Ternary Complexes Formed by Trivalent Lanthanide Ions, Nucleotides, and Biological Buffers

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 adenosine 5‘-monophosphate, guanosine 5‘-monophosphate, cytidine 5‘-monophosphate, and La(III), Ce(III), Pr(III), and Eu(III) with the biologically important secondary ligand zwitterionic buffers (3-[N-morpholinol])-2-hydroxypropanesulfonic acid, 2-(N-morpholino)ethanesulfonic acid, N-2-acetamido-2-aminoethanesulfonic acid, and N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid. Measurements were made in a 1:1:1 ratio. Formation constants for the monohydroxy, dihydroxy, and dimeric ligand complexes for the binary systems Ln(III) + adenosine 5‘-monophosphate, Ln(III) + guanosine 5‘-monophosphate, Ln(III) + cytidine 5‘-monophosphate, and Ln(III) + zwitterionic buffers have been evaluated. 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 nucleotides and their complexes due to stacking interaction was negligibly small; that is, the monomeric normal and protonated ternary complexes were studied.