A 13C nuclear magnetic resonance study of gel-forming (1 goes to 3)-beta-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.
A 13C nuclear magnetic resonance (NMR) study of a resilient gel of a curdlan-type polysaccharide 13140, a (1 goes to 3)-beta-D-glucan, from Alcaligenes faecalis var. myxogenes IFO 13140, was performed in an effort to understand the gel structure. It was found that very broad 13C resonance peaks of line widths ca. 150 (C-1-C-5) and 50 HZ (C-6) are able to be seen in the gel state. Because of a fivefold increase of the peak intensities with respect to those in an aqueous suspension by gelation, these peaks are unequivocally ascribed to certain regions relevant to the gel structure. Those 13 C NMR peaks, however, account for only 20-30 (as viewed from C-1-C-5) and 60% (from C-6) of the total gel, the peak areas of the rest being lost. With respect to those of the disordered low molecular weight acid degraded fraction, fraction II, and laminaran, downfield displacements of C-1, C-3, and C-4 signals are found to take place by amounts of 2.8, 3.2, and 0.9 ppm, respectively, 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 goes to 4)-alpha-D-glucans (Colson, P., Jennings, H.J., and Smith, I.C.P. (1974), J. Am. Chem. Soc. 96, 8081-8087), the observed downfield 13C shifts of the glucosidic bonds in the gel are explained by the presence of the fixed conformation of the preferred dihedral angles, in which internal rotations around the glucosidic bonds are not allowed. Combined with the results of theoretical prediction and the downfield shift of C-4 signals, which is consistent with the presence of an O-4'...O-5 intramolecular hydrogen bond, the observed 13C peaks are ascribed to a region of single helical conformation, whereas the peak-loss portion of 13C NMR signals (70-80 and 40% from C-1 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 as well as the peak positions is also found to take place by stepwise addition of NaOH (greater than 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 by Ogawa et al. (Ogawa, K., Watanabe, T., Tsurugi, J., and Ono, S. (1972), Carbohydr. Res. 23, 399-405).