Role of Biologically Important Zwitterionic Buffer Secondary Ligands on the Stability of the Mixed-Ligand Complexes of Divalent Metal Ions and Adenosine 5‘-Mono-, 5‘-Di-, and 5‘-Triphosphate

Potentiometric equilibrium measurements have been performed at (25.0 ± 0.1) °C and ionic strength I = 0.1 mol dm - 3 KNO 3 for the interaction of the purine nucleotides adenosine 5‘-mono, 5‘-di, and 5‘-triphosphate and Cu(II), Co(II), Ni(II), Mn(II), Zn(II), Ca(II), and Mg(II) with the biologically important secondary ligand zwitterionic buffers 3-( N -morpholino)propanesulfonic acid, 3-[(1,1-dimethyl-2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid, N -(2-hydroxyethyl)piperazine- N ‘-2-hydroxypropanesulfonic acid, piperazine- N, N ‘-bis(2-ethanesulfonic acid), and piperazine- N, N ‘-bis(2-hydroxypropanesulfonic acid) in a 1:1:1 ratio and 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; i.e., 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 have been refined with the SUPERQUAD computer program. In some M(II) mixed-ligand systems, the interligand interactions between the coordinate ligands, possibly H bond formation, have been found to be most effective in deciding the stability of the mixed-ligand complexes formed in solutions.