X-ray structure analysis of the sodium salt of beijeran.

The three-dimensional structure of the sodium salt of beijeran has been determined by X-ray fiber diffraction analysis. The acidic polysaccharide forms an extended twofold helix. Two chains are nestled tightly in a monoclinic unit cell of dimensions a=12.72, b=11.41, c (fiber axis)=24.62 A and gamma=123.7 degree in an antiparallel fashion. In the crystalline lattice, helices are stacked tightly to form a thick sheet along the vertical plane passing through the short diagonal of the basal net. Adjacent sheets associate via a network of sodium ions and water molecules embedded between them. The morphology of sodium beijeran in the solid state is consistent with its observed rheological properties.