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.