Template Synthesis, Spectral and Redox Behavior of Metal Complexes of Macrocyclic Tetraaza Schiff Base and their Interaction with Herring Sperm DNA

Abstract

This paper documents the detailed investigation of the relationship between molecular structure and biological activity of few tetraaza macrocyclic metal (II) complexes equipped by the template condensation of o-bromoaniline, ethylenediamine and salbenz in 2:1:1 ratio with metal (II) salts (1mM). All these complexes are found to be stable in air and soluble in CN3CN or DMSO, and are characterized through spectral (UV-Vis, IR, EPR) and electrochemical methods. A square planar geometry is proposed for Cu(II), Ni(II) and Co(II) complexes, while an octahedral geometry is suggested for Mn(II) and Fe(II). The IR spectra indicate that the NH groups of the amine exist as such even after complexation without deprotonation, and all the complexes show a strong band in 1580-1590 cm-1region corresponding to ν(C=N), due to coordinated azomethine group to the metal. The solution electronic spectra of these complexes show intense LMCT bands around 400 nm. Intense electronic absorption spectra as well as the four line pattern in EPR spectra with broad g⊥suggests that the copper(II) complexes have distorted square planar geometry. On titration with herring sperm DNA, CuN4,CoN4,NiN4and MnN4complexes exhibit an abrupt amend in their electronic spectrum and cyclic voltammogram. The intense intraligand π-π* transition in the region 350–420 nm is found to show hypochromicity on titration with DNA in all these complexes, due to their electrostatic interaction with DNA. All these complexes show one well–defined quasi-reversible redox couple with values ranging from ∆Ep 137 to 337 mV. Their spectral and electrochemical outcome designate that the square planar complexes Cu(II), Co(II) and Ni(II) interact much better than the axially coordinated octahedral complexes Mn(II) and Fe(II). The decrease found in the negative absorption peak, characteristic peak due to helicity of DNA, in circular dichroism studies reflects the perversion in the helical nature of B-DNA upon the addition of complex. The binding of plasmid DNA by these complexes has also been investigated by agarose gel electrophoresis, remarkably Ni(II) complex was found to cleave the DNA double helix.