Metal Ion Complexes Containing Nucleobases and Some Zwitterionic Buffers

Potentiometric equilibrium measurements have been performed at (25.0 ± 0.1) °C and ionic strength I = 0.1 mol·dm^-3 (KNO<INF>3</INF>) 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.