Carrageenans. Part III. Oxidative hydrolysis of methylated κ-carrageenan and evidence for a masked repeating structure

Abstract

After treatment with alkaline borohydride, κ-carrageenan contains equimolar proportions of D-galactose and 3,6-anhydro-D-galactose units, and on methanolysis it gives carrabiose dimethyl acetal (3,6-anhydro-4-O-β-D-galactopyranosyl-D-galactose dimethyl acetal) in a yield which is close to that calculated for a perfectly alternating polymer. The small excess of galactose in the native polysaccharide is attributed to galactose 6-sulphate and 2,6-disulphate units which formally replace a proportion of 3,6-anhydrogalactose units and are converted to 3,6-anhydride by alkaline borohydride. Methylation and other evidence indicates that virtually every galactose unit is 4-sulphated and that about every seventh 3,6-anhydrogalactose unit is 2-sulphated. There is no evidence that κ-carrageenan is branched. Monosaccharide derivatives of 3,6-anhydrogalactose were isolated from the polysaccharide methyl ether by oxidative hydrolysis with a mixture of bromine and sulphuric acid. A simple method for the synthesis of methyl 3,6-anhydrogalactoside monomethyl ethers is described, based on partial methylation followed by separation on a strong base anion-exchange column in the hydroxide form.

It is concluded that κ-carrageenan contains an alternating arrangement of 3-linked β-D-galactopyranosyl units and 4-linked α-D-galactopyranosyl units. Whereas the 3-linked units appear to be present only as the 4-sulphate, the 4-linked units include the 3,6-anhydride, the 3,6-anhydride 2-sulphate, the 6-sulphate and the 2,6-disulphate. The structure is therefore of the ‘masked repeating’ type.