Curdlan conformation

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... 

Figure 2 compares the conformational transition curves of wild-type yeast glucan (branch frequency = 0.20) and PGG (branch frequency = 0.50). Wild-type yeast glucan required approximately 0.1M NaOH to disrupt the triple helical conformation, whereas this transition is observed at approximately 0.04 M NaOH with PGG. This trend is consistent with the observation that curdlan, an entirely linear p-D(l-3)-linked glucan, requires approximately 0.25M NaOH to disrupt the ordered conformation (76). Hence, it is concluded that the highly branched PGG molecules only form weak inter-chain associations resulting in the formation of predominantly single-helical zones. 

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.

In connection with these uses the conformation of curdlan molecule in solution and in gel and the ultrastructure of aggregates of curdlan molecules are considered very important. This paper deals with conformational studies on curdlan molecules by means of light scattering, 13C NMR, X-ray diffraction, and morphological studies using an electron microscope. 

A consideration of the molecular conformation using the wormlike chain model suggests that the curdlan molecule may contain helical portions but, as a whole, takes a random-coil conformation ( ) ... 

Figure 2 shows 1 3C NMR spectra of curdlan(DPn 540) of an water suspension and alkaline solution reported by Saito et al. 8). It is noteworthy that at a concentration of 0.22M NaOH the broad 1 3C NMR signals decrease to less than one-tenth of those of the aqueous suspension. This sharp spectrum is characteristic of random-coil conformation. This is consistent with the results of Ogawa et They determined the dependence of optical... 

The line widths of C-l—C-S 13C resonance peaks in the gel are about 150 Hz, which are rather broad in comparison with those of alkaline solution (0.22M NaOH)(Fig. 2D) and also with those of the lower molecular weight fraction (Fig. 3A). By comparing the integrated peak intensities (with nuclear Overhauser effect sur-pressed) with those of the corresponding peaks in the 0.22M NaOH solution, in which the curdlan molecules are considered to have random-coil conformation, it was estimated that these observed 13C peaks account for only 20-30 and 60% for the C-1--C-5 and C-6 carbons of the total gel, respectively. Hence, the remaining invisible portion of the 13C should be ascribed to the junction zones of the gel net work and to residues located closely to these junctions. 

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). 

There are at least four crystalline forms of cellulose, based on different packing of the primary chain (Blackwell, 1982), and three forms of granular starch, based on the packing of double helices (Noel et al., 1993). The differences are largely in the unit-cell dimensions and the crystallization and precipitation temperatures. One form of starch, precipitated with alcohol, is in a symmetrical molecular arrangement and is readily dispersible in cold water (Kerr, 1950). Mannan and dextran yield different crystals at low and high temperatures, and there was not only a polymorphic difference, but a conformational difference in cellulose (Quenin and Chanzy, 1987). Curdlan appears to have three polymorphs—anhydrous, hydrated, and annealed. 

Curdlan gel formation is heat dependent. This polysaccharide is not soluble in water, but when an aqueous suspension is heated it becomes dear at about 54°C. Furdier heating leads to gel fonnation. The gels are stable over a wide range of pH (3 to 95) and do not melt at temperatures below 100°C. Curdlan gels are formed by cross-linldng, involvii conformational ordering of the exopolysaccharide molecules to give a triple helical structure. 

Thus, regeneration of curdlan form II requires heating with water at 120 °C, and form III requires intensive drying of form II. The conformation changes can be readily monitored by CP-MAS NMR (chemical shift dilferences of up... 

Figure 4.76 Solution conformation of hyaluronan. Note the combination of the two conformational features imposed by two types of glycosidic bonds the two-fold helix imposed by the P-(1 4) link (cf. cellulose) and shallow helix of the disaccharide units typical of a P-(1 3) link (cf. curdlan).

Solution NMR Experiments with Curdlan. In an effort to correlate the solution properties of curdlan with features of molecular conformation, the influence of NaOD concentration was studied by NMR as shown in Figure 3. In 0.1 M NaOD, curdlan displayed relatively broad resonance peaks corresponding to the restricted molecular motion of the triplex. The spectrum recorded at 0.5 M NaOD, above the critical denaturation concentration of 0.2 M, showed considerably improved resolution typical of a random-coil molecular conft)rmation. Similar results were reported by Saito and Kasai 11, 13). 

To probe further the coil conformation of curdlan in 0.5 M NaOD, 2D NMR methods were applied. The 2D heteronuclear correlation... 

CP/MAS NMR of Scleroglucan. Solid, crystalline scleroglucan exists as a single anhydrous polymorph in contrast to the several crystalline forms of curdlan (8). Bluhm (8) proposed that the molecular conformation of scleroglucan is a triplex structure with a repeat distance along the chain... 

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