Preparation, authentication, rheology and conformation of theta carrageenan

Lambda carrageenan was extracted from hand-sorted tetrasporophytes of Gigartina skottsbergii. Theta carrageenan was prepared from it by treatment with alkaline borohydride. Infrared spectroscopy showed complete, or essentially complete, conversion of 4-linked residues to the 3,6-anhydride ring form, and no detectable contamination with gelling carrageenans (iota or kappa). Optical rotation and differential scanning calorimetry (DSC) showed no evidence of conformational transitions on cooling and heating. Solutions of theta carrageenan at 2.0 wt.% in water and in 0.1 M KCl were Newtonian at 20 °C, and substantially less viscous than a 1.0 wt.% solution of the lambda carrageenan precursor. Close Cox–Merz superposition was observed between steady-shear (rotational) viscosity (η) and complex dynamic viscosity (η∗) at equivalent values of shear rate ( γ ˙ /s−1) and frequency (ω/rad s−1), and the variation of log G″ with log ω was linear, with a slope of 1.0, strongly indicating a solution of disordered coils. The absence of double-helix formation, and associated gelation, is explained by sulfation at O(2) of 3-linked residues. A previous report of gel formation by theta carrageenan is attributed to the presence of some gelling carrageenan in the lambda sample from which it was prepared.