Insight into the copper coordination environment in the prion protein through density functional theory calculations of EPR parameters
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Density functional theory (DFT) calculations of Cu(II) electron paramagnetic resonance (EPR) parameters for the octarepeat unit of the prion protein were conducted. Model complexes were constructed and optimized using the crystal structure of the octarepeat unit of the prion protein. Copper g and A tensors and nitrogen hyperfine and quadrupole coupling constants were calculated using DFT. Solvent effects were incorporated using the conductor-like screening model as well as through the inclusion of explicit water molecules. Calculations using the model with an additional axial water molecule added to the coordination sphere of the Cu(II) metal center give the best qualitative agreement for the copper g and A tensors. The S-band experimental EPR spectra were interpreted in light of the DFT calculations of the directly coordinated nitrogen hyperfine coupling constants which indicate that the three directly coordinated nitrogen atoms in the octarepeat unit are not equivalent. These results demonstrate that DFT calculations of EPR parameters can provide important insight with respect to the structural interpretation of experimental EPR data.