Binding of bivalent cations by xanthan in aqueous solution.

The interaction between xanthan and selected bivalent cations (Ca2+, Mg2+, Mn2+, Fe2+, Cu2+, Zn2+, Cd2+ and Pb2+) was studied by means of conductometry, viscometry, and nuclear magnetic resonance spectroscopy. Xanthan from Xanthomonas campestris was studied in comparison with dextran from Leuconostoc mesenteroides. While dextran does not develop specific interactions with the bivalent cations, the analysis of the experimental data shows that xanthan chains (Mn approximately 1.4x10(5) to 2.9x10(6) g/mol) reversibly bind Me2+ species in aqueous solution at pH 6. Conductometric and viscometric titrations show that a single bivalent cation forms a complex which involves two disaccharide units of the main chain together with two side chains. Based on dipolar magnetic interactions between Mn2+ and individual carbon positions of xanthan, a possible structure of a chelate-like complex is proposed which involves the pyruvate units at the terminal ends of the side chains as the main binding sites. According to the stoichiometric relation between cations and disaccharide units, a single bivalent cation is bound between the terminal ends of two side chains, leading to an intramolecular cross-link and a reduced hydrodynamic radius of the overall macromolecule. The results indicate that heavy metal ions (Cd2+ and Pb2+) link stronger to the xanthan chain than lighter cations (Ca2+ and Mg2+), a fact which may be of ecological relevance.