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.

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

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