Thermodynamics of Conformational Ordering of ι-Carrageenan in KCl Solutions Using High-Sensitivity Differential Scanning Calorimetry

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Abstract
Thermodynamic properties of aqueous solutions of ι-carrageenan as affected by KCl (0.15−1.2 M) or ι-carrageenan (0.5−6 mg/mL) content were studied by high-sensitivity differential scanning calorimetry. The polysaccharide was found to undergo two consecutive cooperative conformational transitions, which can be represented by the scheme: [H2]2 ↔ 2H2 ↔ 4C where C is the random coil, H2 is the double helix, and [H2]2 is the double helix dimer. The first transition follows by the “all or none” mechanism. The profile of the second transition resembles that of a second-order phase transition. The parameter σ (of order 1), estimated for this latter transition, suggests that the stacking effect in helices of ι-carrageenan is rather small. The cooperativity of the transition is mainly defined by the loop factor. Free energies of both transitions at 273 K were calculated as a function of salt concentration. These experimental data were found to agree with Manning's theory.