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.

The apparent second-stage dissociation constants of 3-(N-morpholino)propanesulfonic acid, 3-[(1,1dimethyl-2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid, and N-(2-hydroxyethyl)piperazine-N′-(2-hydroxypropanesulfonic acid) were determined at (25.0 ( 0.1) °C and I ) 0.1 mol dm -3 (KNO 3 ) by potentiometric pH titration in pure water and different hydroorganic solvent media. The organic solvents used were methanol, ethanol as amphiprotic hydrogen bond acceptor-donor (HBA-D) solvents, N,Ndimethylformamide, dimethyl sulfoxide, acetone, dioxane as hydrogen bond acceptor solvents, and acetonitrile as (HBA-HBD) solvent. The ESAB2M computer program was used to refine the initial estimates of the apparent second-stage dissociation constants of the three zwitterionic buffers studied. pK a2 * values change with increasing organic content of the solvent mixture. The results obtained are discussed in terms of average macroscopic properties of the mixed solvents. The implications of the results with regard to specific solute-solvent interactions, particularly stabilization of zwitterionic species, are discussed. The effects of coorganic solvents on the acid dissociation equilibria have been interpreted using solvatochromic quantitative values of Kamlet-Taft hydrogen bond acidity and basicity (R, β) and dipolarity-polarizability π * of the solvent.