Characterisation and selectivity of divalent metal ions binding by citrus and sugar beet pectins

A scale of selectivity for the binding of calcium and some heavy metal ions by citrus and sugar-beet pectins was set up by pH-measuremnts. The same order of selectivity was found for the two pectins, decreasing as follows: Cu2+ similar to Pb2+ >> Zn2+ > Cd2+ similar to Ni2+ greater than or equal to Ca2+. Binding isotherms for Ca2+, Cu2+, Ni2+, Pb2+ and Zn2+ ions have shown a greater binding level when the ionic strength decreased and when the pectin concentration increased in the presence of 0.1 M NaNO3. By comparing binding isotherms, the same order of selectivity was found as by pH-studies. Scatchard plots and Hill index evaluation showed for all ions and all pectins anticooperative interactions in water. In the presence of 0.1 M NaNO3, citrus pectins displayed cooperative interactions for all metal ions. In contrast, for sugar-beet pectins, cooperative interactions only occured with Cu2+ and Pb2+. With Ca2+, Ni2+ and Zn2+ sugar-beet pectins displayed Scatchard plots which could not be distinguished from an anticooperative binding. This difference of behaviour could be related to the presence of acetyl groups decreasing the affinity of Me(2+) for sugar-beet pectins. Copyright (C) 1996 Published by Elsevier Science Ltd