Coordination Modes of Histidine Moiety in Copper (II) Dipeptide Complexes Detected by Multifrequency ESR R.Basosi, R.Pogni,

The copper complexes of histidinecontaining peptides naturally occurring in blood plasma [1,2] can be used as a model for metal/protein interactions.In fact a low molecular weight peptide containing histidine or even histidine itself may compete with ceruloplasmin for copper in the blood, significantly altering free ligand levels. For this reason, the coordination behaviour of the histidine moiety in copper (II) dipeptides has been the subject of much investigation [3-7]. From this point of view the study under physiological conditions, namely room temperature and neutral pH, is crucial for maintaining the biological significance of the model. Unfortunately a definitive speciation has so far been hindered by the strong dependence on the ligand-to-metal ion ratio and the multiplicity of possible equilibria, function of pH, in solution. ESR may be used to study paramagnetic metal complexes, and multifrequency ESR in combination with computer simulation is a decisive tool for determining the chemical structure of copper biosystems under physiological conditions. For ESR spectra the requirement of a good fit at different frequencies put constraints on the precision of magnetic parameters and allows definitive assignments, despite the lack of resolution typical of room temperature spectra. In this paper the above method was used in the study of complexes of copper(II) and Glycylhistidine (GlyHis),