Noncovalent interactions and copper(II) coordination in systems containing l-aspartic acid and triamines

Interactions between l-aspartic acids (Asp) and polyamines (PA): 3,3-tri (1,7-diamino-4-azaheptane) or spermidine (Spd, 1,8-diamino-4-azaoctane) are investigated in metal-free systems as well as between Cu(II) ions in ternary systems with Asp and 3,3-tri or Spd. The composition and stability constants of the complexes formed have been determined by a potentiometric method, while the centres of interactions in the ligands have been identified by NMR, UV–Vis, IR, and EPR spectroscopy. The centres are the potential sites of metal ion coordination. In the Asp/PA systems, formation of molecular complexes (Asp)H_x(PA) was observed. Comparison of the log K_e of the adducts showed that the stability of the adducts significantly depends on the steric factor contributed by the length of PA. In the (Asp)H₃(PA) species, an inversion effect was observed where one of the amine groups (deprotonated) of 3,3-tri or Spd becomes a negative reaction centre and reacts with the protonated amino group of Asp. Therefore, depending on pH, the amino group of the PA can act as a positive or negative reaction centre. In the ternary systems of Cu(II)/Asp/PA the heteroligand-protonated complexes and molecular complexes are formed. In the molecular complexes ML?L′, where L = Asp and L′ = PA, the metallation involves oxygen atoms from the carboxyl groups and the amino group of the amino acid, while the fully protonated PA is located outside the inner coordination sphere and reacts with the anchoring binary complex CuH(Asp) or Cu(Asp). Introduction of metal ions into the Asp/3,3-tri system was found to change the character of the interaction and in the Cu(Asp)H₂(3,3-tri) complex, the oxygen atoms from the Asp carboxyl groups do not take part in coordination.