Biophysical properties of Opuntia ficus-indica mucilage

The purified mucilage from Opuntia ficus-indica is a high MW polysaccharide which behaves as a polyelectrolyte. Viscosity of its solution is dependent on the Ca²⁺ ion concentration and on pH, being greatest at alkaline pH. The sedimentation coefficient was dependent on concentration. The molecule had an estimated axial ratio of 256 in water, and this was reduced at low pH and in the presence of high concentrations of Ca²⁺. The molecule was studied with light scattering and CD techniques and its UV spectrum was recorded. All these parameters were influenced by pH and by ion concentration. The gelation properties also changed with pH and with Ca²⁺ giving dense gels in its presence and loose ones in its absence. The results are interpreted in terms of changes in conformation of the molecule, changes in Ca²⁺ binding and degree of ionization of the molecule. An attempt is made to relate the molecular properties to the physiological function of the mucilage in the calcium and water economy of the plant.