Determination of the configuration of 3,6-anhydrogalactose and cyclizable α-galactose 6-sulfate units in red seaweed galactans Original Research Article

A combination of two reported procedures was used in order to determine the configuration of the 3,6-anhydrogalactose present in red seaweed polysaccharides. A mild hydrolysis (to cleave only 3,6-anhydrogalactosyl linkages) was followed by a reductive amination with a chiral amine. Then, the total hydrolysis proceeded, followed by a new step of reductive amination. In this way, using (S)-α-methylbenzylamine as the chiral amine, it was possible to separate and quantitate both enantiomers of 3,6-AnGal and its 2-O-methyl ether as their diastereomeric acetylated aminoalditols. On the other hand, using (S)-1-amino-2-propanol, even though the derivatives of both enantiomers of 3,6-AnGal are not separated, the mixture can be safely quantitated with respect to galactose. Furthermore, a one-pot technique was developed to carry out an alkaline treatment of the polysaccharides, followed by the double hydrolysis–reductive amination procedure, which is useful to determine the proportions of both enantiomers of 6-sulfated 4-linked galactose units in the native polysaccharides. The unexpected presence of small amounts of units of this type belonging to the d-series in a porphyran sample is revealed by this novel procedure.