Egg box model

With different pectins, one found that the activity coefficient of calcium has a value half that of magnesium this is interpreted as the basis of a dimer formation in presence of calcium. The specific interaction of calcium was described as the egg-box model first proposed for polyguluronate in which oxygen atoms coordinated to calcium [46]. Recently, the comparative behaviour of Mg and Ca with homogalacturonan was reexamined [47]. 

In other words, the homopolygalacturonic acids of the cell walls distribute in two equal amounts of high and low affinity exchange sites. This is compatible with the dimer formation predicted by the egg box model between pectin chains in the 2i helical conformation [9, 10]. According to this model, we would associate the high affinity sites with the inner faces of the dimers. 

Grant, G.T., Morris, E.R., Rees, D.A., Smith, P.J.C., Thom, D. (1973). Biological interactions between polysaccharides and divalent cations the egg-box model. FEES Letters, 32, 195-198. 

Figure 2. Egg-box model" of calcium pectate formation. Calcium ions (represented by circles) bound by chain-stacked sections of deesterified pectin molecules.

Substantial research efforts have been made to relate molecular properties to the structures of the junction zones and provide mechanisms for gel formation. Thus we have models for the formation and alignment of double helices for kappa-carrageenan, formation of triple helices for gelatin and egg box models for calcium induced gelation of alginate and pectin to mention a few examples (Morris 1986 Djabourov... 

Fig. 2 Microstructures associated with physically bonded polymer gels. (A) Multihelical junction zones of agar gels and (B) egg-box model junction zones of calcium alginate gels.

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

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