Carrageenan coil-helix transition

The proportionality constant between theoretical helix content and optical rotation is found by fitting the data at the highest concentration of 5.66% measured. Then, theoretical results at other concentrations automatically fit the experimental data with high accuracy. It turned out that, for (n) = 50, the helix initiation parameter cf2 should be as small as 0.001 to obtain good fit. One of the main reasons why t-carrageenan does not form gels is the smallness of this helix initiation probability. The coil-helix transition temperature at dilute limit is fixed at To = 65°C. 

Figure 4.4 Schematic representation of the coil-helix transition of carrageenan and helix aggregation.

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. 

The helix-coil transition of K carrageenan can be also observed by conductivity measurements (Figure 4A). The helix content calculated from the ratio p of the conductivity of the polyelectrolyte to that of a simple reference electrolyte corresponds well with the values of optical rotation (Figure 4B). 

In conclusion it is apparent that a concentration and temperature reversible helix-coil transition takes place in solutions of kappa carrageenans as soon as the total ionic concentration Op becomes lower than a critical value C. This critical value depends on the nature of the counterions. 

For stereoregular charged polysaccharides, the formation of a helical conformation and eventually gelation depend on the ionic concentration, the nature of electrolyte and the temperature. The conformation results from a balance between H-bonds (which stabilize the helical conformation, but which are destabilized when the tenpera-ture increases) and electrostatic repulsions between the charges on the polymer (which are screened by external salt addition). The helix-coil transition for K-carrageenan was demonstrated by different techniques, viz. conductivity. 

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