Isolation and characterisation of a partially methylated galacto–glucurono–xylo-glycan, a unique polysaccharide from the red seaweed Apophloea lyallii

Polysaccharides of the red seaweed Apophloea lyallii were extracted with hot water in a yield of 63.6% w/w of the dry seaweed. The polysaccharides were non-sulphated and were composed of various proportions of d-xylose, 2-O-methyl d-galactose, d-glucuronic acid, d- and l-galactose, 3-O-methyl galactose, and d-glucose. The aldobiuronic acid 3-O-(β-d-glucopyranosyluronic acid)-d-xylopyranose was isolated from the acid hydrolysate of the polysaccharides. The polysaccharides were fractionated by DEAE-sephadex ion exchange chromatography, and two of the fractions obtained were characterised by neutral sugar analyses, linkage analyses, and NMR spectroscopy. A major fraction obtained was shown to be largely composed of a unique (2-O-methyl galacto)–glucurono–xylo-glycan with a repeating unit represented by [α-d-(2-O-Me-Galp)-(1→4)-β-d-GlcAp-(1→3)-α-d-Xylp-(1→4)-]n. The structure of this new glycan was explored by beta-elimination analyses. A minor fraction obtained by ion exchange chromatography contained (1→4)-linked xylose, (1→4)-linked galactose, (1→4)-linked 3-O-methyl galactose, (1,3,4)-linked galactose, and terminal uronic acid in a ratio of approximately 1:0.3:0.5:0.4:0.4.