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Ion Selectivities of Some Extracellular Viscous Polysaccharides from Red Algae

The potassium and sodium ion selectivities of extracellular polysaccharides in red algae were investigated by an elemental quantitative determination of Na+ and K+, the release of these ions in a dialysis experiment, viscosity and potentiometric titration. The elemental analysis indicated that all polysaccharides studied had a higher affinity toward K+ than toward Na+ in a purified and ion-exchanged state, although only carrageenan had such a tendency in a natural state without purification. The release of Na+ from the solution was faster than that of K+ for all polysaccharides studied. Among these, κ-carrageenan released Na+ most readily, in agreement with the K+ ion selectivity of this polysaccharide. The intrinsic viscosity of the potassium salt was always smaller than that of the sodium salt. Also, viscometry revealed a marked difference in the affinity of K+ toward κ- and λ-carrageenans. The potentiometric titration curve, however, showed no difference for either Na+ or K+. These results support all cases of K+ ion selectivity, especially that of κ-carrageenan. Possible mechanisms of the difference in the ion specificity among these polysaccharides are discussed.