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X-ray fiber diffraction patterns from deacylated acetan and glucomannan (konjac mannan) blends are diagnostic of good orientation and modest polycrystallinity. The meridional reflection on the sixth layer line suggests that the binary complex is a 6-fold helix of pitch 55.4 A. A molecular modeling study incorporating this information reveals that a double helix in which one strand is acetan and the other glucomannan is stereochemically feasible. While the backbone and side groups are sufficiently flexible to allow the chains to associate with the same or opposite polarity, the parallel model is superior in terms of unit cell packing. The results are compatible with the observed synergy; namely the weak gelation behavior of the complex. The molecular model can be generalized for the binary system when acetan is replaced by xanthan or glucomannan by galactomannan.

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.

X-Ray diffraction analysis of the sodium salt of the polysaccharide RMDP17, a 2-deoxy rhamsan analog, reveals that it adopts a gellan-like, half-staggered, threefold, left handed, double helix of pitch 57.4 A. The side chain of the branched polymer is hydrogen bonded to the main chain. Sodium ions, linked to the carboxylate groups, promote the association of helices via water molecules. Two helices of opposite polarity occupy a trigonal unit cell of dimensions a=17.6 and c=28.7 A. The packing arrangement displays a series of hydrogen bonds involving main chain and side chain atoms, as well as some water bridges, between the helices.

Rhizobium meliloti mutants produce a linear, acidic exopolysaccharide with alternating galactopyranosyl and glucopyranosyl units having (1-->3) linkages. It has a 4,6-O-pyruvic cyclic acetal group on the alpha-galactosyl and 6-O-acetyl ester group on the beta-glucosyl units. X-ray diffraction patterns from polycrystalline and well oriented specimens of its potassium salt indicate that the polymer forms a 2-fold helix of pitch 15.89 A. The three-dimensional structure has been determined and refined by using the X-ray intensities and the linked-atom least-squares technique. The details of the antiparallel packing arrangement of two helices in an orthorhombic unit cell, a = 14.49, b = 9.79, and c = 15.89 A, reveal that each disaccharide repeating unit is associated with one potassium and three water molecules. The helices are interconnected by a series of ...COO- ...K+ ...W ...COO- ... interactions. Both pyruvyl and acetyl groups, which are on the periphery of the helix, are involved in the association of the polysaccharide chains and thus appear to be an integral component of the galactoglucan in the nodule invasion process.

Welan is the first branched polymer in the gellan family of polysaccharides whose three-dimensional structure has been determined by X-ray diffraction analysis of polycrystalline and well oriented fibers of the calcium salt. The molecule exists as a half-staggered, parallel, double-helix, similar to that of gellan. The side chains fold back on the main chain to form hydrogen bonds with the carboxylate groups. This shielding enhances the stability of the double-helix. Three molecules are organized in a trigonal unit cell of dimensions a = 20.83 and c = 28.69 A with a lateral separation of 12.0 A in each pair; this is 2.9 A larger than in gellan. The double helices are in contact with each other through calcium ions and water molecules via COO-...Ca2+...COO- and COO-...W...Ca2+...COO- interactions, and through side chain-side chain hydrogen bonds. These structural features enable us not only to explain how the side chains in welan are responsible for the enhanced molecular stability relative to gellan, but also to show how essential they are for the associative properties which control the rheology of the polymer.

X-ray diffraction analysis of oriented fibers of the gel-forming, neutral, doubly branched, galactoserich capsular polysaccharide from Rhizobium trifolii has been used to determine and refine the molecular structure to a final R value of 0.28. The polysaccharide forms a 2-fold single helix of pitch 20.2 A, is stabilized by a series of hydrogen bonds that involve the side chains, and has the appearance of a double helix. Packing calculations reveal that the short-range ordering of these "pseudo double helices" is brought about by intermolecular hydrogen bonds that involve the side chains. Complete or partial removal of side chains will weaken the molecular association and is detrimental to the gelation process.

Native gellan, the natural form of the polysaccharide excreted by the bacterium Pseudomonas elodea, has a tetrasaccharide repeating unit that contains L-glycerol 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 crystal structure of native gellan, including ions and water, has been determined and refined to a final R-value of 0.17. The molecule forms of 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 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 carbohydrate 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.

The primary structures of the four bacterial polysaccharides gellan, welan, S-657, and rhamsan are the same with respect to their backbones, but have different side-chains. This difference has a profound influence on their behavior in aqueous media. Solutions of gellan gum form stable aqueous gels under appropriate ionic conditions. By contrast, welan, S-657, and rhamsan do not gel but give very viscous solutions over a wide range of thermal, pH, and salt conditions. X-Ray fiber diffraction analysis and computer modeling of these branched polysaccharides demonstrate that they all have the same half-staggered, double-helical conformations as in the unbranched gellan, suggesting, therefore, that the side chains are responsible for diminishing gelling behavior. Depending on the size and location, the side chains shield the carboxylate groups to varying degrees; this shielding is substantial in welan and S-657, but less in rhamsan. In all cases, side-chain-main-chain interactions within the double helix prevent the carboxylate-mediated aggregation of double helices that is necessary for the gelation.

Gellan gum, useful to the food industry, is chosen as a model system for the investigation of the structural role of water molecules and cations in its gel-forming properties. X-ray fiber diffraction technique has been used to determine the crystal structure of potassium gellan. This gives the three-dimensional structure of the polysaccharide, the locations of ordered water molecules, and of potassium ions. The precise interactions among the three components at molecular level reveal that the water molecules are essential for the stability of the polysaccharide chains, for the binding of cations with the polymer molecules, and for the aggregation of the polymers, all of which are crucial for the gelation process. Computer modeling shows how the calcium ions can directly crosslink adjacent gellan molecules, but the potassium ions cannot, and this explains the stronger gelation properties of calcium gellan, even at very low ionic concentrations. L-glycerate, but not acetyl, groups interfere with the intermolecular association of native gellan molecules and are thus responsible for its weak and rubbery gels.