Pectins junction zones

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. 

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.

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. 

Inserted L-rhamnopyranosyl units may provide the necessary irregularities (kinks) in the structure required to limit the size of the junction zones and produce a gel. The presence of side chains composed of D-xylosyl units may also be a factor that limits the extent of chain association. Junction zones are formed between regular, unbranched pectin chains when the negative charges on the carboxylate groups are removed (addition of acid), hydration of the molecules is reduced (addition of a cosolute to a solution of HM pectin), and/or pectinic acid polymer chains are bridged by multivalent, eg, calcium, cations. 

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... 

Pectin consists predominantly of sequences of galacturonic acid residues (which are quite similar to the G units in alginate), with occasional interruptions by rhamnose residues. At least some of the carboxyl groups are methyl esterified, the precise distribution depending upon the plant source and age, and an important aspect still not fully understood. Reasonably in view of their structural similarity, pectins of low degree of esterification behave like alginates, and gel with divalent ions. The more esterified materials gel under conditions of low pH and decreased water activity, Le. where intermolecular electrostatic repulsions are reduced in this case the junction zones are thermoreversible at, say, 40 C. 

The best known property of pectin is that it can gel under suitable conditions. A gel may be regarded as a system in which the polymer is in a state between fully dissolved and precipitated. In a gel system, the polymer molecules are cross-linked to form a tangled, interconnected three-dimensional network that is immersed in a liquid medium (Flory, 1953). In pectin and most other food gels, the cross-linkages in the network are not point interactions as in covalently linked synthetic polymer gels, but involve extended segments, called junction zones, from two or more pectin molecules that are stabilized by the additive effect of weak intermolecular forces. 

FIGURE 9.8 Structure of the junction zone in high-methoxyl (HM) pectin gels as inferred from x-ray diffraction studies. The hydrogen atoms of the hydrophobic methyl groups are represented by filled circles, and hydrogen bonds are indicated by dotted lines. With permission from Oakenfull and Scott (1984). 

Walkinshaw, M.D. and Arnott, S. 1981b. Conformations and interactions of pectins. II. Models for junction zones in pectinic acid and calcium pectate gels,. /. Mol. Biol., 153 1075-1085. 

Cardoso et al. [13] also compared the dependency of the viscoelastic properties of mature OPE/calcium gels upon the polymer and calcium concentrations to those of the LMP. They showed that, for these variables, both pectin systems exhibited a power law dependence of the G. At pH 7, for the different concentrations of non-esterified carboxyl groups available in the pectin (o-GalA ), the PPE/calcium and citrus LMP/calcium systems exhibited similar dependencies on the calcium concentration (Fig. 8a), with a power law dependence of 2.9-3.3. Still, the gelling ability of OPE/calcium systems was more dependent on the polymer concentration than the citrus pectin. For the different calcium concentrations tested, the corresponding exponents of power law dependency were approximately 3.0 and 1.9 for OPE/calcium and citrus LMP/calcium systems, respectively (Fig. 8b). These results also confirm the lower capability of the pectic olive extracts to form, under similar ionic conditions, elastically effective junctions zones. 

---