Multinuclear NMR study on the sol-gel transition of aqueous κ-and λ-carrageenan solutions: Selective site binding of group I cations

Multinuclear NMR study was carried out to study the interaction between group I cations (Na⁺, K⁺, Rb⁺ and Cs⁺) and κ- and λ-carrageenans. κ- and λ-carrageenans showed a marked difference in their physical properties, particularly in their gel-forming ability. Unlike aqueous λ-carrageenan solution, the gelation of κ- carrageenan produces highly selective binding sites for alkali metal ions, in which Rb⁺, K⁺ and Cs⁺ ions bind more strongly than Na⁺. The alkali metal NMR suggests the occurrence of selective site binding of ions to form the cross-linking domains in κ-carrageenan.