Thermal Denaturation, Renaturation, and Aggregation of a Double-Helical Polysaccharide Xanthan in Aqueous Solution

Xanthan samples prepared in different denaturation and renaturation conditions were studied by circular dichroism (CD) and size exclusion chromatography equipped with a multi-angle light scattering on-line detector (SEC-MALS). It turned out from CD that xanthan looses the double helical conformation in pure water at 80 °C and almost recovers locally the native conformation in the renaturation. On the other hand, SEC-MALS results indicated that the xanthan double helix dissociates into two single chains in the denaturation concentration cd ≤ 1 mg/cm3, but not at cd = 10 mg/cm3 due to the increase in the ionic strength by xanthan itself, stabilizing the xanthan dimer. Furthermore the xanthan single chain dissociated at cd ≤ 1 mg/cm3 reconstructs the intramolecular double helical structure with a hairpin loop in the renaturation, while the xanthan dimer denatured at cd = 10 mg/cm3 aggregates by mismatched pairing. The degree of aggregation is enhanced at higher added salt concentration at the renaturation.