Structural investigation of a polysaccharide released by the cyanobacterium Nostoc insulare

The structural investigation of an extracellular polysaccharide released during photoautotrophic growth by the cyanobacterium Nostoc insulare is reported. After 60 days of cultivation, an average yield of purified, desalted, and freeze-dried released polysaccharide (RPS) of 0.9 g L-1 medium was obtained. The apparent hydrodynamic volume, determined for RPS, was 1.1 x 10(6) Da, and the average molecular weight was 2.8 x 10(6) Da. No sulfate and only traces of pyruvate and acetate groups were detectable. A protein content of only 0.7% indicates a high degree of purity of RPS. The following constituent uronic acids and sugars were identified: glucuronic acid (GlcA), glucose (Glc), arabinose (Ara), and for the first time, cyanobacterial RPSs 3-O-methyl-arabinose (3-O-Methyl-Ara). Adapted from linkage analyses of untreated RPS and of RPS treated by means of reduction of uronic acids, mild acid hydrolysis with oxalic acid, or lithium degradation, respectively, the following partial structure of RPS is proposed, which possesses an arborisation built by 1,3,4-Glcp and a side chain built by 3-O-Methyl-Araf:-> 1)-Glcp-(3 -> 1)-Glcp-[( 3 -> 1)-3-O-Methyl-Araf] 4 -> 1)-GlcAp-(4 ->).