Xanthan chains

Figure 3. Diagram of the effect of shear gradient on shape and orientation of a wormlike xanthan chain and a rigid-rod model.

Can the wormlike model fit other data It was argued above that the Kuhn-equivalent approximation to the xanthan chain (M=10 ) is 50 freely jointed links each 100 nm long. For this chain, the root mean square radius of gyration s can be estimated ... 

Milas M., Rinaudo M., Duplessix R., BorsaU R., Lindner P., Small angle neutron scattering from polyelectrolyte solutions From disordered to ordered xanthan chain conformation. Macromolecules, 28,1995, 3119-3124. 

The main aim of this study was to investigate the effects of the origin and amount of proteins on the aggregation of xanthan chains in solution by means of rheological measurements. Modifications of fermentation conditions led to variations of intrinsic viscosity, Huggins constant and pyruvate content of the xanthan produced. 

It can be seen that the addition of com steep liquor to a xanthan solution led to a drastic decrease in the viscosity in conjunction with an increase of the Huggins constant (Fig.l). This decrease of the macromolecule dimension, due to an entanglement of the polymer chains, and the prevalence of polymer-polymer interactions to solvent interactions, indicate that the polysaccharide molecules are aggregated. A possible explanation of this phenomenon is that proteins from com steep liquor can induce interactions between xanthan chains, forming xanthan-protein complexes. 

In sodium chloride brine, the addition of proteins did not alter the pseudoplastic behavior of xanthan. In sea water, even with a long stabilization time for each measurement, stress values were not identical when the shear rate was increased or decreased, indicating a thixotropic behavior. Protein addition did not significantly enhance this phenomenon. Thus, thixotropy seems to be related rather to the presence of cations inducing an aggregation of xanthan chains than to the addition of proteins. Nevertheless, in sea water, the gel formed by the extra-addition of proteins seems to have a more compact consistency than that of the gel induced by multivalent ions alone. This was probably the result of a synergic effect, e.g. the expulsion of some water molecules from the interior of the gel network by proteins. Experiments such as the determination of the elasticity module could bring some complementary information about the structure of the gel formed in these two cases. 

As described above, the formation of xanthan-protein complexes induced by salts such as calcium or magnesium chloride depends strongly on the quality of the solution. It was previously reported that when xanthan solutions contained less impurities, interactions between macromolecules and calcium ions were less important, and that the pH limit corresponding to precipitation was shifted to basic pH (75). Here, when xanthan C broth was centrifuged, thixotropic phenomena were less apparent. So, for this xanthan solution, it is clearly shown that the presence of cellular components was essential to induce interactions between xanthan chains and cations. It can also be noted that the xanthan broth was frozen before all the experiments and thus, that the cells may have been lysed thereby releasing all their constituents inside the medium. Because of the diversity and complexity of the cellular components, it is difficult to predict which kind of interactions can be involved in the aggregation. 

Effect of the origin of xanthan samples. Different representative samples of xanthan were tested for their ability to display thixotropic behavior in the suitable conditions determined from the aforementioned results. Following the same procedure, all xanthan samples were dissolved in sea water at pH 8 in the presence of added proteins. The flow curves, i.e stabilized values of shear stress at three shear rates, were determined and are shown in Figure 6. Under these conditions, where the aggregation of xanthan chains was favoured, only xanthan C exhibited thixotropic behavior. The other products had the standard behavior of a pseudoplastic fluid and the shear stress values were identical when the shear rate was increased or decreased. 

The addition of proteins can induce an aggregation of xanthan chains according to the ionic composition of the solvent (presence of cations such as calcium or magnesium) and to the pH. Furthermore, the flow behavior of xanthan-protein solutions exhibited some thixotropic phenomena... 

The cells, the nature of which depends on fermentation conditions, had a particular effect on the interactions between xanthan chains, cations and added proteins. 

Figure 31 displays the influence of sodium hydroxide concentration on the power-law index for each of the polymer solutions examined in Figure 30. At a given polymer concentration, the power-law index significantly increased as sodium hydroxide concentration was increased to 1 wt%. The rate of change of the power-law index with sodium hydroxide concentration was greatly reduced at sodium hydroxide concentrations greater than 1 wt%. These results indicate, considering the rod-like shape of xanthan chains, that the hydrodynamic radius of the polymer is significantly...

Figure 16 Proposed interactions of xanthan and gluco- or galactomannan. (a) Attachment of unsubstituted regions of mannan backbone to xanthan hehx. (b) Attachment of mannan backbone through heterotypic junction zones to extended disordered xanthan chain, (c) Attachment of mannan backbone through heterotypic junction zones on xanthan hehx, which uncoils to an extended disordered chain. (Reprinted with permission from Ref. 59. Copyright 1995 American Chemical Society.)...

The main xanthan chain consists ofp-D-(l— 4) glucose units that also occur in cellulose. Side chains (usually trisaccharides) consist of a D-glucuronic acid residue and two D-mannose residues (4-180). To the terminal D-mannose unit of the side chain is attached to P-d-glucuronic acid through - (1 4) glycosidic bond. Glucuronic acid is further linked to a-D-mannose by a (1— 2) bond. The structure is further complicated by the presence of pyruvic add linked, as an... 

The complete biosynthesis of xanthan, a water-soluble bacterial polysaccharide with a cellulose backbone (see Chapter 6, Fig. 6.18 for the structure), has been worked out by Dankert et al. [111-114]. The pentasaccharide repeating unit is attached to a polyprenol pyrophosphate lipid carrier [111], and the polymerization takes place by the addition of the repeating pentasaccharide from the polyprenol pyrophosphate pentasaccharide to the reducing end of the growing xanthan chain [114]. The mechanism of xanthan chain elongation is an insertion mechanism similar to those of murein. Salmonella 0-antigen, the dextrans, mutan, and alteman. 

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