Structural characterization of β-D-(1 → 3, 1 → 6)-linked glucans using NMR spectroscopy

Nondestructive structural analysis of a series of β-D-(1 → 3, 1 → 6)- linked glucans (laminaran, curdlan, yeast glucan, scleroglucan, etc.) was performed using two-dimensional NMR spectroscopy. The relative ratios of H-1 at different AGUs provided the information about DP(n) and DB. The α-, and β-anomeric protons on reducing terminals were assigned at 5.02 ~ 5.03 ppm (J 3.6 ~ 3.7 Hz), and 4.42 ~ 4.43 ppm (J 7.6 ~ 7.9 Hz), respectively, whereas the H-1 protons of internal AGUs and β-(1 → 6)-branched AGUs appeared at 4.56 ~ 4.59 ppm (J 7.6 ~ 7.8 Hz), and 4.26 ~ 4.28 ppm (J 7.6 ~ 10.6 Hz), respectively, in a mixed solvent of 6:1 Me₂SO-d₆-D₂O at 80°C. In the solvent, the OH peaks were eliminated from the spectra allowing the H-1 protons to appear clearly. In addition, the nonreducing terminal H-1 and H-1 at the AGU next to reducing terminal could be assigned at 4.45 ~ 4.46 ppm (J 7.8 ~ 7.9 Hz), and 4.51 ~ 4.53 ppm (J 7.8 Hz), respectively. The DP(n) of the laminaran was 33 (polydispersity 1.12) and the DB was 0.07. The number of glucosyl units in the side chain of laminaran is more than one. The DP(n) and DB of the water-insoluble yeast glucan were 228 and 0.003, respectively. However the DP(n) of water soluble yeast glucan phosphate and curdlan was changed upon the number of freeze-drying processes and the content of water in the mixed solvent, respectively. And the DB of those were calculated as 0.02 and 0, respectively. The DB of scleroglucan was precisely calculated as 0.33, compared with the previously reported data. The H-1s at different AGUs of the various β-D-(1 → 3, 1 → 6)-linked glucans having different DB can be exactly assigned by their chemical shifts in the mixed solvent system. This NMR analysis can be effectively used to determine the DP and DB of polysaccharides in a simple and non-destructive manner. (C) 2000 Elsevier Science Ltd.