OZONOLYSIS FOR SELECTIVELY DEPOLYMERIZING POLYSACCHARIDES CONTAINING BETA-D-ALDOSIDIC LINKAGES

The depolymerization of polysaccharides, particularly those containing acid-sensitive components, into intact constituent repeating units ca n be very difficult. We describe a method using ozonolysis for depolym erizing polysaccharides' containing beta-D-aldosidic linkages into sho rt-chain polysaccharides and oligosaccharides. This method is carried out on polysaccharides that have been fully acetylated whereby beta-D- aldosidic linkages are selectively oxidized by ozone to form esters, f rom which the polysaccharides are subsequently cleaved with a nucleoph ile. Ozone oxidation of aldosidic linkages proceeds under strong stere oelectronic control, and reaction rates depend on the conformations of glycosidic linkages. Thus, beta-D-aldosidic linkages-with different c onformations can have very different reaction rates even in the absenc e of substantial chemical differences. These rate differences allowed for very high selectivity in cleaving beta-D-linkages of polysaccharid es. Several polysaccharides from group B Streptococcus and other bacte rial species were selectively depolymerized with this method. The repe ating units of the group B Streptococcus polysaccharides all contain a n acid-sensitive sia]ic acid residue in a terminal position on a sidec hain and several beta-D-residues including galactose, glucose, and N-a cetylglucosamine; however, with each polysaccharide, one type of linka ge was more reactive than others. Selective cleavage of the most sensi tive linkage occurs randomly throughout the polymer chain, yielding fr agments of controllable and narrowly distributed sizes and the same re peating-unit structure. The average size of the molecules decreases ex ponentially, and desired sizes can be obtained by stopping the reactio n at appropriate time points. With this method the labile sialic acid residue vias not affected.