Svante Nilsson

Effects of small amounts of kappa-carrageenan on the rheology of aqueous iota-carrageenan

Abstract The temperature dependence of the rheology (shear storage and loss moduli) of two ion forms (sodium and potassium) of aqueous iota-carrageenan was investigated in the presence and in the absence of small fractions (≤10%) of added kappa-carrageenan. Both native iota-carrageenan (which already contains kappa-carrageenan as an impurity) and a sample purified by kappa-carrageenase was investigated. In either ion form, pure iota-carrageenan forms a weak gel as a consequence of its conformational transition, and the gel becomes stronger as the helical content increases. No marked ion specificity is seen in the rheology of pure iota-carrageenan. Small fractions of kappa-carrageenan, whether added or present as impurities, have no effect on the rheology of the sodium form but a large effect on the modulus of the potassium form of iota-carrageenan. The storage moduli of the potassium iota/kappa gels fall within the limits expected for a phase-separated gel structure.

On the specificity of the binding of cations to carrageenans: Counterion N.M.R. spectroscopy in mixed carrageenan systems☆

Abstract Counterion n.m.r. relaxation rates and signal intensities from mixed ion forms (sodium and rubidium) and mixed types (kappa and iota) of gel-forming carrageenans are presented. Data from the gel-promoting rubidium ion and the inert sodium ion obtained in the same mixed kappa-carrageenan systems indicate that only the rubidium ion binds to kappa-carrageenan and only in the gel state. Under non-gelling conditions, the transverse and longitudinal relaxation rates of sodium and the longitudinal relaxation rate of rubidium are independent of the ionic composition of samples of kappa-carrageenan. In systems containing mixed iota/kappa-carrageenan of the rubidium form, the large line-broadening effects observed for rubidium correlate with the kappa-carrageenan transition, which occurs in a temperature range well separated from that of the iota-carrageenan transition. The same conclusion holds for natural iota-carrageenan (from Euchema spinosa ), where a small fraction of kappa-carrageenan is responsible for the large effects of ion binding observed in the rubidium relaxation.

Cation specificity and cation binding to low sulfated carrageenans

Abstract A low-sulfated carrageenan from Eucheuma gelatinae and a partially desulfated (by chemical means) furcellaran were studied by polarimetry and Cs-133 NMR. The polarimetric studies show that the cation specificity of the coil-helix transition remains a strong effect even for very low-sulfated carrageenans, whereas the sensitivity to inert salt is considerably weakened compared to unmodified furcellaran. Cs-133 NMR shifts and linewidths reveal a binding of Cs+ to the helical conformation, but not to the random coil. The cation specificity is well predicted by the theoretical model previously used for unmodified furcellaran and kappa-carrageenan, if it is assumed that the intrinsic binding constant and the density of binding sites are independent of the degree of sulfation of the carrageenan helix.

Gelation of (Some) Seaweed Polysaccharides

The mechanism of gelation of carrageenans is discussed in the light of recent experimental and theoretical results. It is concluded that association of helices, rather than the double-helix formation in itself, provides the junctions of the gel network. Although the helical regions are typically much shorter than the carrageenan chains, the average number of helical regions on a chain is insufficient to generate crosslinking. The effects of gelpromoting cations and gel-impeding anions on the gelation (helix aggregation) is caused by the binding of these ions to the carrageenan helices, whereby the electrostatic helix-helix repulsion is affected through the decreased (for cation binding) or increased (for anion binding) charge density of the helix.

Phase separation by surface induced nucleation

The effect of a surface on the phase stability of solvent–solvent and solvent–polymer mixtures is analyzed within a lattice model. The location of the surface nucleation curve, which describes the limit of the metastability due to the presence of a surface, depends in a sensitive way both on the chain length of the polymer and on the interactions with the surface. The metastable phase region, as defined by the binodal and spinodal curves, can be reduced by more than a factor of two due to the presence of a planar surface.