2009_Book_FoodChemistry

4.4 Polysaccharides 297
Ca 2+ ions can be involved to stabilize the conformation.
In this case, two alginate chains are assembled
in a conformation which resembles an
egg box (egg box type of conformation):
(4.128)
It should be emphasized that in all examples the
linear, ribbon-type conformation has a zigzag
geometry as a common feature.
4.4.2.2 Hollow Helix-Type Conformation
Fig. 4.12. Conformations of some β-D-glucans. Linkages:
a1 → 4,b1 → 3,c1 → 2 (according to Rees,
1977)
This conformation is typical for 1,3-linked β-Dglucopyranose
units (Fig. 4.12, b), as occur in
the polysaccharide lichenin, found in moss-like
plants (lichens):
a pectin chain (1,4-linked α-D-galactopyranosyluronate
units):
(4.129)
The formula shows that the helical conformation
of the chain is imposed by a U-form geometry of
the monomer linkages. Amylose (1,4-linked α-Dglucopyranosyl
residues) also has such a geometry,
and hence a helical conformation:
(4.126)
and the same pleated conformation is shown
by an alginate chain (1,4-linked α-L-gulopyranosyluronate
units):
(4.127)
(4.130)
The number of monomers per turn (n) andthe
pitch in the axial direction per residue (h) is
highly variable in a hollow helical conformation.
The value of n is between 2 and ±10, whereas
h can be near its limit value of 0. The conformation
of a β(1 → 3)-glucan, with n = 5.64
and h = 3.16 Å, is shown in Fig. 4.12, b. The
helial conformation can be stabilized in various
ways. When the helix diameter is large,
inclusion (clathrate) compounds can be formed
(Fig. 4.13, a; cf. 4.4.4.14.3). More extended or
stretched chains, with smaller helix diameter,