Journal of Luminescence 127 (2007) 461–468 Europium ion as a probe for binding sites to carrageenans Ana P. Ramos a , Roge´ria R. Gonc - alves a , Osvaldo A. Serra a , Maria Elisabete D. Zaniquelli a,Ã , Kenneth Wong b,1 a Departamento de Quı´mica, Faculdade de Filosofia, Cieˆncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo, Ribeira˜o Preto, Sa˜o Paulo 14040-901, Brazil b Laborato ´rio de Fı´sico-Quı´mica, Centro de Pesquisas de Paulı´nia, Rhodia Brasil, Paulı´nia, Sa˜o Paulo, Brazil Received 11 August 2006; received in revised form 7 February 2007; accepted 8 February 2007 Available online 25 February 2007 Abstract Carrageenans, sulfated polysaccharides extracted from red algae, present a coil-helix transition and helix aggregation dependence on the type and concentration of counterions. In this study, we focus attention on a mixed valence counterion system: Eu 3+ /Na + or K + with different gel-forming carrageenans: kappa, iota, and kappa-2. Results of stationary and time-dependent luminescence showed to be a suitable tool to probe ion binding to both the negatively charged sulfate group and the hydroxyl groups present in the biopolymer. For lower europium ion concentrations, a single longer decay emission lifetime was detected, which was attributed to the binding of europium ion to the carrageenan sulfate groups. An additional decay ascribed to europium binding to hydroxyl groups was observed above a threshold concentration, and this decay was dependent on the carrageenan charge density. Symmetry of the europium ion microenvironment was estimated by the ratio between the intensities of its emission bands, which has been shown to depend on the concentration of europium ions and on the specificity of the monovalent counterion bound to the carrageenan. r 2007 Elsevier B.V. All rights reserved. PACS: 78.55.Bq; 78.47.+p Keywords: Carrageenan; Europium; Luminescence; Ion binding; Polysaccharide; Polyelectrolyte 1. Introduction Ion binding operates in biological systems either to effect a structural change (e.g., in a protein, which triggers a reaction) or as an end in itself (e.g., ion transport). Control is carried out by the specificity of the binding; therefore, competition with other ions ineluctably becomes important to determine the system’s behavior and tuning. Simple polysaccharides bind metal ions and exhibit some of the binding behavior of more complex systems; so they may be useful as a model system. Furthermore, carbohydrate–me- tal complexes are important in their own right in analytical chemistry, in the extraction of heavy metal ions and as support materials for solid electrolytes. Polysaccharides thicken and may gel in the presence of cations and, although the mechanisms and specificity of this viscoelastic behavior are not fully understood, these sought-after properties are widely used in food industry [1], cosmetic and pharmaceutical formulations, and in biotechnological applications [2]. Information that helps understand these mechanisms and optimize the performance of these systems is thus desirable. Carrageenans are one such family of polysaccharides: they are natural, linear, water-soluble, sulfated poly- saccharides consisting [3,4] of disaccharide repeat units of 1-3 b-D-galactopyranose and 1-4 a-D-galactopyranose. Some galactopyranoses have an anhydro-bridge, as in the structures depicted in Fig. 1. These disaccharide repeat units correspond to the idealized primary structure of the main gelling carrageenans. By fractional precipitation, one may obtain k- and i-carrageenan while a third form named k-2 is described as being a hybrid species of i- and k-carrageenan. ARTICLE IN PRESS www.elsevier.com/locate/jlumin 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.02.036 Ã Corresponding author. Tel./fax: +55 16 3602 4838. E-mail address: medzaniquelli@ffclrp.usp.br (M.E.D. Zaniquelli). 1 Present address: Firmenich, Corporate R&D, P.O. Box 239, Route des Jeunes 1, CH-1211 Geneva 8, Switzerland.