Food Chemistry

4.4 Polysaccharides 297

Fig. 4.12.Conformations of someβ-D-glucans. Link-
ages:a 1 → 4 ,b 1 → 3 ,c 1 →2 (according toRees,
1977)

a pectin chain (1,4-linkedα-D-galactopyranosyl-
uronate units):

(4.126)

and the same pleated conformation is shown
by an alginate chain (1,4-linked α-L-gulo-
pyranosyluronate units):

(4.127)

Ca^2 +ions can be involved to stabilize the confor-

mation. In this case, two alginate chains are as-

sembled 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 HollowHelix-TypeConformation..........................

This conformation is typical for 1,3-linkedβ-D-

glucopyranose units (Fig. 4.12, b), as occur in

the polysaccharide lichenin, found in moss-like

plants (lichens):

(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-linkedα-D-

glucopyranosyl residues) also has such a geom-

etry, and hence a helical conformation:

(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 confor-

mation of aβ( 1 → 3 )-glucan, withn= 5. 64

andh= 3 .16 Å, is shown in Fig. 4.12, b. The

helial conformation can be stabilized in var-

ious 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,