The possibilities of the double helical structures of xanthan were re-examined thoroughly. Among the several possibilities, only parallel and antiparallel 51 double helices could be built and packed into a unit cell. The conformation of single chains in the parallel and antiparallel chains are very similar to each other. These double helical structures are compatible with several properties of xanthan in solution. The molecular structure of polysaccharides from Klebsiella serotype K8 has been examined by X-ray diffraction and computer-aided molecular modeling. The most favorable molecular model was not the one which had the maximum number of intra-molecular hydrogen bonds. This result suggests that in the analysis one should not a priori choose the molecular model which has the largest number of intra-molecular hydrogen bonds.
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