Structure of a highly pyruvylated galactan sulfate from the Pacific green alga Codium yezoense (Bryopsidales, Chlorophyta)

Abstract

A polysaccharide fraction consisting of d-galactose, sulfate, and pyruvate in a molar proportion of 4:2:1 was isolated from the green seaweed Codium yezoense by water extraction followed by ion-exchange chromatography. To elucidate its structure, modified polysaccharides were prepared by desulfation, depyruvylation, and by total removal of non-carbohydrate substituents. Structures of the native polysaccharide and of the products of its chemical modifications were investigated by methylation analysis as well as by 1D and 2D 1H and 13C NMR spectroscopy. The polysaccharide devoid of sulfate and pyruvate was subjected to two subsequent Smith degradations to afford a rather low-molecular and essentially linear (1→3)-β- d-galactan. A highly ramified structure was suggested for the native polysaccharide, which contains linear backbone segments of 3-linked β- d-galactopyranose residues connected by (1→6) linkages, about 40% of 3-linked residues being additionally substituted at C-6, probably by short oligosaccharide residues also containing (1→3) and (1→6) linkages. Sulfate groups were found mainly at C-4 and in minor amounts at C-6. Pyruvate was found to form mainly five-membered cyclic ketals with O-3 and O-4 of the non-reducing terminal galactose residues. The minor part of pyruvate forms six-membered cyclic ketals with O-4 and O-6. The absolute configurations of ketals ( R for six-membered ketals and S for five-membered ones) were established using NMR spectral data.