Design of histidine containing peptides for better understanding of their coordination mode toward copper(II) by CD spectroscopy

The systematic investigation of the copper(II) complexes of tripeptides Xaa–Xaa–His, Xaa–His–Xaa and His–Xaa–Xaa, where Xaa = Gly or Ala was performed by combined pH-metry, spectrophotometry, CD and in part EPR spectroscopy. The matrix rank analysis of the spectral data revealed the number of the coloured and optically active species as a basis for the solution speciation. A critical evaluation on the speciation and solution structure of the complexes formed is presented on the basis of their d–d band optical activity. The replacement of a Gly residue with the chiral Ala amino acid allowed us to gain decisive information on the solution structure of the complexes by CD spectroscopy. It was shown that the tripeptides with histidine in the third position formed CuH−2L species with (NH2, 2N−, ImN – where Im stands for imidazole) coordination sphere as a major species, and only the macrochelated CuL complexes as minor species around pH 5.0. In copper(II)–Xaa–His–Xaa tripeptide systems the CuH−1L (NH2, N−, ImN) is the most stable species at physiological pH, but the vacant fourth site around copper(II)ions is offered for further deprotonation, most probably resulting in mixed hydroxo species at low (<5 × 10−4 M) metal ion concentrations, while a tetrameric complex is dominant when the copper concentration exceeds 3 × 10−3 M. The histamine type coordination mode in CuL and CuL2 complexes of His–Xaa–Xaa ligands predominates at low pH. The structural consequences drawn from the CD spectra for the mono and bis parent complexes were supported by theoretical calculations. CD spectra strongly suggest the participation of the imidazole nitrogen both in the Cu2H−2L2 and CuH−2L complexes.