Calcium, complex with polysaccharides

Alkali-earth metals (calcium, barium, and magnesium) complex with polysaccharides extensively (Reisenhofer et al., 1984). Calcium has a smaller atomic and ionic radius than does sodium and, because it has two valence electrons, it is endowed with greater polarizing and bonding ability than Na+. Ca and Ca2+ easily form insoluble complexes with oxygenated compounds. Polysaccharide salts of alkali-earth metals are generally insoluble. 

The action of chlorine in alkaline media is much slower than that of bromine. Lewin29 reported that the rate of oxidation of D-glucose at pH 9.8 by hypobromite is 1360 times higher than that by hypochlorite at the same pH. For cellulose, the ratio is much smaller (33 to 1). The complexity of the latter system is, however, revealed by the variability of this ratio over the pH range of 8-13 at pH 6-7, the action of hypochlorite is actually slightly faster than that of hypobromite. Maltodextrins and starch have been oxidized with alkaline sodium hypochlorite. The resulting oxidized polysaccharide formed stable complexes with calcium cations.30... 

M. Floor, A. P. G. Kieboom, and H. van Bekkum, Preparation and calcium complexation of oxidized polysaccharides. 1. Oxidation of maltodextrin and starch with alkaline sodium-hypochlorite, Starch-Starke, 41 (1989) 348-354. 

Whereas all of the methods proposed for large-scale fractionation of starch that have been discussed depend directly on the ability of amylose to form itLsoluble complexes with polar organic compounds. Cantor and Wimmer s process is based on a totally different principle. If a molecularly disperse solution of starch contains a sufficient amount of calcium chloride and caustic alkali is added, a rapid and quantitative precipitation of the starch occurs, because of the formation of complexes (of calcium hydroxide with the starch polysaccharides) which are insoluble in an aqueous, saturated solution of calcium hydroxide. The same phenomenon is observed with the hydroxides of barium and strontium. 

The conformation of acidic polysaccharides and their interactions with calcium ions was examined by molecular simulation, and the authors demonstrated the existence of specific calcium binding with poly-a-L-guluronate [56, 57], The mechanism of complex formation involves calcium interactions with different oxygen atoms of two adjacent guluronic acid units and with two inter-chain units, as visualized in the egg-box model (Fig. 24.7). The mechanism of gelation is a two-step process first step is a dimer formation, followed by precipitation for small chains, or gelation for long ones formed with different types of blocks. 

The controlled deposition of calcium salts is essential for the development of extracellular structures such as bones, teeth and shell. The process begins with uptake of calcium in the intestine, followed by transport, and then the laying down of structures. A complex system is necessary for the control of all these stages, and involves, for example, vitamin D, parathyroid hormone, calcium-binding proteins for transport, and a range of other proteins and polysaccharides for ordered deposition. Precipitation of calcium salts in the incorrect location can result in stone formation, osteoarthritis, cataracts and arterial disorders. 

---