Roles of potassium ions, acetyl and l-glyceryl groups in native gellan double helix: an X-ray study

Native gellan, the natural form of the polysaccharide excreted by the bacterium Pseudomonas elodea, has a tetrasaccharide repeating unit that contains l-glyceral and acetate ester groups, and forms only weak and elastic gels. Based on X-ray diffraction data from well oriented and polycrystalline fibers of its potassium salt, the crystals structure of native gellan, including ions and water, has been determined and refined to a final R-value of 0.17. The molecule forms a half-staggered, parallel, double helix of pitch 5.68 nm which is stabilized by hydrogen bonds involving the hydroxymethyl groups in one chain and both carboxylate and glyceryl groups in the other. Two molecules are packed in an antiparallel fashion in a trigonal unit cell of side a = 1.65 nm. Although the gross molecular morphology and packing arrangements are isomorphous with those observed in the crystal structure of potassium gellan, which is devoid of any substitutions, native gellan exhibits exceptional changes in its ion binding characteristics with respect to gellan. In particular, the l-glyceryl groups do not allow the gellan-like coordinated interactions of the ions and the carboxylate groups, within and between double helices, which are necessary for strong gelation. These results at the molecular level explain, for the first time, the differences in the behavior of the polymer with and without substitutions.