SELECTIVITY AND COOPERATIVITY IN THE BINDING OF CALCIUM-IONS BY PECTINS

By the use of calcium and sodium specific electrodes, the study of the ionic interactions with well-characterized pectins has shown a greater affinity of pectins towards calcium ions when the degree of methylation of the samples and the ionic strength of the systems decreased. The affinity of pectic chains towards calcium ions increased also with the polymer concentration. Furthermore, the amount of free or bound calcium ions seemed to be independent of the gelation phenomenon, whereas the binding of sodium ions appeared to be strongly affected by this phase transition. It was shown that two types of interactions between the polymer and calcium ions can occur according to the solvent conditions; from an anti-cooperative character in water, due to typical polyelectrolyte effects, they showed a cooperative character in the presence of 0.1 M NaCl.