K-Carrageenans

Consistent with their chemical differences, the molecular structures of i- and K-carrageenans are not identical. A shorter pitch and an offset positioning of the two chains in the kappa helix is compatible with the lack of sulfate group on every 3,6-anhydrogalactose residue. The variations in molecular structures mirror the types of junction zones formed by these polymers and relate to the observed gelation properties. 

Fig. 25.—Stereo view of about a turn of the 3-fold double helix of K-carrageenan (24). The two chains are distinguished by open and filled bonds for clarity. The vertical line is the helix axis. Only three interchain hydrogen bonds per turn among the galactose residues stabilize the helix. The sulfate groups near the periphery are crucial for intermolecular interactions.

Subsequent work by Johansson and Lofroth [183] compared this result with those obtained from Brownian dynamics simulation of hard-sphere diffusion in polymer networks of wormlike chains. They concluded that their theory gave excellent agreement for small particles. For larger particles, the theory predicted a faster diffusion than was observed. They have also compared the diffusion coefficients from Eq. (73) to the experimental values [182] for diffusion of poly(ethylene glycol) in k-carrageenan gels and solutions. It was found that their theory can successfully predict the diffusion of solutes in both flexible and stiff polymer systems. Equation (73) is an example of the so-called stretched exponential function discussed further later. 

Pedersen, J.K.. Carrageenan, pectin and xanthan/locust bean gum gels. Trends in their food uses.Food Chem. 6 (1980) 77-88. 

Ikeda, S., Morris, V. J., and Nishinari, K. (2001). Microstructure of aggregated and nonag-gregated K-carrageenan helices visualized by atomic force microscopy. Biomacromolecules 2,1331-1337. 

Fig. 5.10 TEM micrographs of gel and nanoparticle formation of iron-loaded K-carrageenan at (A) pH 2 and (B) pH 13. In both casestheiron loadingwas 100%. ([57], CopyrightSpringer-Verlag 2000. With kind permission of Springer Science and Business Media).

Grasdalen, H. and Smidsrod, O. (1981) Iodide-specific formation of K-carrageenan single helices-1-127 NMR spectroscopic evidence for selective site binding of iodide anions in the ordered conformations. Macromolecules, 14, 1842-1845. 

Jones, F., Colfen, H. and Antonietti, M. (2000) Interaction of K-carrageenan with nickel, cobalt and iron hydroxides. Biomacromolecules, 1, 556—563. 

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