Site-specific counterion binding and pectic chains conformational transitions in the Nitella cell wall

The types of binding of different mono- and divalent ions to sites of theconstitutive pectic acids of the Nitella cell wallswere investigated by performing ion exchanges at different pH. Theexperimental results were then analysed in the framework of a model derivedfrom the polyelectrolyte theory in which the competitive process ofdissociation of the exchange sites and their complexation by counterionsare taken into account. Divalent ions Ca²⁺ andMn²⁺ interacted specifically with the exchange sitesto give rise to strong thermodynamic association constants. They alsoinduced conformational transitions of the pectic acids which allowed somesite-specific association with monovalent ions, although the latter, in theabsence of divalent ions, interacted only in a purely electrostatic mannerwith the charged sites. The complexation phenomenon of the monovalent ionsalso results in a feedback process which enhances or depletes thesite-specific interactions of the divalent counterions. Changes in thecounterion association with the wall exchange sites will take place withoutmodification in the wall electrostatic field, when divalent ions arepresent at the usual pH. These specific interactions are supported by thevalues of the residual interaction energy, calculated from the variationsof the apparent pKa of the polygalacturonic acids withtheir degree of protonation.