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

Abstract

We have investigated the interactions between polygalacturonate (polyGal) and four divalent cations (M(2+) = Ba(2+), Ca(2+), Mg(2+), Zn(2+)) that differ in size and affinity for water. Our results evidence that M(2+)-polyGal interactions are intimately linked to the affinity of M(2+) for water. Mg(2+) 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 Zn(2+)) or two (for Ca(2+) and Ba(2+)), 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 M(2+)/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 Ca(2+) for water with respect to those of Zn(2+) and Ba(2+), which may induce the formation of cross-links of intermediate flexibility. By comparison, the lower and higher flexibilities of the cross-links formed by Zn(2+) and Ba(2+), respectively, may shift the formation of dimers to higher molar ratios (R*).

Competitive ligand exchange of crosslinking ions for ionotropic hydrogel formation

“…When CLEX was applied to form hydrogels of poly(galacturonate) at neutral pH, a CIC with a strong affinity to Ca 2+ (propylenediamine-N,N,N′,N′-tetraacetic acid (PDTA)) was also necessary (Fig 3D). This is because poly(galacturonate) has a higher affinity for Ca 2+ than alginate, (Log K = c.4.5-5.4 27 ).…”

Theophylline‐loaded pectin‐based hydrogels. II. Effect of concentration of initial pectin solution, crosslinker type and cation concentration of external solution on drug release profile

“…[20][21][22][23] Zinc ions help to develop antibacterial properties, re-epithelialization, and reducing inflammation in topical applications. Calcium is one of the most used ion types in pectin gel structure due to contribution to gel properties.…”

Crystallization of Calcium Carbonate in Alginate and Xanthan Hydrogels

“…The predominant formation of rosette-like calcite was explained by its nucleation on a gelled microparticles template. However, predominant growth of stuck-like calcite morphology in the xanthan systems has been explained by its nucleation directly on ionized carboxylate groups along the backbone of polysaccharide molecules (see SI: Description of molecular gelling process) [38][39][40][41][42]. In comparison to literature data, the systems investigated in this work are additionally complicated by strong gel formation and initial precipitation of metastable and stable solid phases in close physical contact.…”

“…Indeed, alginate contains about one COOH, while xanthan less than 0.4 COOH per sugar unit. The alginate hydrogels are formed by crosslinking their molecules with divalent cations, while the formation of xanthan gel is caused by releasing the water molecules that are attached to polysaccharide molecules and hydrogen bonding of chains [38][39][40][41][42]. The pore size distribution of both gels was controlled by varying the polysaccharide molecule concentration.…”