A conformational model for cyclic β-(1 → 2)-linked glucans based on NMR analysis of the β-glucans produced by Xanthomonas campestris

A cyclic hexadecaglucoside containing 15 β-(1 → 2)-linkages and one α-(1 → 6)-linkage {A. Amemura and J. Cabrera-Crespo, J. Gen. Microbiol. 132 (1986) 2443–2452} was purified from cultures of Xanthomonas campestris. The homogeneity of this glucan preparation facilitated the complete assignment of its 1H NMR spectrum and the assignment of all of the C-1 and C-2 resonances in its 13C NMR spectrum to specific residues within the glucan. The resonances (i.e., H-1, H-2, C-1 and C-2) that are closely associated with the β-(1 → 2) glycosidic bonds of this glucan are dispersed over a relatively broad chemical shift range. This chemical shift dispersion is attributed to the differences in the time-averaged geometry of individual β-(1 → 2)-linked glycosidic bonds in this glucan and is consistent with the hypothesis that these glycosidic bonds have less conformational freedom than do the glycosidic bonds in a linear β-(1 → 2)-linked glucan. The chemical shifts of H-1, H-2, C-1, and C-2 exhibit an alternating pattern when plotted as a function of their locations within the macrocyclic ring, suggesting that the glycosidic bond geometry also alternates in the glucan. A new conformational model for cyclic β-(1 → 2)-linked glucans was developed on the basis of these observations. This model is consistent with the observed spectroscopic features of all cyclic β-(1 → 2)-linked glucans known to be produced by Gram-negative bacteria.