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) degreesC and ionic strength I= 0.1 mol dm(-3) KNO3 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.