CD spectroscopic study on the speciation and solution structure of copper(II) complexes of some tripeptides in combination with potentiometric and spectrophotometric results

The combination of potentiometric, spectrophotometric and CD spectroscopic studies under the same conditions is expected to yield more reliable thermodynamic and structural information for a certain system. The matrix rank analysis of both the spectrophotometric and CD spectra series gives the necessary number of species to be taken into account in the calculations. Based on such speciation diagrams the molar spectra for individual complexes in the copper(II) Gly-Gly-Ala, Gly-Ala-Gly, Ala-Gly-Gly and Ala-Ala-Ala systems were obtained. The results allowed us to assign these spectra to two types of coordination species with the compositions CuLH(-1) and [CuLH(-2)](-). The sign and the intensity of the CD spectra are well related to the chirality and distance of the chiral center(s) in the ligand from the metal ion chromophore and the stability of the metal complexes. They are further on additive within the experimental error in both cases with respect to chiral centers: the sum of the CD spectra of the Gly-Gly-Ala, Gly-Ala-Gly and Ala-Gly-Gly complexes equals those of Ala-Ala-Ala complexes. The CD spectra strongly support the coordination also of the C-terminal part of the peptide in the CuLH(-1) complex of GGA, and even the deprotonation of the Ala peptide group instead of that belonging to Gly cannot be excluded.