Thermodynamics of conformational transition and chain association of iota-carrageenan in aqueous solution: calorimetric and chirooptical data

• Published in: Biopolymers Vol. 45; no. 2; pp. 105 - 117
• Main Authors: Denef, B. (University of Trieste, Trieste, Italy.), Gamini, A, Delben, F, Paoletti, S, Reynaers, H, Vanneste, K
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 01.02.1998
• Subjects: aqueous solutions; CARRAGEENANS; chain association; conformational transition; COUNTERION CONDENSATION THEORY; ELECTROLYTES; ENTHALPY OF MIXING; IONS; microcalorimetry; MOLECULAR CONFORMATION; optical activity; OPTICAL PROPERTIES; PHYSICS; polyelectrolytes; polysaccharide; SODIUM CHLORIDE; SOLUTIONS; THERMAL PROPERTIES; thermodynamics; ι-carrageenan
• ISSN: 0006-3525; 1097-0282
• DOI: 10.1002/(SICI)1097-0282(199802)45:2<105::AID-BIP2>3.0.CO;2-W

Isothermal microcalorimetry, differential scanning calorimetry (DSC), and chiro-optical data obtained for L-carrageenan in NaCl, LiCl, and NaI aqueous solutions are presented. The experiments have been performed as a function of concentration both for the polymer and for the simple salt as a cosolute. The experimental findings consistently show the occurrence of a salt-induced disorder-to-order transition. From microcalorimetric experiments the exothermic enthalpy of transition delta H(tr) is obtained as the difference between the theoretical, purely electrostatic delta H(el) enthalpy change and the actual mixing enthalpy delta H(mix), measured when a L-carrageenan salt-free solution at constant polymer concentration is mixed with a 1:1 electrolyte solution of variable concentration. In the case of added NaCl, the absolute values of enthalpy changes delta H(tr) are in good agreement with those obtained for the opposite process, at comparable polymer and salt concentrations, from DSC melting curves. The microcalorimetric results show that the negative maximum value of delta H(tr) corresponding to the interaction of Li+ counterion with L-carrageenan polyion results to be significantly lower than the corresponding values obtained for Na+ counterion. At variance with the microcalorimetric data, chirooptical results show that the salt-induced disorder-to-order transition, occurring in the 0.02-0.2M salt concentration range, appears to be complete at a concentration of about 0.08-0.1M of the simple ion, irrespective of the polymer concentration and of the nature of added electrolyte