Abstract: The orthogonal array design method was used to optimize three reaction conditions, including esterification reagent, temperature and reaction time, for the sulfation of crude polysaccharides extracted from Nostoc sphaeroides Ku.. tzing by water extraction and subsequent alcohol precipitation. Besides, FTIR spectroscopic analysis was carried out to identify the structural difference of Nostoc sphaeroides Ku.. tzing polysaccharides as a result of the sulfation reaction, and a correlation analysis was done between FTIR A1261/A1418 and degree of substitution (DS) of sulfated polysaccharides, as determined by the barium chloride-gelation method. The optimal sulfation reaction conditions were found to be: 1:4 chlorosulfonic acid-pyridine as esterification reagent for 6 h reaction at 70 ℃. Under the optimal sulfation conditions, the DS of the final products was 1.042. Meanwhile, the sulfated polysaccharide obtained revealed typical sulfated functional groups. The correlation coefficient between FTIR A1261/A1418 and DS of sulfated Nostoc sphaeroides Ku.. tzing polysaccharides was 0.974. Therefore, infrared spectroscopy can be used to characterize the structural difference of sulfated polysaccharides and quantify the DS of sulfate groups.

Key words: polysaccharide, sulfation, degree of substitution