Occurrence of carrageenan structure in an agar from the red seaweed Digenea simplex (Wulfen) C. agardh (Rhodomelaceae, Ceramiales) with a short review of carrageenan-agarocolloid hybrid in the florideophycidae

A fractionation study of crude agar from the red seaweed Digenea simplex led to isolation of a galactan sulfate fraction consisting of a carrageenan-agaroid hybrid-type backbone. This polysaccharide fraction, containing D-galactose (Gal), L-Gal, 3,6-anhydro-D-galactose (D-AnGal), L-AnGal, sulfate and a small amount of xylose, appeared to consist of repeating units of both 4-O-beta-D-galactopyranosyl-3,6-anhydro-L-galactose (agarobiose) and 4-0-p-D-galactopyranosyl-3,6-anhydro-D-galactose (carrabiose) on the basis of results from a partial methanolysis study. Two other repeating disaccharide units in the backbone, 4-O-beta-D-galactopyranosyl-L-galactose and 4-O-beta-D-galactopyranosyl-D-galactose, were also suggested by the methylation study. The sulfate groups were also suggested to be located at O-4 of (1-->3)-linked D-Gal, O-6 of (1-->3)-linked D-Gal, O-6 of (1-->4)-linked L-Gal and/or O-2 of (1-->4)-linked AnGal residues, besides O-3 of (1-->4)-linked D-Gal residue. Co-occurrence of carrageenan and agarocolloid backbone structure among the genera in the Florideophycidae was also reviewed.