2009_Book_FoodChemistry

298 4 Carbohydrates
Fig. 4.13. Stabilizatioin of helical conformations.
a Clathrate compounds, b coiled double or triple helices,
c “nesting” (according to Rees, 1977)
can form double or triple stranded helices
(Fig. 4.13, b; cf. 4.4.4.3.2 and 4.4.4.14.3), while
strongly-stretched chains, in order to stabilize the
conformation, have a zigzag, pleated association
and are not stranded (Fig. 4.13, c).
4.4.2.3 Crumpled-Type Conformation
This conformation occurs with, for example, 1,2-
linked β-D-glucopyranosyl residues (Fig. 4.12,
c). This is due to the wrinkled geometry of the
monomer O-bridge linkages:
4.4.2.5 Conformations of Heteroglycans
(4.132)
The examples considered so far have demonstrated
that a prediction is possible for a homoglycan
conformation based on the geometry
of the bonds of the monomer units which
maintain the oxygen bridges. It is more difficult
to predict the conformation of a heteroglycan
with a periodic sequence of several monomers,
which implies different types of conformations.
Such a case is shown by ι-carrageenan, in which
the β-D-galactopyranosyl-4-sulfate units have
a U-form geometry, while the 3,6-anhydro-α-Dgalactopyranosyl-2-sulfate
residues have a zigzag
geometry:
(4.131)
Here, the n value varies from 4 up to −2 andhis
2–3 Å. The conformation reproduced in Fig. 4.12,
chasn = 2.62 and h = 2.79 Å. The likelihood
of such a disorderly form associating into more
orderly conformations is low. Polysaccharides of
this conformational type play only a negligible
role in nature.
4.4.2.4 Loosely-Jointed Conformation
This is typical for glycans with 1,6-linked β-Dglucopyranosyl
units, because they exhibit a particularly
great variability in conformation.
The great flexibility of this glycan-type conformation
is based on the nature of the connecting
bridge between the monomers. The bridge has
three free rotational bonds and, furthermore, the
sugar residues are further apart:
(4.133)
Calculations have shown that conformational
possibilities vary from a shortened, compressed
ribbon band type to a stretched helix type. X-ray
diffraction analyses have proved that a stretched
helix exists, but as a double stranded helix in
order to stabilize the conformation (cf. 4.4.4.3.2
and Fig. 4.19).
4.4.2.6 Interchain Interactions
It was outlined in the introductory section
(cf. 4.4.1) that the periodically arranged monosaccharide
sequence in a polysaccharide can
be interrupted by nonperiodic segments. Such