Association of κ-Carrageenan Induced by Cs + Ions in Iodide Aqueous Solution: A Light Scattering Study Fabio Cuppo and Harry Reynaers* Laboratory of Macromolecular Structural Chemistry, Department of Chemistry, Catholic University of Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium Sergio Paoletti Department of Biochemistry, Biophysics and Macromolecular Chemistry, University of Trieste, Via L. Giorgieri 1, I-34127 Trieste, Italy Received June 25, 2001; Revised Manuscript Received September 26, 2001 ABSTRACT: Light scattering (LS) investigations have additionally confirmed the intramolecular nature of the fundamental ordered conformation (single helix) of κ-carrageenan in aqueous 0.1 M NaI. The partial replacement of sodium with cesium ions induces a (thermoreversible) intermolecular association of the polysaccharide in the ordered conformation. The association-inducing effect can be observed already at the lowest investigated concentration of the cation. We found no significant change of the thermodynamic properties related to an alleged composition threshold value in the range of the cesium equivalent fraction, X Cs, from 0.12 to 0.40. The extent and the rate of the association process strongly increase with the concentration of cesium. Equilibrium values for the association constant could be obtained except for the case of X Cs ) 0.40, where the process is too massive to allow long-time LS investigation. In the latter condition, some additional stiffening of the chain was noticed. Introduction Carrageenans are a family of linear sulfated polysac- charides extracted from red algae. Three main species can be identified, namely, κ-, ι-, and λ-carrageenan, though other types have been recognized. The present study is restricted to κ-carrageenan, the primary struc- ture of which ideally consists of a regularly alternating R-(1-3)-D-galactose-4-sulfate and -(1-4)-3,6-anhydro- D-galactose (Figure 1). One of the most peculiar features of κ-carrageenan is related to its ability to form ther- moreversible, ionotropic gels, in particular with alkaline metal counterions of high atomic number. 1 Despite its widespread and massive application, mainly by the food industry as a thickening and stabilizing agent, fundamental aspects concerning the mechanism of gelation have been a controversial matter until very recently, 2-12 and further investigation is still needed for a full comprehension of the process. On the other hand, new interesting features have been recently reported, such as the observation of a chiral nematic phase in aqueous κ-carrageenan under specific experi- mental conditions. 13-17 The molecular requirements for gel formation have now been safely assessed, demonstrating that a single- stranded ordered helix is the conformational prerequi- site for any further level of intermolecular organiza- tion. 2-6,10-12,18-22 An intermolecular association of heli- cal stretches follows this stage, ultimately giving rise to the three-dimensional gel network. The same inter- chain association can occur on the microscopic scale without the development of macroscopic gel behavior, e.g., at high dilution and/or in the presence of suitable cosolutes. The type and concentration of ions in solution mediate the whole process: both co-ions and counterions show a specific effect on the conformational and as- sociative behavior of aqueous κ-carrageenan. 6 Some cations, like K + and Cs + , strongly promote the associa- tion of the helices, while others, like Li + and Na + , are much less effective. 1,10,11,22 On the other hand, co-ions can also strongly influence the solution behavior of κ-carrageenan. 21,23 Iodide, in particular, is known to play a key role in establishing a wider range of condi- tions where intramolecular conformational ordering of the polymer develops without intermolecular associa- tion. 2,3,5,6,10,11 It should be noted, however, that associa- tion unavoidably occurs upon increasing iodide concen- tration. 24 The specific action of both cations and anions is balanced depending on the overall salt composition. Thus, for example, in aqueous 0.1 M NaCl, at room temperature, a low-ordered loose-helix 25 is expected, while an ordered single helix is stabilized by NaI under the same conditions of salt concentration and temper- ature. Intermolecular association, however, can be promoted by replacing Na + with gelling cations in iodide solutions. Hence, it is possible to control the conforma- tional-associative properties of κ-carrageenan by ap- propriate modifications of the ionic environment. A better characterization of the molecular properties of κ-carrageenan in aqueous solution is crucial in order to contribute to the elaboration of a self-consistent physical-chemical model for the formation of the gel and of the liquid-crystalline phases. Such a need arises from the controversial results reported in the literature. The discrepancy between experimental data found by different authors can be traced back to the complex * Corresponding author. Figure 1. Ideal repeating unit of κ-carrageenan. 539 Macromolecules 2002, 35, 539-547 10.1021/ma0110913 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/12/2001