SALT-INDUCED EXTENSION AND DISSOCIATION OF A NATIVE DOUBLE-STRANDED XANTHAN

This study shows that xanthan molecules at room temperature may assume at least three different conformations in 0.1 M NaCl aqueous solutions in which the local structure is ordered: (1) the native compact double helix, (2) the extended double helix, and (3) the extended single helix. Experiments including viscosity, low-angle light scattering and optical rotation measurements have been carried out with a fully pyruvated and fully acetylated native laboratory sample supplied as fermentation broth. Two major conformation changes of the native double helix which were found irreversible in our experimental conditions can be induced by treatments at low ionic strength. After treatment in 10-4 M NaCl, xanthan is still a double helix in 10-1 M NaCl, but the backbone of each strand has been extended. After the sample has been in 10-5 M NaCl, the double helix has been dissociated and a single helix sample is obtained. Thus, the denaturing of xanthan is a two-step process. The first step consists of the extension of the two chains inside the double helix, and the second is a dissociation of the native double strand.