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, Mg, Ca, Fe, Fe, Al, Cr) 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 Fe/Al. The specific binding could lead to either precipitation or gelation of λ-car, depending on the way of introducing Fe/Al ions. Fe and Al exhibit the same binding stoichiometry of [M]/[repeating unit] = 1.0, with the former having a relatively larger binding constant. Cr, though having very similar physical properties with Fe/Al, is incapable of binding specifically to Cr. The phenomena could not be interpreted in terms of counterion condensation, and are rather attributable to a mechanism in which hexa-coordination of Fe/Al and entropy-driven cation dehydration play crucial roles in driving the binding of the trivalent metal ions to λ-car.