Chemical methods for the analysis of sulphated galactans from red algae

Methods are reported that facilitate the structural characterization of complex sulphated galactans of the red algae. Two procedures have been developed for the production of alditol acetates from carrageenans and agaroids. Both procedures generate 3,6-anhydrogalactitol acetate from the easily destroyed 3,6-anhydrogalactosyl residues in near quantitative yield. The "double hydrolysis-reduction" method involves preliminary hydrolysis under conditions sufficient to cleave all of the 3,6-anhydrogalactosidic bonds, but mild enough to avoid significant further degradation. The "reductive hydrolysis" method uses the acid-stable 4-methylmorpholine-borane to reduce the 3,6-anhydrogalactose end groups as they are released during acid hydrolysis. An alditol acetate sample can be prepared from a polysaccharide in a single tube, ready for g.l.c. analysis, in less than 2.5 h, i.e. more quickly than by any previous procedure. Problems associated with incomplete methylation of sulphated carrageenans and agaroids by the Hakomori procedure have been overcome by first converting the sulphated polysaccharide into its triethylammonium salt form. The reductive hydrolysis method is effective for the production of partially methylated alditol acetates from the methylated polysaccharides, enabling the rapid determination of the substitution pattern of these polysaccharides. These improved analytical methods have been applied successfully to kappa-, iota-, and lambda-carrageenans, as well as some agars.