Complexes of copper(II) with L-aspartic acid in systems with tetramines and non-covalent interactions between bioligands

Abstract

In metal-free systems, interactions were studied between L-aspartic acid (Asp) and the tetramines 1,11-diamino-4,8-diazaundecane (3,3,3-tet) and 1,12-diamino-4,9-diazadodecane, spermine (Spm), and in the systems of both ligands with Cu(II). The formation of molecular complexes of the (Asp)H _x (tetramine) type was evidenced by results from equilibrium and spectral studies. An increase in the efficiency of the reaction between the bioligands was observed with the increasing length of the polyamine chain. For aspartic acid, the centers of interaction are carboxyl and amine groups, whereas in tetramine molecules it is the amine groups. The effect of inversion was observed in the adducts (Asp)H₄(tetramine) and (Asp)H₃(tetramine). In ternary systems, the presence of molecular complexes of MLL′, protonated complexes MLH _x L′, complexes MLL′, and hydroxo complexes of the MLL′(OH) _x type were found, which were not detected in earlier systems with di- and triamines. In MLL′ complexes, where L = L-aspartic acid and L′ = polyamine, metallation involves the oxygens of carboxyl and amine of the amino acid, while the protonated tetramine is in the external coordination sphere, engaged in noncovalent interaction with the anchoring Cu(Asp). Moreover, in Cu(II)–Asp-tetramine in protonated species, noncovalent bonds were found between ligands which additionally stabilized the complex. At higher pH, hydroxyl groups were more effective in metallation than carboxyl groups from aspartic acid.

Keywords:

• Copper(II)
• L-Aspartic acid
• Tetramine
• Adducts
• Mixed complexes
• Interaction

Acknowledgment

My warm thanks are due to Dr Renata Jastrzab for EPR spectral measurements.