Microstructure of mixed κ- and ι-carrageenan gels viewed by rheology and solid-state NMR

Pure and mixed κ- and ι-carrageenan (CR) were studied by rheology and solid-state nuclear magnetic resonance (NMR) (SSNMR), combining bulk viscoelasticity and microscopic local molecular conformational information for understanding the morphological arrangement of the microstructure of gel network. Independent formation of double helices of κ- or ι-CR was confirmed by rheological data, and two possible models, microphase separation and interpenetrating network (IPN), were also proposed. The cross-polarization magic angle spinning (CPMAS) measurements by SSNMR for mixed gels showed the "new peak" at 95.4 ppm, which was supposed to be mainly composed of concentrated κ-CR chains in phase separation model or the κ-CR with changed conformation due to dense interlacing in the IPN model, in both of which the co-aggregates of κ- and ι-helices may also be incorporated. The molecular dynamic of anomeric carbon of anhydrogalactopyranose (A1-C) region was evaluated by variable contact time (VCT) measurements via fitting for the deconvolution generated "new peak," pure κ-CR at 93.9 ppm and ι-CR 91.8 ppm. The obtained T_1ρ described the degree of homogeneity over a few nanometers and was therefore used to calculate the scale of homogeneity of mixture CR gels to be 1-3 nm. SSNMR was testified to be a practical method for research on multiphase hydrocolloids.