Coil–helix transition of ι‐carrageenan as a function of chain regularity

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Abstract
A series of ι‐carrageenans containing different amounts of ν‐carrageenan (0–23 monomer %) have been prepared from neutrally extracted carrageenan of Eucheuma denticulatum. ν‐Carrageenan is the biochemical precursor of ι‐carrageenan. The conformational order–disorder transition and rheological properties of these carrageenans were studied using optical rotation, rheometry, size exclusion chromatography coupled to multiangle laser light scattering, and high‐sensitivity differential scanning calorimetry. The helix forming capacity of ι‐carrageenan turns out to decrease monotonously with increasing amount of ν‐units. In contrast, the rheological properties of ι‐carrageenan are remarkably enhanced by the presence of a small amount of ν‐units, yielding a maximum twofold increase in G′ at 3% ν‐units. It is concluded that the structure‐forming capacity of ι‐carrageenan, containing a small amount of ν‐carrageenan, is significantly higher than that of pure ι‐carrageenan. This phenomenon is explained in terms of the balance between the helical content and the number of cross‐links between chains, taking into consideration the fact that ν‐units introduce “kinks” in the chain conformation enabling neighboring chains to connect. Increasing amounts of ν‐units increase the number of cross‐links in the network, resulting in increased gel strength. On the other hand, a reduced length of the helical strands weakens the cross‐links between the different chains and, consequently, the gel. © 2002 Wiley Periodicals, Inc. Biopolymers 65: 299–312, 2002