Conformations and flexibility of native and re-natured xanthan in aqueous solutions

Abstract

The conformation and flexibility of sonicated ‘native’ and ‘re-natured’ xanthan have been investigated by size exclusion chromatography (SEC) with coupled multi-angle light scattering and viscosity detectors. ‘Native’ xanthan (NX) refers to xanthan dissolved in moderate ionic strength aqueous solution, which has not been exposed either to high temperature or very low ionic strength, and ‘re-natured’ xanthan (RX) here refers to xanthan which has been heated above the conformational melting temperature and then recooled. The mass distributions of the NX and RX are virtually identical, implying that the RX does not involve aggregates of, or disassociated fragments of, NX. The flexibilities and conformations between NX and RX, however, are strikingly different; RX is far stiffer than NX, the persistence lengths being roughly 1000 Å and 300 Å, respectively, and the mass per unit length M/L of the RX is roughly double that of NX. With estimated M/L of 200 Da/Å and 98 Da/Å, respectively, the results strengthen the notion that RX is double stranded, whereas as NX appears single stranded. The nature and mechanism of formation of the double-stranded form is still unclear, and a few speculative scenarios are suggested. Finally, preliminary results on the kinetics of xanthan self-association in HCl are presented which illustrate the complexity of such processes in xanthan.