Other Polysaccharides

Other polysaccharides whose solutions yield liquid crystalline phases include the bacterial polysaccharide xylan and the extracellular fungal polysaccharide produced by schizophyllum commune 

3 Polymra-s Comprising Both Rigid and Flexible Units or Sequences 

7 It will be evident that theory could be brought into agreement with experiment through arbitrary choice of an equivalent segment equal in length to the persistence of the schizophyllan triple helix. Independent justification for this choice is not apparent, however. 

Mesomorphic behavior has been observed in a number of random copolymers consisting of two kinds of units, one rigid and rodlike and the other flexible , in the sense of offering a variety of conformations and, hence, capable of disrupting perpetuation of the axis of the rigid unit. The p-hydroxybenzoate (PHB) unit is prototypal of a rigid unit it has often been incorporated with ethylene terephthaiate (ET) as the flexible unit present as the minor constituent (0.3-0.4 mole fraction). 

While the possibility of this dispersion of micro-domains of the nematic phase in an isotropic phase cannot be dismissed, concrete evidence for morphologies of this kind in nematogenic copolymers is not prominently in evidence. The longer sequences of rigid units undoubtedly are responsible for promotion of liquid crystallinity but, as theory suggests they appear to be uniformly dispersed Sequences of units that include many flexible members and hence are not rodlike may assume a role analogous to that of the solvent in a lyotropic system The nematic copolymer should, on this basis, consist of a single phase. 

The majority of polysaccharides considered in previous Sections of this Chapter have been examined in sufficient detail to permit their classification, at least provisionally, in terms of basal chains of sugar residues, and hence as members of a relatively small number of families of related molecular species. The exudate gum from the Nigerian tree Combretum leonense29,30 has been studied in considerable detail, and, on the basis of methylation and partial-hydrolysis data, the partial structures 92 to 94 have been proposed. However, the relation- 

The classification of exudate gums and mucilages in terms of basal chain-types provides a convenient framework for the consideration of polysaccharides of highly complex structure. Such a scheme also serves to emphasize the relationship of these to other groups of plant 

Partial-hydrolysis Products from Combretum leonense Gum and Beech Tension-wood Polysaccharide 

Dehydrophytosphingosines with Ci8-Sphingosine and with 2-Amino-2-deoxy-D-glucose and D-Calactose. 386 

Glycosphingolipids of Plant Materials and Micro-organisms 

Oat /3-glucan (or oat lichenin) is constituted similarly to the barley polysaccharide, but contains (1 — 4)- and (1 — 3)-linkages in the proportions of60 2 1 or61 3 1. Partial, acid hydrolysis of the polysaccharide yields cel-lobiose, laminaribiose, cellotriose, and the two trisaccharides XLVI and XLVII containing both (1 — 3)- and (1 — 4)-linkages.61 On the basis of these results, the following partial structure (XLVIII) has been proposed.61 

Chitin is a nitrogen-containing polysaccharide that forms the shells of crustaceans and the exoskeletons of insects. It is similar to cellulose, except that the hydroxyl group at C-2 of each glucose unit is replaced by an acetylamino group, CH3CONH—. 

Pectins, which are obtained from fruits and berries, are polysaccharides used in making jellies. They are linear polymers of D-galacturonic acid, linked with 1,4-a-glycosidic bonds. D-Galacturonic acid has the same structure as D-galactose, except that the C-6 primary alcohol group is replaced by a carboxyl group. 

Numerous other polysaccharides are known, such as gum arabic and other gums and mucilages, chondroitin sulfate (found in cartilage), the blood anticoagulant heparin (found in the liver and heart), and the dextrans (used as blood plasma substitutes). 

Some saccharides have structures that differ somewhat from the usual polyhy-droxyaldehyde or polyhydroxyketone pattern. In the final sections of this chapter, we will describe a few such modified saccharides that are important in nature. 

Chitin is found in crustacean sheiis and insect exoskeietons pectins, found in fruits, are used in making jeiiies. 

Increasing side-chain substitution of the mannose chain with galactose gives rise to a family of mucilages (Fig. 2.11). Such galactomannans are found as major storage components in the endosperm of seeds of Leguminosae (see Ta- 

---

Although most seeds contain starch as the principal food reserve, many contain other polysaccharides and some have industrial utility. The first seed gums used commercially were quince, psyUium, flax, and locust bean gum. However, only locust bean gum is stiU used, particularly in food appHcations quince and psyllium gums are only used in specialized appHcations. 

