Binary and ternary metal complexes of Congo red with amino acids Tuba S x is x manog˘ lu a , Su¨heyla Pura a , A.S. Bas xtug˘ b, * a Istanbul University, Engineering Faculty, Department of Chemistry, Avcılar, 34850 Istanbul, Turkey b Marmara University, Faculty of Pharmacy, Department of Basic Pharmaceutical Sciences, Haydarpas xa, 34668 Istanbul, Turkey Received 15 February 2005; received in revised form 22 March 2005; accepted 12 May 2005 Available online 7 July 2005 Abstract The stability constants of the 1:1 binary complexes of Ag C , Ni 2C , Co 2C ,Y 3C metal ions with glutamic acid (Glu), leucine (Leu) and Congo red (CR) and their 1:1:1 ternary complex and the protonation constants of the ligands were determined potentiometrically at a constant ionic strength of I Z 0.10 mol dm ÿ3 (NaClO 4 ) in aqueous solutions at 15.0, 25.0 and 35  C. The thermodynamic parameters DG o , DH o and DS o were reported for the complex formation reactions. The enthalpy changes of all complexation are found to be negative but entropy changes positive. The driving forces for the formation of AgeCR, NieCR, CoeCR are the enthalpy decreases. But the driving force for YeCR is the entropy decreases. The driving force of NieGlu, Coe Glu, NieLeu and CoeLeu systems are about equally enthalpy decrease and entropy increase while AgeGlu and AgeLeu complexation are enthalpy stabilized. But, the driving force of YeGlu and YeLeu systems are the entropy increase. For formed ternary complexes the reactions are all enthalpy-driven. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Congo red dyes; Amino acids; Thermodynamic parameters; Binary and ternary complexes; Metal complexes; Congo red complexes 1. Introduction In the recent decades, considerable studies have been carried out on the interaction between various metal ions and ligands that take place in biochemical pro- cesses. Furthermore, some researches focus on mixed- ligand complexes of amino acids and sulfonated azo dyes and investigations including the biological aspects have been made on the metal complex systems formed by amino acids and azo dyes [1,2]. In this work, Glutamic acid (Glu) and Leucine (Leu) were chosen as the amino acids and Congo Red (CR) as sulfonated azo dye. Leu and Glu are very important bioligands for human health. Leu (2-amino-4 methyl pentanoic acid) is a neutral amino acid. It has one carboxyl and one amino group. Many proteins include a large amount of Leu. Glu (2-aminoglutaric acid) is an acidic amino acid. It has one amino and two carboxyl groups. Glu is present in the vegetable proteins such as wheat, corn, soybean, etc. The human body contains a relatively R Group NH 2 Leu CH 3 CH 3 CH CH 2 CH COOH 5 4 3 2 1 R Group NH 2 | Glu HOOC CH 2 CH 2 CH COOH 5 4 3 2 1 * Corresponding author. Fax: C90 216 345 2952. E-mail address: asbastug@marmara.edu.tr (A.S. Bas xtug˘). 0143-7208/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2005.05.006 Dyes and Pigments 70 (2006) 136e142 www.elsevier.com/locate/dyepig