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 the constitutive pectic acids of the Nitella cell walls were investigated by performing ion exchanges at different pH. The experimental results were then analysed in the framework of a model derived from the polyelectrolyte theory in which the competitive process of dissociation of the exchange sites and their complexation by counterions are taken into account. Divalent ions Ca2+ and Mn2+ interacted specifically with the exchange sites to give rise to strong thermodynamic association constants. They also induced conformational transitions of the pectic acids which allowed some site-specific association with monovalent ions, although the latter, in the absence of divalent ions, interacted only in a purely electrostatic manner with the charged sites. The complexation phenomenon of the monovalent ions also results in a feedback process which enhances or depletes the site-specific interactions of the divalent counterions. Changes in the counterion association with the wall exchange sites will take place without modification in the wall electrostatic field, when divalent ions are present at the usual pH. These specific interactions are supported by the values of the residual interaction energy, calculated from the variations of the apparent pKa of the polygalacturonic acids with their degree of protonation.