Potentiometric studies of binary and ternary complexes of Zn(II) with triethylenetetramine as primary ligand and selected amino acids and DNA units as secondary ligands

The acid-base equilibria of triethylenetetramine. (Trien) and the formation equilibria of binary and ternary complexes of Zn(II) with Trien as primary ligand and some selected arnino acids and DNA units as secondary ligands have been investigated. The results showed the formation of a 11 Zn (Trien)²⁺ complex. At higher pH, the Zn (Trien)²⁺ complex is hydrolysed to give Zn (Trien) (OH)⁺ and Zn(Trien)(OH)₂ complexes. The fraction of the monohydroxo species attains a maximum of 81.3% at pH 10. The stability constantsK _Zn(Trien)A ^Zn(Trien) for the ternary complexes were determined. Histidine coordinates in a histamine-like way. Lysine and ornithine ligate by the two amino groups. Serine and methionine are bound in a glycine-like mode. However, penicillamine, cysteine and glutathione ligate partly like mercaptoethylamine and partly like mercaptopropionic acid. In the case of DNA complexes, inosine is bound through the n₁ atom. However, uracil, undine, thymine and thymidine ligate through the N₃ atom. The relative stabilities of ternary complexes are compared with those of the corresponding binary complexes in terms of logK values. The concentration distribution diagrams of the complexes are evaluated.