On the Relationship between Protein-Forced Ligand Fields and the Properties of Blue Copper Centers

Abstract

The stable conformation of blue copper proteins forces the metal ion into a unique geometrical ligand arrangement. Spectroscopic data suggest that at least 70 kJmol⁻¹ would be required to twist the protein into a conformation that would allow the cupric ion to occupy a square planar site. The observation that the γ S→ Cu(x²-y²) transition energy does not change with increasing ligand field strength is interpreted in terms of a model in which Cu(xz, yz)ligand back bonding enhances the splitting between the Cu(x²-y² ) and Cu(xz, yz) levels in blue sites with high chemical reduction potentials, thereby providing an explanation of the special stability of the cuprous state.