Site-specific counterion binding: application of the standard Poisson-Boltzmann cell model to ionic polysaccharides of the plant cell wall

A numerical method is presented for analysing the effects of the competitive processes of dissociation and complexation that occur at the level of the functional groups of charged polysaccharides during ion exchange experiments carried out on plant cell walls. The interactions between the exchange sites and the mobile ions are handled by the standard Poisson-Boltzmann cell model. The dissociation and the complexation are described by mass-action laws. Uncomplexed counterions are involved in the formation of a cylindrical double layer. This latter type of interaction determines the dissociation degree of the polyion. A least-squares algorithm is used to evaluate the complexation constants of the counterions from experimental data. The need of introduction of the complexation constant of the divalent counterions has been clarified by a comparison between theoretical titration curves calculated with and without specific site binding. The fraction of complexed sites does not agree with that predicted from Oosawa's theory of the condensation. The accumulation coefficients, calculated from the optimised theoretical titration curves, clearly demonstrate the competition between the protonation and the complexation processes at the level of the pectic acids of the wall.