Pectin covalently cross-linked

The pectin network revisited.--The importance of the interconnections of the pectic polysaccharides to the integrity of the pectin network has been highlighted by the recent discovery that RG-II is present in primary walls as a mixture of monomers and dimers [54]. The dimers are covalently cross-linked by borate diesters [55,56]. If single molecules of homogalacturonan are covalently attached to both RG-I and RG-II, the covalently cross-linked RG-n dimers would explain how the network of the three types of pectic polysaccharides is covalently connected and covalently cross-linked. 

The boron requirement and wall pectin content of many plants are correlated [54-56]. There is increasing evidence that a function of boron essential for normal plant growth and development is to covalently cross-link wall pectins (see Figure 5A). The structural organization of a boron cross-linked pectin network is likely to be a factor that contributes to the physical and biochemical properties of the wall because boron deficiency results in abnormal walls. For example, the results of recent studies provide evidence that the pore size of pectin-rich cell walls is determined in large part by borate ester cross-linking of RG-II [57]. 

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. 

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