Helix carrageenan

Fig. 24.—(a) Stereo view of slightly over a turn of the 3-fold double helix of i-carrageenan (23). The two chains are distinguished by open and filled bonds for clarity. The vertical line is the helix axis. Six interchain hydrogen bonds per turn among the galactose residues stabilize the double helix. The sulfate groups lined up near the periphery are crucial for intermolecular interactions. 

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. 

Rees and coworkers158 showed that, at 15°, i-carrageenan forms a gel whose 13C-n.m.r. signals are so broad that they cannot be detected, in contrast to those given by the solution at 80° (see Fig. 28). At the lower temperature, segmental motion is restricted by frequent, interunit junction-zones in a double-helix structure, in contrast to the gel of a /8-D-(l— 3)-linked D-glucopyranan, where the intermolecular association is not so complete, and portions of the polymer are sufficiently mobile to provide broad signals.159... 

Figure 7. Mutually perpendicular views of the (a) agarose, (b) iota-carrageenan, and (c) kappa-carrageenan double-helix structures. The two chains are shown with open and full bonds, and the 06—02 hydrogen bonds by broken lines. (Reproduced with permission from ref. 28. Copyright 1989 Elsevier.)...

Figure 6.5 shows experimental data relating to the self-assembly of sodium K-carrageenan, as induced by cooling in the presence of 0.1 M NaCl, and occurring simultaneously with the coil-to-helix transition for the same polysaccharide (Semenova et al., 1988). In what follows we consider this system in some detail. 

FlG. 2.—Schematic Form of the Proposed Mechanism for Gelation of i- and K-Car-rageenan. [In t-carrageenan gels, aggregation of double-helix junction-zones does not occur.]... 

A similar structure has been established for the gelforming carrageenans from red seaweed. The X-ray data suggest that three of the disaccharide units form one turn of a right-handed helix with a pitch of 2.6 nm. A second chain with a parallel orientation, but displaced by half a turn, wraps around the first helix.124 Such... 

Rees et al. (25) have extensively examined the solution conformation and interactions of a number of polysaccharides, especially carrageenan fractions. Kappa-carrageenan is unusual as it forms gels when a solution of its potassium salt is cooled. Rees conceives that double helix formation occurs in solution and, interlocking helices develop in the gel state. 

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