Study of structural and dynamic characteristics of copper(II) amino acid complexes in solutions by combined EPR and NMR relaxation methods.

Structural features and dynamical behaviour of the copper(ii) bis-complexes with glycine, d-alanine, d-valine, l-serine, l-aspartic acid, l-glutamic acid, l-lysine, l-proline, and sarcosine were studied by combined EPR and NMR relaxation methods. The cis and trans isomers were unambiguously assigned and characterized by EPR data. It was found that addition of a salt background has an influence on the cis-trans isomer equilibrium in favour of the formation of the cis isomer. By comparison of NMRD, DFT computations, and structural data it was shown that only one water molecule is coordinated in the axial position of these complexes. The increased exchange rates of this molecule found for Cu(l-Asp)2(2-), Cu(l-Glu)2(2-), Cu(l-LysH)2(2+), and Cu(l-Pro)2 were attributed to its pushing out by side chain groups of the ligands. By simulation of NMRD profiles an increase of lifetimes of the copper(ii) 2nd coordination sphere water molecules was revealed in the presence of additional carboxylic, alcoholic, or ammonium groups of the ligands, as well as the pyrrolidine ring of proline. The very short lifetimes of the 2nd coordination sphere water molecules (4-13 ps at 298 K) were explained in terms of the Frank-Wen structural model by the existence of cavities which draw in quickly enough water molecules from the 2nd coordination sphere.

Complex formation, chemical exchange, species structure, and stereoselective effects in the copper(II)-L/DL-histidine systems.

The formation of copper(II) complexes with L- and DL-histidine (HisH) has been studied by means of pH-potentiometry and spectrophotometry over a wide range of pH (2-14), ligand-to-metal ratio (1 : 1-15 : 1), and temperature (15-55 °C) in aqueous solutions with 1.0 mol dm(-3) KNO(3) as background. Formation constants and spectral characteristics of 13 complex types were found. Fine stereoselective effects have been detected with preferential coordination of two ligands with identical configuration in Cu(His)(HisH)(+) and opposite configuration in Cu(His)(2). The stereoselective effect for Cu(His)(HisH)(+) is explained by hydrogen bond formation between the carboxyl and imidazolyl groups of neighboring ligands at cis-arrangement of amino groups (3N(eq)-form). The opposite sign of stereoselective effect for Cu(His)(2) is derived from favourable axial coordination of the imidazole group in meso-form with cis-structure (3N(eq)N(ax)-form). A significant tetrahedral distortion was revealed for the first time in the prevalent cis-isomer of the Cu(L-His)(2) 4N(eq)-form. These findings were confirmed by EPR data and DFT computations at the B3LYP/TZVP level. The prevalence of cis-isomers for these complexes has been assigned to the rather strong trans effect of the amino groups. The structures of other detected complexes are briefly discussed on the basis of spectroscopic data. Chemical exchange reactions in the copper(II)-L/DL-hishidine systems have been investigated by the NMR relaxation of water protons. A unique proton exchange reaction with short-term proton dissociation from the coordinated imidazolyl group catalyzed by hydroxide ion was characterised for the first time. The discovered enantioselective effects in the ligand exchange reactions between Cu(His)(2) and HisH or His(-) species were attributed to the associative substitution mechanism.