THERMODYNAMICS OF CONFORMATIONAL TRANSITION AND CHAIN ASSOCIATION OF IOTA-CARRAGEENAN IN AQUEOUS-SOLUTION - CALORIMETRIC AND CHIROOPTICAL DATA

Isothermal microcalorimetry, differential scanning calorimetry (DSC), and chirooptical data obtained for iota-carrageenan in NaCl, LiCl, and NaI aqueous solutions are presented. The experiments have been perfor med as a function of concentration both for the polymer and for the si mple salt as a cosolute. The experimental findings consistently show t he occurrence of a salt-induced disorder-to-order transition. From mic rocalorimetric experiments the exothermic enthalpy of transition Delta H-tr is obtained as the difference between the theoretical, purely el ectrostatic Delta H-el enthalpy change and the actual mixing enthalpy Delta H-mix measured when a iota-carrageenan salt-free solution at con stant polymer concentration is mired with a I:I electrolyte solution o f variable concentration. IPL the case of added NaCl, the absolute val ues of enthalpy changes \Delta H-tr\ I are in good agreement with thos e obtained for the opposite process, at comparable polymer and salt co ncentrations, from DSC melting curves. The microcalorimetric results s how that the negative maximum value of Delta H-tr corresponding to the interaction of Li+ counterion with iota-carrageenan polyion results t o be significant lower than the corresponding values obtained for Nacounterion. Ar variance with the microcalorimetric data, chirooptical results show that the salt-induced disorder-to-order transition, occur ring in the 0.02-0.2M salt concentration range, appears to be complete at a concentration of about 0.08-0.1M of rite simple ion, ii-respecti ve of the polymer concentration and of the nature of added electrolyte . (C) 1998 John Wiley & Sons, Inc.