Gelation junction zones

Consistent with their chemical differences, the molecular structures of i- and K-carrageenans are not identical. A shorter pitch and an offset positioning of the two chains in the kappa helix is compatible with the lack of sulfate group on every 3,6-anhydrogalactose residue. The variations in molecular structures mirror the types of junction zones formed by these polymers and relate to the observed gelation properties. 

The HM and LM pectins give two very different types of gels the mechanisms of stabilization of the junction zones in the two cases are described and few characteristics given. The different molecular characteristics (DE, distribution of methoxyl or acetyl substituents, neutral sugar content or rhamnose content) play an important role on the kinetic of gelation, mechanical properties of the gel formed and also on the experimental conditions to form the stronger gels. All these points were briefly discussed. 

FlG. 2.—Schematic Form of the Proposed Mechanism for Gelation of i- and K-Car-rageenan. [In t-carrageenan gels, aggregation of double-helix junction-zones does not occur.]... 

It is noteworthy that although WAXS data indicate that DDOA and CAB exhibit comparable types of molecular organization in their bulk solid states 93. their gel fibers are very different. Junction zones of DDOA gels are dry and crystalline, while those of CAB and CAQ are swollen lyotropic microdomains whose organization is clearly different from that of the solid 114]. ALS and AZOC gelators confirm that the cholesteryl group is an important element in... 

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. 1. —Schematic Mechanism of Gelation by a Polymer Solution. [The fine details are not meant to apply universally for example, the polymer need not have the random-coil conformation in the sol, and junction zones in the gel may engage a greater part of the chain length and may also engage more than two chains.]...

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