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Potentiometric equilibrium measurements have been performed at (25.0 ± 0.1) °C and ionic strength I = 0.1 mol·L−1 KNO3 for the interaction of Eu–Glycylhistidine (Glyhis) with five biologically important zwitterionic buffers (Z) (HEPES, EPPS, PIPES, POPSO and HEPPSO). The formation constants for the binary Eu(III) + Glyhis or Eu(III) + Z systems and ternary Eu(III) + Glyhis + Z systems have been evaluated, and the lowest value for formation constant is assigned to the system Eu(III) + Glyhis + PIPES. The electrochemical behavior of the Eu–Glyhis complex with selected zwitterionic buffers was studied on the surface of a glassy carbon electrode using cyclic, differential pulse and square wave voltammetric techniques. The values of the standard heterogeneous rate constant (K s), diffusion coefficients of the reduction (D red) and oxidation (D ox) processes were calculated for all the studied systems. The data reveal that PIPES is the most suitable of these zwitterionic buffers for study of the biological behavior of Glycylhistidine in its europium complexes.

Potentiometric measurements have been performed at (25.0 ± 0.1) °C and ionic strength I = 0.1 mol·L−1 KNO3 for the interaction of guanine, thymine, uracil, aminouracil, guanosine, inosine, cytidine, guanosine 5′-monophosphate, adenosine 5′-diphosphate, adenosine 5′-triphosphate and palladium Pd(II) with the biologically important secondary ligands glycine, leucine, N,N-bis(2-hydroxyethyl) glycine, N-[tris(hydroxymethyl)methyl] glycine, d-2-t-butylglycine and Boc-N-methyl-l-leucine. The formation of various 1:1:1 ternary complexes and 1:1:2 quaternary complex species as well as the polynuclear species was inferred from the potentiometric pH-titration curves. The experimental conditions were selected such that self-association of the nucleobases, nucleosides or nucleotides and their complexes due to stacking interaction is negligibly small; that is, the normal monomeric protonated ternary complexes were studied. Formation of the ternary complexes of some systems was confirmed using cyclic voltammetry, differential pulse voltammetry and conductance measurements in solution.

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,N-bis(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 ΔG° values of the monomeric normal and protonated ternary complexes were calculated and discussed.