K + and Na + effects on the gelation properties of n-Carrageenan M.R. Mangione a,b , D. Giacomazza a , D. Bulone a , V. Martorana a , G. Cavallaro b , P.L. San Biagio a, * a CNR-IBF at Palermo, Via U. La Malfa, 153, 90146 Palermo, Italy b DCTF, University of Palermo, Via Archirafi, 32, 90132 Palermo, Italy Received 6 August 2004; accepted 26 August 2004 Available online 27 September 2004 Abstract The effects of K + , Na + ions and their mixture on the conformational transition and macroscopic gel properties of n-Carrageenan system have been studied using different experimental techniques. The macroscopic gelation properties of n-Carrageenan were found to be dependent upon cosolute type. Indeed, a more ordered and strong gel was obtained in the presence of K + with respect to Na + ions. The gel properties obtained using mixtures of two cosolutes are shown to depend on the [K + ]/[Na + ] ratio. D 2004 Elsevier B.V. All rights reserved. Keywords: n-Carrageenan; Na + and K + ion effects; Self-assembly; Gelation; Fractal dimension 1. Introduction The n-Carrageenan comes from a family of linear water- soluble polysaccharides extracted from different species of marine red algae with a primary structure based on an alternating disaccharide repeating unit of a-(1-3)-d-galac- tose-4-sulphate and h-(1-4) 3,6-anydro-d-galactose. It is largely used as thickening, gelling agent or texture enhancer or stabilizer in food, pharmaceutical [1,2] and cosmetic [3] industries. Indeed, in aqueous solutions and in the presence of several cations, n-Carrageenan forms, on cooling, thermoreversible gels. It is known that, in the presence of K + , thermoreversible gelation of n-Carrageenan involves a coil-to-double helix conformational change, followed by aggregation of the ordered molecules in an infinite network [4]. One of the most interesting feature of this system is that macroscopic properties of n-Carrageenan gels are affected by the concentration and species of cations in solution. Cations are known to affect the polymer conformational properties but the relation between conformational change and final gel structure in this system is still subject of debate. In addition to the practical interest, the under- standing of mechanism leading from molecular conforma- tional change to macroscopic aggregation is, at the present time, relevant in pathological protein coagulation [5,6] and polymeric aggregation [7,8]. In the present work, we study in detail, using different experimental techniques (static light scattering (SLS), optical rotatory dispersion (ORD), rheology and turbidity), the effects of K + , Na + and their mixture on the conforma- tion/gelation properties of n-Carrageenan system. Several studies are found in the literature, concerning the specific effect of some cations (K + , Rb + and Cs + ) on gelation properties of n-Carrageenan [9–11], while the effect of Na + on its conformational change and aggregation is still a matter of debate. In the presence of Na + , n-Carrageenan is found in the coil state at room temperature [12], whereas in the presence of K + , it can be in the coil or helix state depending on salt concentration and temperature [13,14]. Potassium ions seem to have a specific effect and to bind to the polymer also in the disordered state [10]. More recently, it was suggested that the presence of Na + in solution can induce a conformational transition in n-Carrageenan at high salt concentration and low temperature only [15–18]. Finally, only few literature data concern the effects of simultaneous presence of different cations on the gelation properties of n-Carrageenan. Indeed, results obtained by 0301-4622/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.bpc.2004.08.005 * Corresponding author. Tel.: +39 91 6809311; fax: +39 91 6809312. E-mail address: sbiagio@pa.ibf.cnr.it (P.L. San Biagio). Biophysical Chemistry 113 (2005) 129 – 135 www.elsevier.com/locate/bpc