Synthesis, spectroscopic and voltammetric studies of mixed-ligand copper(II) complexes of amino acids

Two CuII complexes of the type [Cu(L1)(L2)] (where L1 = tryptophanate or phenylalaninate; L2 = cysteine thiolate) have been prepared and characterised, and their spectrophotometric and voltammetric behaviour has been investigated. The results obtained by means of FT-IR, e.s.r., u.v.–vis. spectroscopy and by voltammetry revealed the existence of two different [Cu(L1)(L2)] complexes. A significance decrease in the g|| value and, concomitantly, an increase in the d–d transition energy was observed when a mixed-ligand complex is present. The observed anisotropic g-values indicate the presence of CuII in a tetragonally distorted octahedral geometry. Formation of a mixed-ligand copper complex can be considered as a type of synergism in the presence of cysteine. The redox state CuII or CuI of copper in the Cu(L2) complex depends on the analysing conditions, i.e., cysteine forms a CuII complex under aerobic conditions and a CuI complex in anaerobic media. Tryptophan or phenylalanine is bound to CuII ions in the Cu(L1) complexes.