Structural and rheological properties of kappa (κ)-carrageenans covalently modified with cationic moieties

Cationized kappa-carrageenans containing 2-hydroxy-3-(trimethylammonium)propyl groups with various degrees of substitution (0.13–0.75) were synthesized by reaction of sulfated polysaccharides with 3-chloro-2-hydroxypropyltrimethyl ammonium chloride (QUAB 188) in alkaline solutions through the generation of the corresponding 2,3-epoxy reagent in situ. The structure of the modified algal polysaccharides was characterized without any further treatment or after methanolysis and/or enzymatic depolymerization, by means of spectroscopic tools (FT-IR, NMR, Mass spectrometry) and high-performance size exclusion chromatography (HPSEC). Significant differences in the rheological properties of these cationized kappa-carrageenans have been found depending on the DS values and the presence of KCl salt. Despite their lower molecular weights in comparison with native polysaccharides, cationized kappa-carrageenans with a DS of 0.75 exhibited high viscosity and gelling behaviors mediated by the high density of quaternary ammonium groups.