Specific binding of trivalent metal ions to λ-carrageenan.

Carrageenans are a family of sulphated cell wall polysaccharides extracted from seaweeds and are widely used in different industrial sectors. Relative to κ-carrageenan (κ-car) and ι-carrageenan (ι-car), the ionic binding behavior of λ-carrageenan (λ-car) is far less studied. In this work, the interaction and binding behavior between λ-car and metal ions of different valency (Na+, K+, Mg2+, Ca2+, Fe2+, Fe3+, Al3+, Cr3+) have been investigated. In contrast to the non-specific interaction of the monovalent and divalent cations, specific binding has been identified between λ-car and Fe3+/Al3+. The specific binding could lead to either precipitation or gelation of λ-car, depending on the way of introducing Fe3+/Al3+ ions. Fe3+ and Al3+ exhibit the same binding stoichiometry of [M3+]/[repeating unit] = 1.0, with the former having a relatively larger binding constant. Cr3+, though having very similar physical properties with Fe3+/Al3+, is incapable of binding specifically to Cr3+. The phenomena could not be interpreted in terms of counterion condensation, and are rather attributable to a mechanism in which hexa-coordination of Fe3+/Al3+ and entropy-driven cation dehydration play crucial roles in driving the binding of the trivalent metal ions to λ-car.