298 4 Carbohydrates
Fig. 4.13. Stabilizatioin of helical conformations.
aClathrate compounds,bcoiled double or triple he-
lices,c“nesting” (according toRees, 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-TypeConformation.............................
This conformation occurs with, for example, 1,2-
linkedβ-D-glucopyranosyl residues (Fig. 4.12,
c). This is due to the wrinkled geometry of the
monomer O-bridge linkages:
(4.131)Here, the n value varies from 4 up to−2andhis
2–3 Å. The conformation reproduced in Fig. 4.12,
chasn= 2 .62 andh= 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-JointedConformation
This is typical for glycans with 1,6-linkedβ-D-
glucopyranosyl units, because they exhibit a par-
ticularly great variability in conformation.
The great flexibility of this glycan-type confor-
mation 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:
(4.132)4.4.2.5 ConformationsofHeteroglycans...........................
The examples considered so far have demon-
strated that a prediction is possible for a homo-
glycan 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 byι-carrageenan, in which
the β-D-galactopyranosyl-4-sulfate units have
a U-form geometry, while the 3,6-anhydro-α-D-
galactopyranosyl-2-sulfate residues have a zigzag
geometry:(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 InterchainInteractions
It was outlined in the introductory section
(cf. 4.4.1) that the periodically arranged mono-
saccharide sequence in a polysaccharide can
be interrupted by nonperiodic segments. Such