Binding of Divalent Cations to Polygalacturonate: A Mechanism Driven by the Hydration Water

We have investigated the interactions between polygalacturonate (polyGal) and four divalent cations (M2+ = Ba2+, Ca2+, Mg2+, Zn2+) that differ in size and affinity for water. Our results evidence that M2+-polyGal interactions are intimately linked to the affinity of M2+ for water. Mg2+ interacts so strongly with water that it remains weakly bound to polyGal (polycondensation) by sharing water molecules from its first coordination shell with the carboxylate groups of polyGal. In contrast, the other cations form transient ionic pairs with polyGal by releasing preferentially one water molecule (for Zn2+) or two (for Ca2+ and Ba2+), which corresponds to monodentate and bidentate binding modes with carboxylates, respectively. The mechanism for the binding of these three divalent cations to polyGal can be described by two steps: (i) monocomplexation and formation of point-like cross-links between polyGal chains (at low M2+/Gal molar ratios, R) and (ii) dimerization (at higher R). The threshold molar ratio, R*, between these two steps depends on the nature of divalent cations and is lower for calcium ions (R* < 0.1) than for zinc and barium ions (R* > 0.3). This difference may be explained by the intermediate affinity of Ca2+ for water with respect to those of Zn2+ and Ba2+, which may induce the formation of cross-links of intermediate flexibility. By comparison, the lower and higher flexibilities of the cross-links formed by Zn2+ and Ba2+, respectively, may shift the formation of dimers to higher molar ratios (R*).