Tuning Xanthan Viscosity by Directed Random Coil-to-Helix Transition.

Xanthan gum is a polysaccharide that is widely used as a thickening agent in numerous food, cosmetic, and technical applications. Therefore, the knowledge of the molecular interplay that builds up and stabilizes water-binding networks is crucial for the optimization of xanthan thickening performance. Using atomic force microscopy, rheometry, and inductively coupled plasma optical emission spectroscopy, we show a clear correlation between xanthan thickening properties and the ability to form characteristic secondary structures as well as the valence and amount of cations coordinated at the polysaccharide side chain. Based on these findings and the Debye-Hückel theory, we derive an ion-interaction model in which divalent cations mediate bridging of adjacent single polymer strands due to chelate-like coordination building stable secondary structures. We furthermore demonstrate in a cation exchange assay that xanthan secondary structures can be modified in a directed and reversible manner, which, in turn, alters its thickening properties.