Structural studies of green algal polysaccharides with special reference to Ulva lactuca

Abstract

The water-soluble sulphated polysaccharide (containing ca. 1% of starch) of the green seaweed Ulva lactuca has been isolated in 24% yield. The starch-free polysaccharide onhydrolysis gave D-glucose, D-xylose, L-rhamnose (22%) and D-mannose together with D-glucuronic acid (24.4%) andsulphate ester groups (19%). After repeated desulphation and reduction a nearly neutral material containing 1.3% ofsulphate (SO4) and 5.4% of uronic anhydride (U.A.) wasobtained. Methylation of this material provided evidence of end group, glucuronic acid/glucose, xylose and rhamnose, of 1,4-linked rhamnose and xylose, of 1,3,4-linked rhamnose and xylose, and of 1,3,6-linked glucose. Tentative evidence of 1,3- and 1,4-linked glucuronic acid/glucose, of 1,3-linked xylose and of 1,2,3-1inked rhamnose was also obtained. Partial hydrolysis of a second nearly neutral sample ofthe starch-free polysaccharide gave a trisaccharide whichwas characterised as O-L-rhamnopyranosyl xylosyl (1 3)-D-glucose. In contrast, the water-soluble sulphated starch-free and starch-containing polysaccharides were partially hydrolysed separately and gave in both cases O-D-glucosyl (1 > 3)-D-xylose. The acidic fraction inthese hydrolysates comprised mainly glucuronosyl (1 >4)-L-rhamnose with smaller proportions of glucuronosyl (1 >3)/( 1->4)-D-xylose and a tetrasaccharide tentatively characterised as O-D-glucuronosyl (1 > 4)-L-rhamnosyl (1 > 3) -0-D-glucuronosyl (1 --> 3)-D-xylose.Smith Degradation of the sulphated starch-containing polysaccharide gave degraded polymeric material (7.2%) together with neutral and acidic fragments. From the neutral fragments glycerol, erythritol, 2-0-P-D-xylosyl-glycerol and 2-0-B-D-glucosylerythritol were separated and characterised. Tentative evidence for the presence of glycollic aldehyde and 2-0-D-xylosylerythritol was also obtained. That the free erythritol was not derived solely from the contaminating starch was proved by a parallel Smith Degradation of the starch-free polysaccharide. The acidic fragment comprised a single oligosaccharide containing glucose, xylose, rhamnose and glucuronic acid together with glycerol and erythronic acid. A possible structure for this fragment is advanced. All the above results are correlated with one another and their bearing on the overall structure of the macro-molecule discussed.