Electrochemical and spectroscopic studies on copper(II) complexes of macrocyclic ligands as models for square-pyramidal metal active sites of copper(II) complexes of bleomycin and glutathione

Abstract

The properties of Cu(II) complexes of saturated macrocyclic ligands with square-pyramidal array were examined by cyclic voltammetry, electron spin resonance, visible absoption and magnetic circular dichroism spectroscopy. All ligands used with N 4, N 5, N 6, N 4S and N 4O donor sets coordinated to Cu(II) to form a chromophore by the same donor set N 4 at the equatorial plane. The values of λ max vs. E 1 2 and g∥ vs. 1/E 1 2 showed excellent linear correlations; however, the complexes containing O and S ligation exhibited marked deviations from the correlations. Of special interest is the fact that the physicochemical data of the 15N 5–Cu(II) complex are substantially similar to those from the naturally occurring bleomycin–Cu(II) complex, in which the axially coordinated primary amino group plays a significant role for antitumor activity. With replacement of the axial N donor with the O and S donor, a marked positive shift was recognized in the E 1 2 values in the order of S > O > N. A positive shift of E 1 2 value, which is a feature of the apical S-donor in the square-pyramidal 15N 4S–Cu(II) complex, was consistently seen for the glutathione–Cu(II) complex.