Xylogalactans from Lithothamnion heterocladum, a crustose member of the Corallinales (Rhodophyta)

The main polysaccharides from the red seaweed Lithothamnion heterocladum share the general characteristics of corallinans (agar-like xylogalactans), even though this seaweed is crustose, whereas those previously studied in detail were articulated seaweeds. After fractionation by cetrimide precipitation, several fractions were separated and characterized by sugar composition, other components, methylation, ethylation, desulfation-methylation, and NMR analyses. The main group of fractions carry the agaran disaccharidic repeating unit [→3)-β-d-Galp-(1 → 4)-α-l-Gal-(1→] substituted mainly on O-6 of the β-d-Gal unit by β-xylosyl side stubs, and less with sulfate or methoxyl groups, and also on O-2 of the α-l-Gal unit with methoxyl or sulfate, or less on O-3 of the same unit with methoxyl groups. These features are common to those of the polysaccharides of the four members of the order Corallinales already studied. However, a sugar not very common to the order appears in substantial proportions: 2,3-di-O-methyl-d-galactose, which acts as a side stub. Another peculiar issue is the presence of significant amounts of 4-linked α-l-Gal units glycosylated on O-6 or O-3. These results confirm that within a common pattern of corallinean xylogalactans, each species has minor characteristics of their own.