O-ACETYLATION AFFECTS THE BINDING-PROPERTIES OF THE CARBOXYL GROUPS ON THE VI BACTERIAL POLYSACCHARIDE

The capsular polysaccharide of Salmonella typhi and Citrobacter freund ii (Vi) is a linear homopolymer of (1 --> 4)-linked 2-acetamido-2-deox y-alpha-D-galactopyranosyluronic acid partially O-acetylated at the C- 3 position. The physico-chemical properties of the carboxyl groups of the Vi polysaccharide, as a function of different degrees of O-acetyla tion, were studied by potentiometric titration, circular dichroism, an d their reaction with the bulky nucleophile 2-nitro-phenylhydrazine (N PH). Potentiometric titrations with K+ and Ca2+ hydroxides showed that the difference in the free energy of binding between the two cations (Delta G(K)(Ca)) was inversely proportional to the degree of O-acetyla tion. Similar cationic effects were found when measuring circular dich roism. Moreover there was also an inverse relation between the degree of O-acetylation and the extent of binding of NPH to the carboxyl grou ps. These data all indicate that O-acetyl groups hinder the associatio n of carboxyls with cations and nucleophilic reagents. This provides a possible explanation for the importance of the O-acetyl and the relat ive unimportance of the carboxyl groups in contributing to the immunol ogic properties of the Vi.