Solvent and ion effects on the molecular interactions of kappa-carrageenan

The conformational helix transition and gelation of κ-carrageenan was studied in aqueous solutions of glycerol and sorbitol using rheology and optical rotation. The hydrogen-bonding ability and dielectric constant of the solvent were found to play an important role on the temperature of the transition and on the interactions of ions with the polymer. As the hydrogen bonding ability of the solvent is decreased, the helix is stabilized due to poorer polymer-solvent interactions. The poorer solvent quality also results in a different mechanism of gelation—randomly oriented helical network in glycerol as opposed to ordered aggregates in water. Ion pairing is more prominent in glycerol and sorbitol relative to water due to the lower dielectric constant of these solvents. Specific ion interactions were not observed for carrageenan in glycerol.; The interactions of counterions with carrageenan was studied using 133Cs-NMR. A change in the mobility of the cesium ions on lowering the temperature revealed a mobile gel to a frozen “glass” transition. This was substantiated by rheology measurements that showed the modulus to remain constant in the frozen state, indicating the inability of the polymer strands to re-arrange and synerese. Conditions under which syneresis would occur can be predicted through NMR.; A possible explanation for the specificity of ion “binding” to the helix was proposed: differences in the energy loss due to dehydration of the ion and energy gain from coordination of the ion to oxygen atoms lining the interior of a helical “channel” could result in high selectivities. More work needs to be done to conclusively prove this.