On the Conformational Transitions of Native Xanthan

Potentiometric titrations of aqueous solutions of native, biochemically purified xanthan are analyzed using Henderson—Hasselbalch plots. In addition to the well-known sigmoidal conformational transition, a new consecutive and more gradually evolving conformational transition is observed at low ionic strength and elevated temperatures. As far as we are aware, this transition has not been reported earlier. Rotational viscosity experiments confirm that the first transition induces a severe reduction of the relative viscosity, but the second transition causes a more dramatic change in the viscosimetric behavior: the well-known shear-thinning effect vanishes in the shear rate range investigated. This result shows the inequality of the final states after the first and the second transitions and demonstrates the necessity for the application of a three-state model. Combining the titration results with optical rotation and titration data from the literature, we prove the similarity of the first transition for native and sonicated xanthan. An empirical relation between the midpoints of the first transition, T_m and θ_m, is established which helps to predict the occurrence of a melting point of xanthan solutions at various temperatures and degrees of dissociation.