DFT calculations of the EPR parameters for Cu(II) DETA imidazole complexes

The electron paramagnetic resonance (EPR) parameters for Cu(ii) diethylenetriamine imidazole complexes, which serve as empirical models for copper-containing proteins, were calculated using density functional theory (DFT). The orientations of three different types of imidazole ligands, imidazole, 1-methylimidazole and 4-methylimidazole, were investigated by rotating the ligand about the Cu(ii) imidazole bond. The calculated EPR values indicate that the imidazole ligands studied are oriented approximately +/-45 degrees with respect to the ligand plane. EPR parameters calculated using the B3LYP density functional in conjunction with the conductor-like solvent model (COSMO) show the best agreement with the experimentally determined EPR values. Good agreement with electron spin echo envelope modulation (ESEEM) data is achieved when an explicit water molecule is located near the remote nitrogen atom of the imidazole ligand. The implications of these DFT calculations for interpreting experimental pulsed EPR data for copper proteins containing imidazole ligands are discussed.