Potentiometric equilibrium measurements have been performed at (25.0 ± 0.1) °C and ionic strength I = 0.1 mol·dm-3 (KNO3) for the interaction of guanine (2-amino-6-oxypurine), thymine (2,4-dihydroxy-5-methylpyrimidine), adenine (6-aminopurine), uracil (2,4-dioxypyrimidine), hypoxanthine (6-oxypurine), and Cu(II), Ni(II), Co(II), Mn(II), Zn(II), Ca(II), and Mg(II) with the biologically important secondary ligands glycine (aminoethanoic acid), bicine [N,N-bis(2-hydroxyethyl)glycine], tricine [N,N,N-tris(hydroxymethyl)methylglycine], and ADA [N-(2-acetamido)-3-iminodiacetic acid] in a 1:1:1 ratio. The experimental conditions were selected such that self-association of the nucleobases and their complexes was negligibly small; that is, the monomeric normal and protonated complexes were studied. The formation of various 1:1:1 mixed ligand complexes was inferred from the potentiometric titration curves. Initial estimates of the formation constants of the resulting species and the acid dissociation constants of guanine, thymine, adenine, uracil, hypoxanthine, and the secondary ligands glycine, bicine, tricine, and ADA have been refined with the SUPERQUAD computer program. Confirmation of the formation of ternary complexes of the type Cu(II) + NB + Z in solution has been carried out using differential pulse polarography (DPP), square wave voltammetry (SWV), cyclic voltammetry (CV), and UV−visible spectroscopic measurements.

The apparent dissociation constants of the nucleobases guanine, thymine, uracil, hypoxanthine, and cytosine were determined at (25.0 ± 0.1) °C and I = 0.1 mol·dm-3 (KNO3) by potentiometric pH titration in pure water and different hydroorganic solvent media. The organic solvents used were methanol and ethanol as amphiprotic hydrogen bond acceptor−donor (HBA-D) solvents, N,N-dimethylformamide (DMF), dimethyl sulfoxide, acetone, and dioxane as hydrogen bond acceptor solvents. A computer program was used to refine the initial estimates of the apparent dissociation constants of the five purine and pyrimidine nucleobases. The results obtained are discussed in terms of average macroscopic properties of the mixed solvents. The effects of organic cosolvents on the acid dissociation equilibria have been interpreted using the solvatochromic quantitative values of Kamlet−Taft hydrogen bond acidity and basicity (α, β) and dipolarity polarizability π* of the solvent. The free energy of transfer of the protons from water to mixed solvent has been calculated for the nucleobases under investigation.