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

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 H-1 and C-13 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)-beta-D-galactan. A highly ramified structure was suggested for the native polysaccharide, which contains linear backbone segments of 3-linked beta-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. (c) 2006 Elsevier Ltd. All rights reserved.