Spectroscopic studies and a structural model for blue copper centers in proteins

Abstract

Low temperature absorption, circular dichroism, and magnetic circular dichroism spectral studies of the blue copper proteins Rhus vernicifera stellacyanin, bean plastocyanin, and Pseudomonas aeruginosa azurin have been made. Low energy bands attributable to the d-d transitions 2B2-.2E and 2B2-.2B1 in a flattened tetrahedral (D2d) copper-(II) center are observed in these proteins at about 5000 and 10,000 cm-1, respectively. The band positions accord well with ligand field calculations based on a tetrahedral structure that is distorted approximately 6° toward a square plane. The ligands in this flattened tetrahedral coordination unit in bean plastocyanin are identified from various spectroscopic experiments as His-38, Cys-85, His-88, and a deprotonated peptide nitrogen (N*) a few residues above His-38. Based on a comparison of absorption and CD intensities, the characteristic bands at about 13,000, 16,000, and 22,000 cm-1 in the blue proteins are assigned to the ligand to metal charge transfer transitions [pi]S->d(x^2-y^2), [sigma]S->d(x^2-y^2) and [pi](N*)->d(x^2-y^2), respectively, in the flattened tetrahedral CuN2N*S unit.