O-acetylation affects the binding properties of the carboxyl groups on the Vi bacterial polysaccharide.
The capsular polysaccharide of Salmonella typhi and Citrobacter freundii (Vi) is a linear homopolymer of (1-->4)-linked 2-acetamido-2-deoxy-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-acetylation, were studied by potentiometric titration, circular dichroism, and their reaction with the bulky nucleophile 2-nitro-phenylhydrazine (NPH). Potentiometric titrations with K+ and Ca2+ hydroxides showed that the difference in the free energy of binding between the two cations (delta GKCa) was inversely proportional to the degree of O-acetylation. Similar cationic effects were found when measuring circular dichroism. Moreover there was also an inverse relation between the degree of O-acetylation and the extent of binding of NPH to the carboxyl groups. These data all indicate that O-acetyl groups hinder the association of carboxyls with cations and nucleophilic reagents. This provides a possible explanation for the importance of the O-acetyl and the relative unimportance of the carboxyl groups in contributing to the immunologic properties of the Vi.
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Laboratory of Developmental and Molecular Immunity, National Institute of Child Health and Development, National Institutes of Health, Bethesda, MD 20892.
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D000107:Acetylation; D001665:Binding Sites; D002118:Calcium; D002151:Calorimetry; D002236:Carbohydrate Conformation; D002942:Circular Dichroism; D016971:Citrobacter freundii; D006834:Hydrazines; D010659:Phenylhydrazines; D011135:Polysaccharides, Bacterial / Q000737:chemistry* / Q000378:metabolism; D011188:Potassium; D011199:Potentiometry; D012485:Salmonella typhi
D006834:Hydrazines; D010659:Phenylhydrazines; D011135:Polysaccharides, Bacterial; C054154:capsular polysaccharide, Salmonella; C060574:2-nitrophenylhydrazine; D011188:Potassium; D002118:Calcium