A carbon-13 nuclear magnetic resonance study of gel-forming (1→3)-β-D-glucans. Evidence of the presence of single-helical conformation in a resilient gel of a curdlan-type polysaccharide 13140 from Alcaligenes faecalis var myxogenes IFO 13140

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Abstract
A 13C NMR study of a resilient gel of a curdlan-type polysaccharide 13140, a (1 .fwdarw. 3)-.beta.-D-glucan, from A. faecalis var. myxogenes IFO 13140, was performed to understand the gel structure. Very broad 13C resonance peaks of line widths .apprx. 150 (C-1.sbd.C-5) and 50 Hz (C-6) are seen in the gel state. Because of 5-fold increase of the peak intensities with respect to those in an aqueous suspension, these peaks are unequivocally ascribed to certain regions relevant to the gel structure. Those 13C NMR peaks account for only 20-30 (as viewed from C-1.sbd.C-5) and 60% (from C-6) of the total gel, the peak areas of the rest being lost. Of the disordered low molecular weight acid degraded fraction, fraction II, and laminaran, downfield displacements of C-1, C-3 and C-4 signals occurred by amounts of 2.8, 3.2 and 0.9 ppm, respectivley, while the remaining peaks (C-2, C-5 and C-6) are unchanged. In view of similar differences of chemical shifts between cyclodextrins and linear (1 .fwdarw. 4)-.alpha.-D-glucans, the observed downfield 13C shifts of the glucosidic bonds in the gel are due to the presence of the fixed conformation of the perferred dihedral angles, in which internal rotations around the glucosidic bonds are not allowed. Combined with the results on theoretical prediction and the downfield shift of C-4 signals, which is consistent with the presence of an O-4''...O-5 intramolecular H bond, the observed 13C peaks are ascribed to a region of single helical conformation, but the peak-loss portion of 13C NMR signals (70-80 and 40% from C-1.sbd.C-5 and C-6, respectively) is presumably ascribed to the multiple-helical junction zones for the gel structure and their vicinities. The variation of the line widths and the peak positions also occurred by stepwise addition of NaOH (> 0.22 M). The onset of the conformational transition, helix to random coil, is in good agreement with the change of viscosity, specific rotation, optical rotatory dispersion and absorption maximum shift by complex formation with Congo Red reported previously.