Medium Effect and Thermodynamic Studies for the Proton-Ligand and Metal-Ligand Formation Constants of the Ternary Systems M II + Adenosine-5′-triphosphate (ATP) + Asparagine Ahmed A. A. Boraei,* Fouad Taha, and Ali H. Mohamed Department of Chemistry, Faculty of Science, Minia University, El-Minia 61519, Egypt Said A. Ibrahim Department of Chemistry, Faculty of Science, Assiut University, Assiut 71516, Egypt Proton-ligand and metal-ligand stability constants formed in the ternary system Ni II + adenosine-5′- triphosphate (ATP) + asparagine are determined pH-metrically in different aqueous-organic solvent mixtures at 25 °C and at a constant ionic strength I ) 0.10 mol dm -3 (KNO 3 ), and are discussed in terms of both the proportion and solvent characteristics. The organic solvents used are ethanol, DMF, dioxane, and acetonitrile (AN). It was concluded that solvent effects such as stabilization of the proton by ion- solvent interaction as well as the basicity of the medium, in addition to the electrostatic effect, have a profound influence on the proton-ligand formation constants for ATP and asparagine. On the other hand, the electrostatic effect of the medium and the hydrogen-bonding interaction seem to play the major role in the formation of the binary and ternary metal complexes. Moreover, the thermodynamic functions ∆H, ∆G°, and ∆S° associated with the formation of proton-ligand and the 1:1 binary as well as the 1:1:1 ternary metal complexes in the system M II + (ATP) + asparagine (where M II ) Cu II , Ni II , and Co II ) in pure aqueous media were also determined and discussed. Introduction Adenosine-5′-triphosphate (ATP) is a biologically impor- tant ligand that plays a key role in the metabolism of organisms providing transphosphorylation in the presence of metal ions. 1 Further, numerous metal ions form ATP adducts which could be involved in many enzymatic processes; 2 hence, the complex formation of transition metals with this compound is of fundamental interest for bioinorganic chemistry. Thus, a considerable interest has been focused on the study of the binary metal complexes formed with ATP. On the other hand, ternary complexes of some transition metal ions with ATP and some secondary ligands have been studied using several techniques. 1,3-9 Recently, we have reported a study on the formation of the binary and ternary metal complexes of some divalent and trivalent transition metal ions with ATP and some mono- and dicarboxylic amino acids. 10 Although a little attention 11 has been given to studying the medium effect on the ionization processes of ATP, a scanning of the literature reveals that no studies appear to have been made on the medium effect on the binary as well as the ternary metal complex formation containing the biologically important ligand ATP. Moreover, the literature is lacking studies concerning the thermodynamic properties of the formation of proton-ligand and metal-ligand complexes containing ATP. Therefore, this article is devoted to study the medium effect on the formation constants of the proton-ligand as well as the 1:1 binary and 1:1:1 ternary mixed ligand complexes formed between Ni II ion, ATP, and asparagine in different aqueous solutions containing varying propor- tions of organic solvents at 25 ( 0.1 °C at the constant ionic strength I ) 0.10 mol dm -3 of KNO 3 . The organic solvents used are ethanol as an amphiprotic, DMF as a dipolar aprotic, acetonitrile as a low basic dipolar aprotic, and dioxane as a low basic, low polarity solvent. The study adopts the Irving and Rossotti technique 12 for the deter- mination of stability constants of the different proton- ligand as well as the binary and ternary mixed ligand complexes. The stabilities of the different proton-ligand, binary, and mixed ligand complexes have been discussed in terms of both the nature and the proportion of the organic solvent used. Moreover, the thermodynamic func- tions ∆H, ∆G°, and ∆S° associated with formation of the proton-ligand as well as the 1:1 binary and 1:1:1 ternary mixed ligand complexes in the ternary systems M II + ATP + asparagine (where M II ) Co II , Ni II , and Cu II ) in pure aqueous media at the constant ionic strength I ) 0.10 mol dm -3 (KNO 3 ) were also determined and discussed. The structures of the ligands used in this study are Experimental Section Materials and Solutions. Adenosine-5′-triphosphate tetrahydrate (ATP) in the form of the disodium salt and * To whom correspondence should be addressed. Fax: 002 086 342601. E-mail: rumenia@rusys.eg.net. 267 J. Chem. Eng. Data 2001, 46, 267-275 10.1021/je000221k CCC: $20.00 © 2001 American Chemical Society Published on Web 01/25/2001