The Copper–Poly-L-histidine Complexes. II. Physicochemical Properties

Soluble Cu(II) complexes of poly-L-histidine (PLH) formed at pH 5 and 14 at a molar ratio of His/Cu = 4 were investigated. Titration of PLH showed that in the presence of Cu(II) ions four protons are released per Cu(II) ion in the pH range of 3.0–4.5 and a fifth proton is released between pH 4.5 and 6.5. Absorption spectra in the visible range show a maximum at 543 mμ (∈ 83) for the complex formed at pH 5 and at 530 mμ (∈ 200) at pH 14. Circular dichroic spectra show that these bands are composite, the main band being positive at pH 5 (λ_max 536 mμ (∈_L–R 0.62)) and negative at pH 14 (λ_max 532 mμ (∈_L–R –0.63)). The two complexes also differ in their epr spectra. It is concluded that in the complex formed at pH 5 the Cu(II) ion is bound to three imidazoles and one peptide nitrogen, whereas at pH 14 four consecutive peptide nitrogens occupy a distorted coordination square and one imidazole group binds at an axial site. These structures were also built from space-filling models.