A single-crystal electron paramagnetic resonance study at 95 GHz of the type 1 copper site of the green nitrite reductase of Alcaligenes faecalis

An electron-spin-echo-detected electron paramagnetic resonance (EPR) study has been performed for the type 1 copper site of the green nitrite reductase of Alcaligenes faecalis. Accurate g values and principal directions of the g tensor have been obtained. The z principal axis of the g tensor makes an angle of 60 degrees with the direction from copper to the S delta of the axial methionine. The direction of this axis is compared with that of the blue type 1 proteins azurin and M121Q azurin and of the green type 1 protein M121H azurin, and interpreted in terms of the d orbital at copper in the wave function of the unpaired electron. This d orbital is oriented such that the overlap with the lone-pair orbitals of the ligands is maximum. The relation between the absorption spectrum in the visible, the copper-thiolate interaction, and the rhombicity of the EPR spectrum of type 1 proteins is discussed in terms of the trigonal and tetragonal character of the copper sites and the position and strength of the axial ligand.