Computer Simulation of the EPR Spectra of Mononuclear and Dipolar Coupled Cu(II) Ions in Nitric Oxide- and Nitrite-Treated Hemocyanins and Tyrosinase

1. EPR spectra of nitrite-treated Helix pomatia hemocyanin (at 9 and 35 GHz) and of Cancer magister hemocyanin (at 9 GHz) are reported. The spectra show the presence of mononuclear Cu(II) ions. 2. From computer simulations of the mononuclear Cu(II) EPR spectra of the nitrite-treated hemocyanins, using a spin Hamiltonian with orthorhombic symmetry and assuming an anisotropic linewidth it is concluded that these Cu(II) sites have a rhombic distortion. The anisotropic linewidth and superhyperfine structure, which is resolved in some circumstances, can be attributed to coupling with nitrogen ligands. 3. The ΔM = 1 EPR spectra near g = 2 and the ΔM = 2 EPR spectra near g = 4 due to the dipolar coupled Cu(II) ions in NO-treated hemocyanins and tyrosinase have also been simulated, neglecting exchange coupling. The overall symmetry of the dipolar coupled system was found to be close to axial, with the Cu(II)-Cu(II) direction being close to the direction of the z-axis of the g tensors of the individual ions. 4. The calculated Cu(II)-Cu(II) distances were in all cases near 6 Å: for Agaricus bispora tyrosinase 5.9 ± 0.2 Å, for Cancer magister hemocyanin 5.7 ± 0.2 Å and for Helix pomatia hemocyanin 6.4 ± 0.3 Å.