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

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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.