Novel 4-O-β-D-xylopyranosyl-3,6-anhydro-L-galactopyranosyl disaccharide units in a polysaccharide from the red alga Pyrophyllon subtumens

Thalli of the endemic epiphytic New Zealand red seaweed Pyrophyllon subtumens are known to contain a high level of xylose and a notable amount of arabinose but the extracted polysaccharide has not been characterised. The linkage/substitution of individual sugars within the water-soluble polysaccharide extract and various derivatives were determined by chemical and spectroscopic methods. No 3-linked sugars nor any D-galactose were found, which excluded agar-, carrageenan- or mixed 3-linked/4-linked β-D-xylan-type polysaccharides found in many other red macroalgae. Instead, the polysaccharide backbone contained predominantly 4-linked β-D-xylopyranosyl, 4-linked 3,6-anhydro-L-galactopyranosyl and 4-linked L-galactopyranosyl units. Some of each type of sugar were sulfated at various positions. Some xylosyl units were substituted at the 2- or 3-position with L-arabinosyl units. The polysaccharide is complex and likely contains a range of structures. However, partial sequencing was successfully used to recover and identify a novel disaccharide 4-O-D-xylopyranosyl-3,6-anhdydro-L-galactopyranose, which indicates a unique →4)-β-D-Xylp-(1 → 4)-3,6-anhydro-L-Galp-(1 → repeat unit in the polysaccharide.