Spectral Properties of Mixed Ligand Copper(II) Complexes Containing l -α-Amino Acids

Absorption and circular dichroism (CD) spectral behaviors of ternary copper(II) complexes containing two l-α-amino acids or an l-α-amino acid and iminodiacetate (ida), 2,2′-bipyridine (bpy), 2-pyridinecarboxylate (pea), or 2,6-pyridinedicarboxylate (pdea), have been studied comparatively, and synthetic studies have given as crystals the mixed ligand complexes, [Cu(ida)(l-arg)]·2H2O, [Cu(ida)(l-orn)]·2H2O, [Cu(ida)(l-lys)]·1/4H2O, [Cu(pdca)(l-arg)]·H2O, and [Cu(pdca)(l-orn)]·2.5H2O, where l-arg, l-orn, and l-lys denote l-arginine, l-ornithine, and l-lysine, respectively. The CD magnitudes of the systems Cu(ida)(l-arg), Cu(ida)(l-orn), and Cu(ida)(l-lys) were found to be enhanced appreciably as compared with the magnitudes estimated from the values for the corresponding binary systems. These ternary systems showed further magnitude enhancements in 50% aqueous ethanol. The enhanced magnitudes observed for Cu(l-glu)(l-arg), where l-glu is l-glutamate, were also intensified in 10, 20, and 50% aqueous ethanol, whereas they were weakened nearly to the estimated values in aqueous solutions with higher ionic strengths (μ=0.1 and 0.5(NaClO4)). These observations support the view that the pairs of ligands constituting the above ternary complexes are fixed around copper(II) by electrostatic ligand-ligand interactions between the oppositely charged side chains. Pyridine-containing ligands, bpy, pea, and pdea, in the ternary systems with l-α-amino acids affected the signs and the magnitudes of the CD spectra.