CONFORMATIONAL TRANSITIONS AND MODIFICATIONS IN IONIC EXCHANGE PROPERTIES INDUCED BY ALKALINE IONS IN THE NITELLA-FLEXILIS CELL-WALL

The decrease in the cationic exchange capacity (CEC) concomitant to th e replacement in the Nitella wall of adsorbed Mn2+ ions was measured i n different mixtures of alkaline ions. At low external concentrations, the loss of CEC is enhanced in presence of Li+ ions but is weaker whe n Na+ ions are present in the exchange mixtures. The relative affinity of the wall exchange sites for alkaline ions was Na+ > K+ > Rb+ great er-than-or-equal-to Cs+ > Li+. As the CEC is progressively reduced, th e wall discrimination between the different alkaline ions tends to can cel out except for the Na+-K+ pair. The wall preference for K+ is then increased. A diminution of the effective pK of the polygalacturonic a cids constitutive of the wall is also observed, while increasing the C EC loss. The simple disruption of divalent cation crosslinks cannot fu lly explain the CEC leakage at low monovalent concentrations. It is su ggested that the alkaline ions also cleave H bonds or solvatation-like bonds between the cell wall polyuronides and then cause a concomitant unfolding of the short pectic chains which involves their solubilizat ion.