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The conformational properties of xanthans with partially hydrolyzed side chains were investigated 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 beta-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 beta-mannose in the side chains. Modification of the side chains 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 (delta Hcal) increased linearly with the fraction of beta-mannose, again indicating that a significant part (up to 80%) of delta Hcal 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 sigma by means of the Zimm-Bragg theory (OR data) or from the ratio between delta Hcal and the van't Hoff enthalpy (delta HvH) using DSC data showed a correlation between sigma and the content of beta-mannose, but the two methods gave different results when the content of beta-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 beta-mannose. Application of the Manning polyelectrolyte theory does not readily account for the observed delta Hcal values, neither does it provide new information on the nature of the ordered and disordered conformations in xanthan. |
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