Abstract
The study of Cu(II)–poly(L‐arginine) complexes by potentiometric titration, as well as by optical, circular dichroism, and infrared spectra, provides information about the nature of ligands and the coordination sphere around the metal ion. Three different complexes have been identified. The first, which is formed below pH 8, contains two guanidinium nitrogens and two water molecules at the corners of the coordination square. The constant of the overall process as determined by the Gregor method equals 2.0 ± 0.1 × 10^−9^. The two other complexes form between pH 8 and 10.5 and they contain two guanidinium and two peptide nitrogens as nearest ligands. One of them is a monomer and the other probably a dimer, which differ in the symmetry of the coordination sphere around the cupric ion. The optical spectra of the three complexes show an absorption band at 260 nm that we have assigned to a charge‐transfer transition between a σ metal nitrogen (amine) molecular orbital and a d~x~2~−y~2 metal orbital. The spectra of the two complexes containing peptide nitrogens exhibit another absorption band at 320 nm, which we have assigned to a charge transfer from a π orbital of the amide group to the d~x~2~−y~2 metal orbital.