B. Denef 1 * A. Gamini 1 Thermodynamics of F. Delben 1 Conformational Transition S. Paoletti 1 H. Reynaers 2 and Chain Association of K. Vanneste 2 i-Carrageenan in Aqueous 1 Department of Biochemistry, Biophysics Solution: Calorimetric and and Macromolecular Chemistry, Chirooptical Data University of Trieste, Via L. Giorgieri 1, I-34127 Trieste, Italy 2 Laboratory of Macromolecular Structural Chemistry, Department of Chemistry, Catholic University of Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium Received 1 April 1997; accepted 25 August 1997 Abstract: Isothermal microcalorimetry, differential scanning calorimetry ( DSC ) , and chiro- optical data obtained for i-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 DH tr is obtained as the difference between the theoretical, purely electro- static DH el enthalpy change and the actual mixing enthalpy DH mix , measured when a i- 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 ÉDH tr É are in good agreement with those obtained for the opposite process, at compa- rable polymer and salt concentrations, from DSC melting curves. The microcalorimetric results show that the negative maximum value of DH tr corresponding to the interaction of Li / counter- * On leave from the Laboratory of Macromolecular Struc- tural Chemistry, Department of Chemistry, Catholic University of Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium Correspondence to: S. Paoletti Contract grant sponsor: University of Trieste, Catholic Uni- versity of Leuven, Italian Ministero dell’Universita ` e della Ri- cerca Scientifica e Tecnologica Biopolymers, Vol. 45, 105–117 (1998)  1998 John Wiley & Sons, Inc. CCC 0006-3525/98 / 020105-13 105 5539 / 8k3b$$5539 12-04-97 00:18:55 bpas W: Biopolymers