Multi-Microwave Frequency ESR in the Structural Characterization of Copper(II) Dipeptide Complexes

Because of the multiplicity of possible equilibria in metal-peptide solutions, a new study using multifrequency EPR in combination with computer simulations and second-derivative displays was attempted. This approach permitted the determination with good precision of a set of spin Hamiltonian input parameters for the Cu-GlyHis and Cu-HisGly complexes. Two different arrangements in the first coordination sphere of copper were determined. In the case of the Cu-GlyHis complex, the simultaneous presence of the monomer and the bis complex and their relative percentages were established. For the Cu-HisGly complex, histamine-like coordination is proposed with a limited deviation from planarity. Spectral results obtained at room temperature for fast-tumbling copper complexes provided confirmation of the biological validity of the models. Molecular orbital coefficients characteristic of metal-ligand bonds were derived for effective D4h symmetry. The influence of the coordination modes on the covalent and ionic character of the metal-ligand bonds is discussed. © 1993, American Chemical Society. All rights reserved