Hollow Helix-Type Conformation

This conformation is typical for 1,3-linked P-D-glucopyranose units (Fig. 4.12, b), as occur in the polysaccharide lichenin, found in moss-hke plants (lichens)  

The formula shows that the helical conformation of the chain is imposed by a U-form geometry of the monomer linkages. Amy lose (1,4-linked a-D-glucopyranosyl residues) also has such a geometry, and hence a helical conformation  

The number of monomers per turn (n) and the 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 P(1 3)-glucan, with n = 5.64 and h = 3.16 A, 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. 

This conformation occurs with, for example, 1,2-linked p-D-glucopyranosyl residues (Fig. 4.12, c). This is due to the wrinkled geometry of the monomer 0-bridge linkages  

the n value varies from 4 up to —2 and h is 2-3 A. The conformation reproduced in Fig. 4.12, c has n = 2.62 and h = 2.19 k. 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. 

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