Cross-linking by calcium ions

In plants and fungi, the extracellular matrices are more generally of polysaccharides, although they are present in animals too. The polysaccharides are modified celluloses, etc. and are often cross-linked by calcium ions. 

Fig. 2 Supramolecular natural polymeric hydrogels discussed in this chapter, (a) Chemical structure of the most-repeated sequence in collagen, forming the a-chain that folds in a three-stranded superhelix [135]. These superhelices bundle to fram the collagen fiber, (b) Representative chemical structure of fibroin and the antiparallel p-sheet formation connected by hydrophilic linkers, (c) Chemical structure of alginic acid, cross-linked by calcium ions (highlighted), (d) Left Top view of two a-helixes of keratin forming a coiled coll by hydrophobic interactions. Right Overview of subsequent formation of the fibril. The left part is adapted from [57] with permission of The Royal Society of Chemistry...

From the experimental data it can be concluded that the films have a diameter of about 5-10 xm and consisted of stearic acid, calcium stearate, and calcium carbonate (mainly calcite, no experimental information could be given concerning amorphous calcium carbonate at this point). The viscoelastic data pointed to a glassy state with a crystalline hardness. It is important to note that the films did not consist of a stearate monolayer, cross-linked by calcium ions (chalk soaps) as control experiments with CaCl2 observed. Derived from other control experiments it became clear that the growth of the films was due to a specific interaction of crystalline (or precrystalline) calcium carbonate and stearic acid. 

Dropping or spraying a sodium alginate solution into a calcium chloride solution produces microcapsules. The divalent calcium ions cross-link the alginate, forming gelled droplets. These gel droplets can be permanently cross-linked by addition to a polylysine solution. Lim and Sun developed this method for the encapsulation... 

Fig. 2. Egg box model for cross-linking of polypectate chains by calcium ions. Reprinted from reference 25.

The hydrogel has been synthesized by means of free radical polymerization of PVME [1] by y-radiation. A PVME-sodium alginate emulsion was obtained by mixing a 30% (by weight) PVME aqueous solution with a 1% sodium alginate water solution. Then, the emulsion was cross-linked by means of Ca " " ions in a 1M calcium chloride (CaCl2) bath solution and submitted to y-radiation of 0.91 M rad/h for 24 hours. The final PVME hydrogel samples were cut in the form of parallelepiped of various dimensions and left to equilibrate in a bath of de-ionized water at room temperature. 

Polyelectrolytes may also be precipitated through cross-linking by multivalent counterions. An example is the precipitation of alginate by the addition of calcium ions or other di- and trivalent metal ions. ... 

The polysaccharide can exist in a number of chemical forms. It is composed of uronic acids. If the acid groups are in the acid form (-COOH), the polysaccharide is called alginic acid, which is water insoluble. If the acid groups are in the car-boxylate form (-COO ) it is as the alginate or sodium salt (-COONa) or sodium alginate, which is water soluble. If the sodium ions are replaced by a divalent metal ion such as calcium, barium, or strontium, the polysaccharide is cross-linked by the metal ions to form gels. 

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