it
will shed light on the gel-setting mechanism of
macromolecules in general.
Initially, a periodic sequence of altering units of
β-D-galactopyranose- 4-sulfate (I, conformation
4 C 1 )andα-D-galactopyranose-2,6-disulfate (II,
conformation 4 C 1 ) is built up in carrageenan
biosynthesis:
Fig. 4.14. Schematic representation of a gel setting process
(according to Rees, 1977)
(4.134)
When the biosynthesis of the chain is complete,
an enzyme-catalyzed reaction eliminates sulfate
from most of α-D-galactopyranose-2,6-disulfate
(II), transforming the unit to 3,6-anhydro-α-
D-galactopyranose-2-sulfate (III, conformation
1 C 4 ). This transformation is associated with
a change in linkage geometry. Some II-residues
remain in the sequence, acting as interference
sites. While the undisturbed, ordered segment of
one chain can associate with the same segment
of another chain, forming a double helix, the
nonperiodic or disordered segments can not
participate in such associations (Fig. 4.14).
In this way, a gel is formed with a threedimensional
network in which the solvent
is immobilized. The gel properties, e. g., its
strength, are influenced by the number and distribution
of α-D-galactopyranosyl-2,6-disulfate
residues, i.e. by a structural property regulated
during polysaccharide biosynthesis.
The example of the τ-carrageenan gel-building
mechanism, involving a chain–chain interaction
of sequence segments of orderly conformation,
interrupted by randomly-coiled segments corresponding
to a disorderly chain sequence, can
be applied generally to gels of other macromolecules.
Besides a sufficient chain length, the
structural prerequisite for gel-setting ability is
interruption of a periodic sequence and its orderly
conformation. The interruption is achieved by
insertion into the chain of a sugar residue of
a different linkage geometry (carrageenans,
alginates, pectin), by a suitable distribution of
free and esterified carboxyl groups (glycuronans)
or by insertion of side chains. The interchain
associations during gelling (network formation),
which involve segments of orderly conformation,
can then occur in the form of a double helix
(Fig. 4.15,a); a multiple bundle of double helices
(Fig. 4.15,b); an association between stretched
ribbon-type conformations, such as an egg box
model (Fig. 4.15,c); some other similar associations
(Fig. 4.15,d); or, lastly, forms consisting
of double helix and ribbon-type combinations
(Fig. 4.15,e).
Fig. 4.15. Interchain aggregation between regular conformations.
a Double helix, b double helix bundle,
c egg-box, d ribbon–ribbon, and e double helix, ribbon
interaction