Sol-gel transition diagram and theoretical study of & kappa;-carrageenan in the presence of calcium ions

The gelation and melting temperatures of calcium-induced ?-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 ?-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 ?-carrageenan model with Ca2+ ions were analyzed, and the results showed that ?-carrageenan can exhibit intra- and intermolecular hydrogen bonding and coordination with Ca2+ ions. The interaction energies that can present conformers of the ?-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.

Automated summary

The gelation and melting temperatures of calcium-induced ?-carrageenan gels were measured, and the theoretical interactions between dimeric unit models of ?-carrageenan and Ca2+ ions were analyzed. The addition of CaCl2 increased the gelation and melting temperatures, indicating thermal hysteresis. The study showed that ?-carrageenan can form intra- and intermolecular hydrogen bonding and coordinate with Ca2+ ions, suggesting that the coil-helix transition is promoted by Ca2+ ions.