Abstract
Direct visual evidence obtained by atomic force microscopy demonstrates that when xanthan is adsorbed from aqueous solution onto the heterogeneously charged substrate mica, its helical conformation is distorted. Following adsorption it requires annealing for several hours to restore its ordered helical state. Once the helix state reforms, the AFM images obtained showed clear resolution of the periodicity with a value of 4.7nm consistent with the previously predicted models. In addition, the images also reveal evidence that the helix is formed by a double strand, a clarification of an ambiguity of the xanthan ultrastructure that has been outstanding for many years.
Highlights
Xanthan is a bacterial polysaccharide produced by Xanthamonas campestris (GarciaOchoa et al 2000)
The present study reveals new images of xanthan at sub-molecular resolution revealing the fine detail of its secondary structure development, which has enabled the process of charge screening to be investigated in a manner never previously reported
The value of the height of this chain is slightly larger than the predicted width (1.8nm) of xanthan in the double helical form (Millane 1990)
Summary
Xanthan is a bacterial polysaccharide produced by Xanthamonas campestris (GarciaOchoa et al 2000). The terminal mannose units may contain a pyruvic acid substitute and the -linked mannose units may have an acetyl group at position O-6 (Phillips & Williams 2009). Another two recent studies have shown that there can be more heterogeneity of xanthan’s repeat unit than was previously assumed in terms of the ratio of the charge groups within xanthan’s sidechains depending upon the fermentation conditions: There are 6 different patterns of attachment of pyruvate and acetate groups to the pentasaccharide repeat unit, and the relative abundance of these affects the stability of the ordered structure The charged groups on the sidechains play a vital role in xanthan’s aqueous solubility and its structural conformation
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