Structural and spectroscopic studies of the copper site of stellacyanin

X-ray absorption spectroscopic studies at different pH values of the structure of copper site in oxidized and reduced Rhus vernicifera stellacyanin reveals that the fourth ligand in the oxidized protein is O- or N-donating. This ligand is absent in the inner coordination sphere but shows a distant interaction at 2.7 Angstroms from the copper atom. Reduction of the stellacyanin removes a histidine ligand by nearly 0.2 Angstroms from the copper atom. The reduced protein lacks the S-donating fourth ligand.