Cell wall polysaccharides from Australian red algae of the family Solieriaceae (Gigartinales, Rhodophyta): highly methylated carrageenans from the genus Rhabdonia

The hot water-soluble polysaccharides from Rhabdonia coccinea and R. verticillata were characterised by a combination of constituent sugar analysis, sulphate and pyruvate content assays, infrared (IR) spectroscopy, linkage analysis, and C-13-nuclear magnetic resonance (NMR) spectroscopy. These revealed unique polysaccharides belonging to the red algal galactan family. The polysaccharides had IR spectra resembling that of l-carrageenan, but were rich in 6-O-methylgalactose (ca. 31 mol% and 17 mol% for R. coccinea and R. verticillata, respectively). Data from C-13-NMR spectroscopy provided evidence that the polysaccharides were carrageenans rather than agarocolloids. The preparations contained mainly l-carrageenan, partially methylated at C(O)6 of the 3-linked galactose residue. The polysaccharide from R. verticillata also contained significant quantities of 3-O-methylgalactose and pyruvate. The unusual sugar 3-O-methylgalactose occurred primarily as main-chain 4-linked residues, with a small proportion in the form of terminal residues. Other structural variations occurred in the polysaccharides of both species.