Structural features of the bioactive cyanobacterium Nostoc sp. exopolysaccharide.

Microalgal biopolymers are studied mainly in terms of physico-chemical characterization, biological effects as well as possible biotechnological applications. Due to the significant antitussive, bronchodilator, anti-inflammatory and immunomodulatory effects of the previously isolated crude extracellular polysaccharide (EPS) produced by the cyanobacterium Nostoc sp., the purified biopolymer and its oligosaccharides, obtained after partial acid hydrolysis, were subjected to an in-depth NMR structural study. Analyses of the data obtained by chemical methods and NMR showed that the EPS backbone is composed of the repeating unit [→4)-β-D-Xylp-(1 → 4)-β-D-Glcp-(1 → 4)-α-L-Arap-(1 → 3)-β-D-Manp-(1→], in which about 60% of glucose units are substituted at C6 by uronic acids, in particular by the unusual unsaturated 3-O-lactyl-4-deoxy-α-erythro-hex-4-enopyranuronic acid, and to a lesser extent by β-D-glucuronic acid and 3-O-lactyl-β-D-glucuronic acid. These findings, structural features and identified biological effects, suggest the potential use of this biopolymer in the medical-pharmaceutical field.