Thermodynamic stereoselectivity assisted by weak interactions in metal complexes. Copper(II) ternary complexes of cyclo-L-histidyl-L-histidine and L- or D-amino acids in aqueous solution

Abstract

The thermodynamic stereoselectivity and spectroscopic characteristics of copper(II) ternary complexes with L- or D-amino acids (Xaa) and cyclo-L-histidyl-L-histidine, cyclo(-His-His-), have been investigated. Ternary complex formation with L-amino acids is more enthalpy and less entropy favoured than that of the analogous D-amino acids. For aromatic amino acids these differnces increase going from phenylalanine to tryptophan, suggesting involvement of aromatic ring-stacking interactions between one of the dipeptide imidazoles and the Xaa aromatic side chain in the mixed-ligand complexes. Enthalpy changes are the driving force for the thermodynamic stereoselectivity. A detailed analysis of the CD spectra of these ternary systems resulted in a correlation between Δ(Δε), the difference between the molar CD coefficients at the wavelength where it is maximized, and the ΔG° and ΔH° values. On the basis of EPR spectra, it is excluded that the differences between the mixed-ligand complexes with L- or D-amino acids are due to a different degree of solvation. It is proposed that hydrophobic and ring-stacking interactions, responsible for stereoselectivity, are favoured by a certain degree of tetrahedral distortion of the tetragonal copper(II) chromophore formed by the two imidazole nitrogen donors of cyclo(-His-His-) and the nitrogen and carboxylate oxygen donors of the L- or D-amino acids.