Structure of pectin

This paper will not described the chemical structure of pectins which is a difficult problem [1] even if the physical properties in solution and ability to form gel must be directly related with the distribution of the units along the chain. The functional properties of pectins are not only related to the neutral sugar content (up to 15 %) but also to the distribution of structural blocks having very different contibutions. 

Effect of Chemical Structure of Pectins on Their Interactions with Calcium... 

In pectic acids, all carboxyl groups are free, or at least not present as the methyl ester. Under suitable conditions, pectins will form jellies with sugar and acid, whereas the low-ester pectins will form gels with traces of polyvalent ions. The general structure of pectin is ... 

Figure 5. Tentative structure of pectin fragments released in apple juice by enzymic liquefaction (modified hairy regions MHR). (Reproduced with permission from reference 90, copyright 1987 Food Hvdrocolloids).

Specific enzymatic degradation has been useful in determining the structures of pectins, gums and other plant polysaccharides (115), the mucopolysaccharides of higher animals (116), and other complex carbohydrates (117) including those from microorganisms 118, 119, 120). In these heterogeneous polymers enzyme treatment is most useful for deter-... 

Figure 1. Structure of pectin. X is H or CHg. The amount of esterification varies with the source and method of extraction.

Figure 1. Molecular structure of pectins (a-(1 4)-linked D-galacturonic acid)...

The knowledge of only the primary structure of complex carbohydrates is no longer sufficient to understand and explain their function and specificity [61]. The three-dimensional structures of pectin and polyuronic acid determine their interactions with other ions, molecules and macromolecules and are significant for their function and biological activity [88,102-103] Computational chemistry methods may greatly help in the determination of the three-dimensional structure of polysaccharides allowing to xmderstand and explain the behaviour of the PGA chains and the diffusion of ions inside. We apphed [89-92] Molecular Dynamics (MD) to the study of the motion of small (6-24 units) Polygalacturonic Acid (PGA) chains either protonated or complexed by Na" and Ca ions. 

Figure 3.4 shows the structures of pectin and cellulose. Also shown arc the sugars that make up the polysaccharide backbone and side chains of the heterogeneous compound hemiceJIulose. 

Eliminations and other reactions do not necessarily take place only on the polymeric chain or only on the side groups. Combined reactions may take place, either with a cyclic transition state or with free radical formation. The free radicals formed during polymeric chain scission or during the side chain reactions can certainly interact with any other part of the molecule. Particularly in the case of natural organic polymers, the products of pyrolysis and the reactions that occur can be of extreme diversity. A common result in the pyrolysis of polymers is, for example, the carbonization. The carbonization is the result of a sequence of reactions of different types. This type of process occurs frequently, mainly for natural polymers. An example of combined reactions is shown below for an idealized structure of pectin. Only three units of monosaccharide are shown for idealized pectin, two of galacturonic acid and one of methylated galacturonic acid ... 

A representative structure of pectin is illustrated in Figure 9.2. Pectins are composed of a a-(l—>4) linked D-galacturonic acid backbone interrupted by single a-(l—>2) linked L-rhamnose residues (BeMiller, 1986). Depending on the pectin source and the extraction mode, the carboxyl groups of galacturonic acid units are partly esteri-fied by methanol and, in certain pectins, are partially acetylated. The mole-percent of rhamnose may be different depending on the source of pectins (Ryden and... 

In order to study the composition and structure of pectins and their changes during ripening, storage and processing, various procedures have been developed for fractional extraction of pectins. A typical laboratory scheme for sequential extraction of pectin from plant cell wall materials is summarized in Figure 9.3. (Selvendran and O Neill, 1987 Voragen et al., 1995, Vierhuis et al., 2000). Not every step in the scheme is used by all of the researchers, and decisions depend on the source materials. 

For a more detailed discussion of the chemical structure of pectins, see reference... 

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