بحث
Ternary Complexes of La(III), Ce(III), Pr(III) or Er(III) with Adenosine 5_-mono, 5_-di, and 5_-triphosphate as Primary Ligands and some Biologically Important Zwitterionic Buffers as Secondary Ligands

Adel S. Orabi Hassan A. Azab F. Saad Hani Said
 

Abstract

 Equilibrium constant measurements have been performed potentiometrically at (25 } 0.1) °C and an ionic strength I = 0.1 mol・dm-3 KNO3 for the interaction of La(III), Ce(III), Pr(III) and Er(III) with the purine nucleotides adenosine 5_-mono, 5_-di, and 5_-triphosphate and with the biologically relevant secondary ligand zwitterionic buffers 3-(cyclohexyl amino)-1-propanesulfonic acid (CAPS), 3-(cyclohexylamino)-2-hydroxy- 1-propane sulfonic acid (CAPSO), N-tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid (TAPS), 3-[N-bis(hydroxyethyl)amino]-2 hydroxypropanesulfonic acid (DIPSO), N,Nbis( 2-hydroxyethyl)glycine (BICINE), and N-(2-acetamido)-2-iminodiacetic acid (ADA) in a 1:1:1 ratio. The formation of various 1:1:1 normal and protonated mixed-ligand complex species was inferred from the potentiometric pH titration curves. The experimental conditions were selected such that self-association of the purine nucleotides and their complexes was negligibly small; that is, the monomeric normal and protonated ternary complexes were studied. Initial estimates of the formation constants of the resulting species and the acid dissociation constants of adenosine 5_-mono-, 5_-di-, and 5_-triphosphate and the zwitterionic buffer secondary ligands were refined with the Superquade computer program. In some Ln(III) mixed-ligand systems, interligand interactions between the coordinating ligands, possibly involving H-bond formation, have been found to be the most important factors in deciding the stability of the mixed-ligand complexes in solutions. The thermodynamic _G° values of the monomeric normal and protonated ternary complexes were calculated and discussed.

Keywords  Zwitterionic buffer ・ Purine nucleotides ・ Formation constants ・ Binary and ternary complexes ・ Lanthanide ion ・ Thermodynamic






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