Interaction of divalent cations with polyuronates

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Abstract
The interaction of some divalent cations (Ca2+, Cu2+, Pb2+, Zn2+ and Cd2+) with two polyuronates (alginate and polygalacturonate) has been studied by thermodynamic and structural methods. From the thermodynamic results, the enthalpy of interaction is claimed to reveal the changes in the conformation and/or association of polymeric chains, while the dilatometric results are related with the solvent release due to ion binding. The degree of cooperativity of the ion-binding process (with Ca and Pb) is evidenced by the calorimetric and dilatometric data and, more quantitatively, by the Scatchard plot of the single-ion activity data (with Cu). Theoretical evaluation of the electrostatic energy change accompanying the process of mixing polyuronates with ions and/or of the conformational energy of the polysaccharide is carried out to discriminate the cases in which the binding process is complicated by other effects (chain aggregation or conformational change). Further structural information is obtained by studying the perturbation on the circular dichroic bands of the chromophore arising from the binding process.

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