Coil-helix transition of ι-carrageenan as a function of chain regularity

A series of ι-carrageenans containing different amounts of v-carrageenan (0-23 monomer %) have been prepared from neutrally extracted carrageenan of Eucheuma denticulatum. v-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 v-units. In contrast, the rheological properties of ι-carrageenan are remarkably enhanced by the presence of a small amount of v-units, yielding a maximum twofold increase in G′ at 3% v-units. It is concluded that the structure-forming capacity of ι-carrageenan, containing a small amount of v-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 v-units introduce "kinks" in the chain conformation enabling neighboring chains to connect. Increasing amounts of v-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. Chemicals/CAS: Carrageenan, 9000-07-1; Gel