NMR of Na+, glycine and HDO in isotropic and anisotropic carrageenan gels

Abstract

The chemical-interaction environment of several gels made from carbohydrate polymers consisting of unmodified commercial-grade ι- and κ-carrageenan and varying in their NaCl and polysaccharide concentrations was studied by solution-state ¹H, ²H, and ²³Na NMR spectroscopy. All gels showed isotropic NMR spectra at low gel concentrations (1% ‘w/w’) inside 10 mm NMR tubes. Anisotropy was introduced by stretching the gels, and the degree of alignment depended on the extent of stretching as well as gel and NaCl concentration. For Na⁺ a strong binding component of the anisotropy in ι- and less in κ-carrageenan gels was found, in contrast to a partial binding of glycine, and a spatial and gel-concentration dependent anisotropic effect for deuterated water (HDO). This finding is explained by the possibility of electrostatic interaction between Na⁺ and ionic sulfate groups in the carrageenan polymer, HDO probably only interacts via hydrogen bonding, while glycine could interact by both means. The lifetime of bound Na⁺ is likely to be small but bigger than for hydrogen-bonded HDO; thus Na⁺ would be more exposed to the electric field gradient tensor (caused by stretching the gel) than HDO, resulting in larger anisotropic spectral splitting and explaining the strong NaCl-concentration dependence of ²³Na⁺ NMR anisotropy, which was not evident with HDO in ²H NMR spectra.