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.