Curdlan helix structure

Accordingly, the gel networks of linear Curdlan consist mainly of molecular chains with a single helix structure. The triple helix content is at most 10%. The crosslink structure is formed by association of these triple and single helices. On the other hand, the main structure of the branched glucan is a triple helix and the partial association of these chains functions as a crosslink structure. Thus, the model based on x-ray diffraction data of the thermally treated Curdlan, stating that the crosslink structure consists of a triple helix, is incorrect. This is why NMR data capable of providing information on structure in situ is desired. To be described, the dynamics data from nuclear magnetic relaxation time measurement can also provide information on the differences in these network structures. 

Fig. 21.—Structure of the 6-fold anhydrous curdlan III (19) helix, (a) Stereo view of a full turn of the parallel triple helix. The three strands are distinguished by thin bonds, open bonds, and filled bonds, respectively. In addition to intrachain hydrogen bonds, the triplex shows a triad of 2-OH - 0-2 interchain hydrogen bonds around the helix axis (vertical line) at intervals of 2.94 A. (b) A c-axis projection of the unit cell contents illustrates how the 6-0H - 0-4 hydrogen bonds between triple helices stabilize the crystalline lattice.

The dimensions of the xylan unit cell are slightly different a = b = 1.340 nm, (fibre axis) = 0.598 nm.) Atkins and Parker T6) were able to interpret such a diffraction pattern in terms of a triple-stranded structure. Three chains, of the same polarity, intertwine about a common axis to form a triple-strand molecular rope. The individual polysaccharide chains trace out a helix with six saccharide units per turn and are related to their neighbours by azimuthal rotations of 2ir/3 and 4ir/3 respectively, with zero relative translation. A similar model for curdlan is illustrated in Figure 6. Examinations of this model shows that each chain repeats at a distance 3 x 0.582 = 1.746 nm. Thus if for any reason the precise symmetrical arrangement between chains (or with their associated water of crystallization) is disrupted, we would expect reflections to occur on layer lines which are orders of 1.746 nm. Indeed such additional reflections have been observed via patterns obtained from specimens at different relative humidity (4) offering confirmation for the triple-stranded model. 

The X-ray evidence favours a model for the curdlan gel based on triple-stranded molecules. This structure is present in both the gel prepared from aqueous suspension and from alkali solution. The alkali gel contains the proposed seven-fold triple-stranded model (Figs. 7 and 14a) which converts to the more symmetric six-fold triple helix by annealing (Figs. 6 and 14b). This... 

Figure 6.3 (Top) The chemical structures of schizophyllan, curdlan, and a representative model of schizophyllan triple helix, (a) TEM image of as-grown-SWNT/s-SPG composite, and (b,c) its magnified picture, (d) The original image of (c) was Fourier filtered to enhance the contrast of the composite.41 (Reprinted with permission from M. Numata et al., J. Am. Chem. Soc. 2005, 127, 5875-5884. Copyright 2005 American Chemical Society.)...

Fig. 4.24 Molecular structures of p-(l->3)-glucans. The subset (a) shows the repeating unit of schizophyllan, one of the p-(l->3)-glucans, for example. The natural p-(l-43)-glucans take the form of a right-handed triple helix. The subset (b) shows the helix of curdlan the helix pitch and diameter are 1.88 and 1.56 nm, respectively. Here, the circles represent glucoses. The subset (c) shows how the hydrogen bonds are formed between the adjoining glucoses.

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