Helix curdlan triple

The mounting evidence in the case of the curdlan triple helix (13)is that the chains can untwist slightly (or twist tighter) under different conditions giving rise to quite different x-ray diffraction patterns as the symmetry is destroyed. These changes should not be confused with complete untwining of the chains. 

Curdlan is a bacterial polysaccharide made by Agrobacterium biovar [87,88,89]. It is a linear (1 3)- -glucan (MW 73,000) that forms a triple helix. Curdlan is insoluble in cold water. When aqueous dispersions of curdlan are heated, two types of gels form. First curdlan dissolves. When the solution reaches 55-66 °C, then is cooled, a reversible gel forms. The gel melts when held at about 60 °C. When the thermoreversible gel is heated to a temperature above 80 °C, an irreversible gel forms. Heating to higher temperatures results in stronger irreversible gels. Transition temperatures are a function of concentration. 

Scleroglucan exists in a triple hehcal conformation that is highly stable (314). The D-glucopyranosyl side groups project to the outside of the helix (312) and prevent the aggregation of hehces, which would result in insolubiUty, as in the case of curdlan vide infra). The transition from helix to coil occurs... 

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.

Figure 2 Effect of branch frequency on glucan conformation. Conformational characterization of glucans was carried out as described in the experimental section. Curdlan is a linear p(l-3)linked glucan Yeast glucan has a 30% P(l-6) branch frequency and PGG-R glucan has a 50% p(l-6) branch frequency. The Congo Red-single/triple helix complex absorption maxima are indicated.

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.)...

Curdlan adopts a triple helix conformation in the dispersed and solid states (Deslandes et al., 1980) and reverts to a random coil in 0.25-M sodium hydroxide (Stipanovic and Giammatteo, 1989). Some glucans are ordered in dilute alkali and disordered at higher concentrations (Ogawa et al., 1972). 

Another polysaccharide which displays interesting hydration phenomena is (l->-3 )-B-D-g 1 ucan, often called paramylon ( ), curdlan ( ) or laminaran (16). The molecular crystalline arrangement o P this polysaccharide consists of a triple helix formed by three intertwining 6j hel ices. (, 23) Two polymorphs... 

The beta-l,3-glucans are dramatically different from the glucans described so far. This polymer is extremely flexible, and occurs in many instances in nature. The polymer goes by the names of curdlan, pachyman, laricinan, schleroglucan, paramylon, lentinan, laminarin, callose and schizophylan. The prevalent form appears to be a triple helix, with n > 6 and b 0.3 nm for each of the three strands ( ). If triacetate derivatives are made, n > 6 and h = 0.31 to 0.36 nm (10). The acetate helices are single. Of some interest is the lack of allowed 2-fold conformations on the n-b map (Figure 5). The allowed zones close up when n = 20 or so, but some small adjustments in the monomer shape could allow an infinite number of monomeric units per helix repeat. 

C-1-0 and C-3-0 linkages is allowed (6). We attempted here to compare the chemical shifts of the gel with those of solid state curdlan, the conformation of which is mainly single helix at ambient temperature and triple-helix at higher temperature above 120°, as studied by X-ray diffraction (14>I5)- The newly emerging technique, cross-polarization magic angle spinning (CPMAS) ... 

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.

Scleroglucan (from Sclerotinia sclemtiomm) is a 1 3 -D-glucan with additional 1 —> 6 (3-links (Fig. 13.9) that confer water solubility to it under ambient conditions, but do not significantly interfere with a triple helix gelling process, similar to that of curdlan [46]. Similar polysaccharides can also be extracted from other sources such as waste yeast. 

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. 

Curdlan is a microbial polysaccharide that occurs naturally as a linear (triple-helix) polysaccharide composed of 1,3-P-hnked D-glucose units, produced by a strain of Mcaligenes faecalis (Figure 2.38). It is a neutral, bacterial polysaccharide without branched chains. It is insoluble in water and alcohol but soluble in alkaline solution and dimethyl sulfoxide (DMSO) [274-276]. It occurs as a tasteless powder, stable in dry state. It was reported as a support matrix for enzyme immobilization, through activation with epichlorohydrin that can be covalently linked to the available amino, hydroxyl, and suHhydryl enzyme groups [277]. It has the specific character to form an irreversible gel by heating of a water suspension [278]. Its water-insoluble nature helps to improve a material s water barrier capabihty, and its solubility in... 

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