An EPR and voltammetric study of simple and mixed copper(II) complexes with L- or D-glutamate and L-arginate in aqueous solution

EPR CW measurements at room and low temperature have been carried out on the systems containing 63copper(II) nitrate, L-Arg and L-or D-Glu or DL- racemic mixtures of both amino acids in aqueous solution or in mixed solvents. For these copper(II) bis and ternary complexes, RT EPR spectra revealed that mixtures of cis and trans conformers are really present in aqueous solution. In most cases, for these systems, the cis conformer is more abundant at the lowest temperature (275 K). Increasing gradually the temperature from 275 K to 305 K the cis/trans ratio tends towards the trans conformer. This latter is less abundant in the case of [Cu(L-Glu)2], because of the possible interaction of negative side chain carboxylate anions with the water molecules axially bound, and for [Cu(L-Arg)(D-Glu)], probably owing to a possible electrostatic interaction between the two opposite charged side chains which are on the same side of the CuN2O2 coordination plane. Low temperature frozen solution EPR spectra run on these systems show the presence of only one absorbing specie thus giving reason to the stabilization of one conformer over the other, in particular, the cis conformer. There are no evident differences in the anisotropic Spin Hamiltonian parameters derived from the frozen EPR solution spectra of these systems, which are characterized by a tetragonally elongated octahedral stereochemistry. SWV measurements confirmed the EPR spectroscopic data, actually giving rise to the observation of slightly asymmetrical peaks, which revealed two near formal redox potentials assigned to the two conformers.