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The influence of acetyl and pyruvic acid substituents on the solution and interaction properties of xanthan.
| Content Provider | Semantic Scholar |
|---|---|
| Author | Shatwell, Karolyn P. |
| Copyright Year | 1989 |
| Abstract | A range of xanthans (Na+ salt form) with varying levels of acetyl and pyruvate substitution were prepared by culturing different strains of Xanthomonas campestrLs, and by chemical deacetylation and depyruvylation. The polymers were characterized chemically using a number of colorimetric and chromatographic methods; the molecular weights were determined by light scattering and from intrinsic viscosity data. A range of physical techniques were used to study the solution and interaction properties of the materials, concentrating on four in particular. These were ps.646 (4.5% acetyl, 4.4% pyruvate), ps.1128 (7.7%, 1.7%), ps.PX061 (14.3%, 0.3%) and ps.556 (1.6%, 6.0%). Optical rotation (OR) was used to characterize the helix-coil transition behaviour of the polymers. In deionized water there was a strong correlation between Tm and the levels of acetyl and pyruvate substitution. The degree of conformational order and the thermal stability of the polysaccharides increased with the ionic strength. Sample ps.556 exhibited an unusual two-phase OR transition in the presence of NaCl. The flexibility of ps.646, ps.1128, ps.PXOg]_ and ps.556 were compared, using the Smidsrtfd and Haug B-value method and static light scattering. According to the B-value method the polymers with high pyruvate and low acetyl showed the greatest flexibility. However, the Kuhn segment lengths measured by light scattering in 0.1M KC1 did not vary significantly. The effect of salt on specific viscosity (7] SpQ) was studied using the same four materials. Addition of salt to a 0.2% solution of xanthan produced an initial sharp fall in viscosity attributed to the decrease in hydrodynamic volume resulting from charge shielding. This was followed by a gradual increase in viscosity believed to be due to enhanced macromolecular association. Viscosity data for ps.556 determined over a range of polysaccharide concentrations and ionic strengths (^20mM NaCl), together with limited data for ps.646 and ps.PXOg^, were shown to fit a master curve of T] SpQ versus c [T) ] . The data for ps.1128 however, did not superpose apparently because of the formation of stable high molecular weight aggregates upon freeze drying. Mechanical spectrometry was used to study the solution and interaction properties of xanthan, locust bean gum, guar gum and konjac mannan. Solutions (2.0%) of xanthan in aqueous NaCl showed typical weak gel network behaviour but in deionized water the behaviour resembled that of an entanglement system. The glucoand galactomannan solutions (1.0%) all showed entanglement behaviour both in deionized water and salt solution. The majority of xanthans (0.5%) interacted with locust bean gum and konjac mannan (1.0%) in deionized water to form a strong, thermoreversible gel network. The xanthan/konjac mannan gels had significantly higher melting and setting temperatures than the xanthan/locust bean gum gels, and the transition from the liquid to the gel state on cooling was much sharper. The xanthan/locust bean gum mixed systems formed gels down to a much lower total polysaccharide concentration. Mixtures of xanthan and guar gum did not gel but instead showed entanglement behaviour. Acetyl substitution appeared to inhibit gelation with both locust bean gum and konjac mannan and addition of NaCl also produced a significant reduction in gel strength. A correlation was found between Tm and gel strength for the xanthan/locust bean gum systems only and denaturation of the xanthan helix was shown to be unnecessary for the interaction with both locust bean gum and konjac mannan. Limited studies on ps.BD9A, a mutant polymer believed to lack the terminal mannose residue from the trisacharide side-chain, indicated that the terminal sugar residue plays an important role both in stabilizing the ordered helical conformation of xanthan and in the gelling interaction with locust bean gum. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.era.lib.ed.ac.uk/bitstream/handle/1842/11910/Shatwell1989.pdf;sequence=1 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
