Interactions of ordered water and cations in the gel-forming polysaccharide gellan gum

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.