ISSN:
0006-3525
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The conformational properties of xanthans with partially hydrolyzed side chains were in vestigated by optical rotation, CD, and differential scanning calorimetry (DSC). All variants displayed the well-known temperature-driven, cooperative order-disorder transition, and both optical rotation and DSC showed that the transition temperature was essentially independent of the content of terminal β-mannose. It was found that up to 80% of the changes in the specific optical rotation accompanying the transition reflects conformational changes linked to the terminal β-mannose in the side chains. Modification of the sidechains also affected the CD when xanthan was in the ordered state, but in this case the data suggest that the glucuronic acid is the major component determining the magnitude of the CD signal. DSC measurements showed that the transition enthalpy (ΔHcal) increased linearly with the fraction of β-mannose, again indicating that a significant part (up to 80%) of ΔHcal reflects conformational changes in the side chains. The conformational transition of the xanthan variants generally showed a higher degree of cooperativity (sharper transition) than unmodified, pyruvated xanthan. Calculation of the cooperativity parameter σ by means of the Zimm-Bragg theory (OR data) or from the ratio between ΔHcal and the van't Hoff enthalpy (ΔHvH) using DSC data showed a correlation between σ and the content of β-mannose, but the two methods gave different results when the content of β-mannose approached 100%. The ionic strength dependence of the transition temperature, expressed as d (log I)/d(T-1m), was nearly identical for intact xanthan and a sample containing only 6% of the terminal β-mannose. Application of the Manning polyelectrolyte theory does not readily account for the observed ΔHcal values, neither does it provide new information on the nature of the ordered and disordered conformations in xanthan. © 1993 John Wiley & Sons, Inc.
Additional Material:
12 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/bip.360330115
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