Structural characterization of the released polysaccharide of desiccation-tolerant Nostoc commune DRH-1

Abstract

The structure of the viscous extracellular polysaccharide (glycan) of desiccation-tolerant Nostoc commune DRH-1 was determined through chromatographic and spectroscopic methods. The polysaccharide is novel in that it possesses a 1-4-linked xylogalactoglucan backbone with D-ribofuranose and 3-O-[(R)-1-carboxyethyl]-D-glucuronic acid (nosturonic acid) pendant groups. The presence of D-ribose and nosturonic acid as peripheral groups is unusual, and their potential roles in modulating the rheological properties of the glycan are discussed. Nosturonic acid was present in the glycans of N. commune from diverse geographic locations, suggesting that this uronic acid is an integral component of this cosmopolitan anhydrophile.

FIG. 1

Synthesis of the α- and β-anomers of methyl 3-O-[2-(1-hydroxy)propyl]-d-glucopyranoside (structure 3), starting from both diacetone d-glucose and the purified polysaccharide. Redn, reduction. See text for details.

FIG. 2

Structures of the oligosaccharides isolated from the polysaccharide.

FIG. 3

Anomeric and ring proton regions of an absolute-value COSY spectrum (non-PFG) of trisaccharide 5. The important correlations are indicated (see inset for key).

FIG. 4

The β-anomeric and ring proton regions of a 500-MHz gradient-enhanced absolute-value COSY spectrum of a mixture of pentasaccharides 7 and 8. The boxed assignments indicate protons with HMBC correlations to anomeric carbons on adjacent rings (see inset for key).