Sol-gel transition diagram and theoretical study of κ-carrageenan in the presence of calcium ions

The gelation and melting temperatures of calcium-induced κ-carrageenan gels in the range of ionic strength 0–40 mmol/L were measured by small deformation oscillatory rheology. Theoretical studies of the interactions between dimeric unit models that form the κ-carrageenan polymer chain with Ca²⁺ cations were also carried out by means of quantum chemistry techniques using density functional theory with M06-2X/6-311G**. Gelling (on cooling) and melting (on heating) temperatures moved to progressively higher temperatures upon the progressive addition of CaCl₂, yielding thermal hysteresis. The interactions of the κ-carrageenan model with Ca²⁺ ions were analyzed, and the results showed that κ-carrageenan can exhibit intra- and intermolecular hydrogen bonding and coordination with Ca²⁺ ions. The interaction energies that can present conformers of the κ-carrageenan model were located between 25.1 and 46.02 kJ/mol. Coordination between the Ca²⁺ cation and the polymer as well as the intra- and intermolecular interactions of two sugars may be one of the factors by which Ca²⁺ ions could promote the coil-helix transition, resulting in the formation of more aggregates acting as junction zones.