The common hemiceUulose components of arborescent plants are listed in Table 3. Xylans, arabinogalactans, and pectic substances are common to all while only traces (if at all) of glucomaimans are found in the cell walls of bamboo. Other polysaccharides are found in trace amounts in wood as well as in bark, growing tissues, and other specialized parts of trees. 

For some appUcations, microbial polysaccharides have supplemented or replaced those derived from plants or algae in other instances, microbial polysaccharides have been developed for specific appUcations that cannot be met by other polysaccharides. Further information is available (5—24). 

Historically, dietary fiber referred to iasoluble plant cell wall material, primarily polysaccharides, not digested by the endogenous enzymes of the human digestive tract. This definition has been extended to iaclude other nondigestible polysaccharides, from plants and other sources, that are iacorporated iato processed foods. Cellulose [9004-34-6] (qv) is fibrous however, lignin [9005-53-2] (qv) and many other polysaccharides ia food do not have fiberlike stmctures (see also Carbohydrates). 

A number of other polysaccharides, such as glycogen, dextran, chitin, etc., possess interesting structures for chemical modification [103,104]. Dextran has been used as a blood plasma substitute. Although it can be converted to films and fibers, chitin s relatively small resource restricts its commercialization. 

Unlike many of the catalysts that chemists use in the laboratory, enzymes are usually specific in their action. Often, in tact, an enzyme will catalyze only a single reaction of a single compound, called the enzyme s substrate. For example, the enzyme amylase, found in the human digestive tract, catalyzes only the hydrolysis of starch to yield glucose cellulose and other polysaccharides are untouched by amylase. 

Many different types of carbohydrate-containing molecules are located on the surface of microbial cells. Some of these are components of die microbial cell wall and are limited to certain types of micro-organisms such as bacterial peptidoglycan, lipopolysaccharides, techoic adds and yeast mannans. Other polysaccharides are not... 

The presence of organic acid substituents in exopolysaccharides increases the lipophilidty of the molecule. In addition, for some exopolysaccharides with relatively high organic acid contents, their interaction with cations and with other polysaccharides may be influenced. Several amino adds have also been found in bacterial exopolysaccharides, including serine and L-glutamic add (Figure 7.1). 

The alternative large scale recovery method to precipitation is ultrafiltration. For concentration of viscous exopolysaccharides, ultrafiltration is only effective for pseudoplastic polymers (shearing reduces effective viscosity see section 7.7). Thus, pseudoplastic xanthan gum can be concentrated to a viscosity of around 30,000 centipoise by ultrafiltration, whereas other polysaccharides which are less pseudoplastic, are concentrated only to a fraction of this viscosity and have proportionally lower flux rates. Xanthan gum is routinely concentrated 5 to 10-fold by ultrafiltration. 

Among other polysaccharides studied were those elaborated by Neisseria per-flava (starch-type polysaccharide), Polytomella coeca (a starch richer in amy-lopectin than most natural starches), Pseudomonas morsprunorum (Wormald) (levan), Acetobacter acetigenum (cellulose), Aerobacter aerogenes (NCTC 8172) (Klebsiella Type 164), Bacillus megaterium. Bacterium pruni, and Bacterium prunicola (polyfructoses of the levan type). 

Other polysaccharides of primary cell walls.-A complex mixture of enzymes including endopolygalacturonase, pectin methylesterase, and/or pectin lyase solubilizes a mixture of polysaccharides from the primary cell walls of fruits [57-64]. Food scientists have referred for some 15 years to this mixture of polysaccharides as the hairy region to describe the highly branched character of the polysaccharides in the fraction and to emphasize the contrast to unbranched homogalacturonan. The recent discovery of rhamnogalacturonan hydrolase [65,66], which selectively cleaves the backbone of RG-I, led to the realization that the hairy... 

A regular fine structure causing maximal hydrogen bonding between the molecules reduces the possibility of reversible processes and therefore reduces the gel-forming properties of the compound. Other polysaccharides with different types of monomers or with branched chain structures can be treated in a similar manner. However, much more study of their fine structure and the accessibility of their functional groups is necessary. 

Most of the xanthan gum used in oil field applications is in the form of a fermentation broth containing 8% to 15% polymer. The viscosity is less dependent on the temperature in comparison with other polysaccharides. 

An underlying assumption is that dextran is a representative polysaccharide and that results derived from its study can be applied to other polysaccharides. Effected modifications are intended to occur throughout the dextran material rather than only at the surface. This is achieved by employing solutions containing dissolved dextran. 

---