Structural fragments of an immunomodulatory active exopolysaccharide produced by the green microalgae Dictyosphaerium...

Microalgal metabolites represent source of valuable compounds with wide range of biological effects and industrial applications. Eight sugar units found in more than twenty methylated derivatives, of which Gal, Glc, Fuc, Xyl and GlcA were dominant, suggest complex structure of D. tetrachotomum exopolysaccharide (DtF). Almost 20 % of Glc is partially methylated as 2-OMe-Glc. Ten fractions obtained by ion-exchange chromatography were analysed by chemical and spectroscopic methods. Fractions DtF1, DtF4 and DtF5 were isolated in the highest yields. The lower molecular weight DtF1 biopolymer was identified as the neutral β-D-gluco-3,6-β-D-galactan with fragments of unsubstituted 1,3-linked βGalp alternating 1,3,6-linked βGalp branched at O3 by OMe and βGlcp. The high molecular weight fractions DtF4 and DtF5 were rich in Gal, Glc, Fuc, Xyl and GlcA residues. Methylation results support 2,4-fuco-4-β-D-galactan backbone terminated by Glc, Xyl and GlcA residues. The results indicate at least two types of polymers in DtF; a minor neutral β-D-gluco-3,6-β-D-galactan and a dominant acidic one with a 2,4-fuco-4-β-D-galactan backbone. Partial acid hydrolyses of DtF, DtF4 and DtF5 led to identification of oligomers. Each one has at the reducing end Fuc substituted at O2 by 2-OMe-αGlc and at O4 by substituents R = βGlcpA; βGalp; βGalp (1 → 3)βGalp (1→ and Xylp (1 → 4)βGalp (1 → 3)βGalp (1 → .