Binding of calcium ions on acid polysaccharides of peach gum and distribution pattern of carboxyl groups in the macromolecule

The acid polysaccharide of peach gum and its macromolecular products of a gradual degradation by acid hydrolysis were characterized by molecular weight (Mw) and molar ratios of both uronic acids and neutral saccharide units. The single-ion activity coefficient of calcium ions (γCa2+) bound to these acid polysaccharides and to the corresponding aldobiouronic acids was measured and CD spectra of potassium and calcium salts of these compounds were recorded. It was ascertained that Ca2+ ions are bound to carboxyl groups of polysaccharides under study by an electrostatic bond in molecular disperse solutions. The mean distance of the neighboring carboxyl groups b = 0.83 to 1.00 nm, calculated from the measured values γCa2+ indicates a very close arrangement of uronic acid units, which does not undergo alteration with the stepwise degradation of the macromolecule. The obtained results let us conclude that the main chain of the polysaccharide is substituted in longer segments by D-glucuronic acid units and its 4-O-methyl derivative (monomeric side chains), at least at each second D-galactose unit.