Coil conformation, curdlan

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. 

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

NMR spectroscopy allowed to determine that the native SSG glucan from Sclerotinia sclerotiorum IFO 9395 and regenerated SSG obtained by neutralization of the alkaline solution had different conformations, since their C-3 signals appeared at 86 and 89 ppm, respectively. These values imply that while regenerated SSG possesses helical structure similar to that of curdlan, the native SSG in solid state has random-coil conformation [69]. 

It is expected that full peak-areas are revealed at 0.22M NaOH, because no cross-links can exist in curdlan of random-coil 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... 

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). 

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). 

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