Physical, Chemical, Antigenic, and Immunologic Characterization of Polygalacturonan, Its Derivatives, and Vi Antigen from Salmonella typhi

Abstract

The immunogenicity, physical and chemical properties of three polygalacturonans:— pectin, O -acetylated pectin, and Vi— varied significantly depending on the presence of acetyl groups. The O -acetyl and N -acetyl groups that stabilize Vi to acid hydrolysis are also essential immunodeterminants. Cations at carboxyl groups have lesser effects on the immunologic properties of Vi. Based on the Courtald–Koltum space-filling model, it is observed that the bulky O - and N -acetyl groups dominate the molecular surface of the Vi or the O -acetylated pectin. As a result, these hydrophobic groups also shield the carboxyl groups from interaction with other molecules present as counterions in solvent-positive charged molecules or as receptors on cell surfaces. The binding of multivalent cations with polyanions consists of two major types: (1) the electrostatic binding of the cations with negative charges on the polyanions results in the same free energy of binding as the monovalent cation, and (2) intermolecular cooperative binding or the chelating effect. Potentiometric titration showed that when the Vi or O -acetylated pectin was highly O -acetylated, the binding of the carboxyl groups to Ca 2+ and K + was similar. The immunological properties of Vi are related to its content of O -acetylated groups. The binding activity of carboxyl groups may be affected sterically by the acetyl groups in close proximity.