Effects of ions on the disorder-order transitions of gel-forming polysaccharides

Recent advances in the understanding of the effects of various cations on the disorder order transitions of the charged carrageenans (iota- and kappa-carrageenan and furcellaran) are reviewed, with special emphasis on the occurrence and origin of cation specificity. Analysis within a general electrostatic model, the Poisson-Boltzmann cell model, shows that, for charged polysaccharides, the stabilizing effect of divalent ions on the ordered conformation is expected from non-specific electrostatic interactions alone. On the other hand, if the ordered conformation contains a site which may specifically bind certain cations (as for kappa-carrageenan), even a weak intrinsic binding constant may have large consequences for its stability in the presence of binding ions. The ordered conformation of furcellaran contains sites very similar to those in kappa-carrageenan, which specifically bind certain monovalent cations. Iota-carrageenan, on the other hand, contains no ion-binding sites, but is influenced only by non-specific electrostatic interactions. Ion-specific effects seen in conventionally prepared iota- carrageenan samples are due to small impurities of kappa-carrageenan structure. The nature of the binding site in kappa-carrageenan and furcellaran is unclear. It is concluded that it contains more than one functional group in the carrageenan helix, but not necessarily the sulfate group.