Electron microscopy of native xanthan and xanthan exposed to low ionic strength

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Abstract
Optical rotation data indicate that xanthan can exist both in an ordered and a disordered conformation. Using molecular weights obtained from light scattering measurements and contour length distributions obtained from electron micrographs, we find that a native, filtered xanthan exposed to low salinity (< 10−4M NaCl) and subsequently returned to 0.1M NaCl has a highly elongated structure with a mass per unit length of 1950 ± 200 Dalton/nm. Our data thus suggest that the ordered conformation of this xanthan is double stranded. We find that native, filtered xanthan in 0.1M NH4Ac has a nearly similar structure, but exists in part as aggregates of varying shape and size. Electron micrographs of these xanthans in 10−4M NH4Ac (the disordered conformation) display a mixture of species ranging from unaggregated single‐ or perfectly matched double‐stranded species, to double‐stranded chains branching into its two subunits as well as double‐stranded chains with different degrees of mismatching. This study suggests that the perfectly matched antiparallel or parallel double‐stranded chain constitutes the lowest free energy state of the ordered conformation of xanthan in dilute aqueous solution.