Junction zone

The discussion of Fig. 1 (see p. 269) will now be resumed to enquire into the way in which the chains are held together in junction zones, and the extent to which association can be explained in terms of our knowledge of polysaccharide structures and conformations. 

Some gels do have properties (such as an ability to heal ) that suggest that their junctions are continually breaking and re-forming, 

The methods available for the characterization of junctions will be illustrated by particular examples in the following Sections, but an overall view of the type of information that may be obtained will first be attempted. 

These and other qualitative inferences will be more fully discussed for particular examples. 

If a meaningful equilibrium constant, K, could be written for this process, the number of junctions should change with temperature in a way that is determined by Van t Hoff s equation  

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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. 

More informations are given in references 1 and 4. The conformation of the polymeric chain forming the junction zones where described previously [33]. 

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. 

X-ray scattering enhances the cation-rich zones and thus allows the determination of the network crosslinks, the so-called junction zone or egg-boxes... 

Immunogold localization of the pectic epitope has been performed on different types of cells cell suspensions, roots, shoots, meristems, coleoptiles, pollen grains, protoplasts from different species carrot, sugar beet, tobacco, oat... The pattern of labeling was always the same polygalacturonic acid was essentially located on the material expanded at three-way junctions between cells or lining intercellular space, but was not found in primary walls. No epitope could be located close to the plasma membrane (Fig. lO.a). Middle lamellae far from junction zones and walls of meristematic cells were never labeled. 

Fig. 10. Intercellular junction zones of carrot cells grown in suspension have been observed in electron microscopy after immunogold labeling with the 2F4 antibody, (a) no treatment of the sections prior to labeling the gold particles are restricted to the center of the junction zones (b) enzymatic (pectin methyl esterase) deesterification of the E.M. grids before labeling the deesterified pectins present in the primary walls now bind the probe. Scale bars = 1 pm.

Figure 7. The egg-box structure for the junction zones of dilute calcium pectate gels two galacturonan chains in the twofold (2i) helical conformation with calcium ions (shaded circles) locked between them.

Thus pectins in muro contain most elements of the cable model but have additional features due to esterification (acetyl- as well as methyl-) and branching. The ionic junction zones are similar to those of calcium pectate gels in vitro but also contam methyl-esterified junctions, and most of the single chains probably have a relatively high degree of methyl-esterification. 

Groups in equatorial positions seem to be more readily accessible for chain-to-chain reactions than groups in axial positions. Carboxyl groups seem not to share directly in the formation of the junction zones. They are important as regulators of the potential of the chain molecules. 

Rees and coworkers158 showed that, at 15°, i-carrageenan forms a gel whose 13C-n.m.r. signals are so broad that they cannot be detected, in contrast to those given by the solution at 80° (see Fig. 28). At the lower temperature, segmental motion is restricted by frequent, interunit junction-zones in a double-helix structure, in contrast to the gel of a /8-D-(l— 3)-linked D-glucopyranan, where the intermolecular association is not so complete, and portions of the polymer are sufficiently mobile to provide broad signals.159... 

This result is directly proportional to the minimum number of carboxylic acid sites required to form a stable junction zone on polymers. The number of calcium cations bound must also be directly related to the stability of the junction and its thermoreversibility. Finally, it must be also pointed out that it is not the pH, but the degree of neutralization, a, which controls calcium binding. 

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