Christer Viebke

Characterisation of kappa- and iota-carrageenan coils and helices by MALLS/GPC

Abstract Iota- and kappa-carrageenans were degraded by ultrasonication to obtain samples of different molecular weights. The coil and the helix conformations of the degraded and non-degraded samples were investigated by multi-angle laser light scattering (MALLS) coupled to gel permeation chromatography (GPC), and the weight-average molecular weight, Mw, as well as the z-average radii of gyration, Rg,z, was obtained for all samples in both conformations. The conformational state was controlled by temperature and/or electrolyte content. All measurements were done under non-aggregating conditions, and an approximate doubling of the weight-average molecular weight was seen for both iota- and kappa-carrageenan helices over the entire molecular weight range studied, implying a double-helix formation. Zimm plots for non-degraded kappa-carrageenan coils and helices confirmed the approximate doubling of the molecular weight for the helix conformation.

On the structure of aggregated kappa-carrageenan helices. A study by cryo-TEM, optical rotation and viscometry

Cryo-transmission electron microscopy (cryo-TEM), viscometry and optical rotation have been used to study the microstructure of dilute solutions of helical kappa-carrageenan under varied electrolyte composition (0.1 M of NaI, CsI and mixtures of the two salts). Microfibers (300-400 nm in length) were observed above a critical cesium content, while below that no microstructure was seen in the micrographs. A drastic increase in the specific viscosity and an onset of thermal hysteresis in the optical rotation above the same critical cesium content provide clear evidence of aggregate formation. This leads to the conclusion that the observed microfibers are aggregates consisting of several helices. At high proportions of cesium, further aggregation of the microfibers was observed by cryo-TEM.

Adsorption of galactomannans onto agarose

New direct evidence for a mixed association between aggregated agarose and aqueous galactomannan is presented. Gel permeation chromatography of two galactomannans, locust bean gum (lbg) and guar gum (gg), on a column packed with unmodified and highly aggregated agarose showed that the eluted amount of lbg was greatly reduced compared to the injected amount, whereas the loss of gg was smaller. The loss of material was significantly smaller, especially for lbg, with sodium iodide (rather than sodium nitrate) in the eluent salt solution. No loss of material for either galactomannan under any salt condition was found on a column packed with a hydroxylated polymethacrylate gel. The results strongly suggest that galactomannans adsorb onto aggregated agarose, and that the adsorption depends on the chemical nature of both the adsorbent and the substrate.

A light scattering study of carrageenan/galactomannan interactions

New experimental results that support the formation of mixed aggregates in the carrageenan/galactomannan system are presented. Kappa- and iota-carrageenan were degraded by ultrasonication to obtain low molecular weight samples. These samples were mixed with galactomannan in various proportions and studied in dilute solutions by multi-angle laser light scattering. Two different galactomannans were used, locust bean gum and guar gum. The ion specificity of the carrageenans was used to control their conformation and association. This made it possible to separately study how double helices and aggregates interact with the galactomannans. Under self-aggregated conditions, kappa-carrageenan was seen to associate with locust bean gum, but not with guar gum. Iota-carrageenan did not show any signs of association under any conditions.

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.