ON THE CONFORMATIONAL TRANSITIONS OF NATIVE XANTHAN

Potentiometric titrations of aqueous solutions of native, biochemicall y 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 o bserved at low ionic strength and elevated temperatures. As far as we are aware, this transition has not been reported earlier. Rotational v iscosity experiments confirm that the first transition induces a sever e reduction of the relative viscosity, but the second transition cause s 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 a nd the second transitions and demonstrates the necessity for the appli cation of a three-state model. Combining the titration results with op tical rotation and titration data from the literature, we prove the si milarity of the first transition for native and sonicated xanthan. An empirical relation between the midpoints of the first transition, T(m) and theta(m), is established which helps to predict the occurrence of a melting point of xanthan solutions at various temperatures and degr ees of dissociation.