Copper L-edge spectral studies: a direct experimental probe of the ground-state covalency in the blue copper site in plastocyanin

Modelling the spectroscopy and dynamics of plastocyanin

“…A variety of spectroscopic techniques have been applied to study these proteins, predominantly by Solomon and co-workers. [10][11][12][13] Also of interest are model systems, such as CuCl 2À 4 , which have been used to inform the interpretation of the protein spectra. 7 In the ultraviolet (UV) and visible regions of the spectrum, ligand field and charge transfer excitations occur, arising from d ' d and d ' ligand excitations, respectively.…”

Determination of the geometric structure of the metal site in a blue copper protein by paramagnetic NMR

“…The electronic structure of the blue copper site of plastocyanin has been studied extensively during the last decades both by experimental approaches (5,16,17,38) and quantum chemical calculations (5,10,(25)(26)(27)47). It was found that the unpaired electron spin density is located predominately at the atomic orbitals of the four atoms in the trigonal NNS-plane, i.e., the copper atom, the Cys S ␥ ligand, and the two aromatic His N ␦ ligands (5,16,17,(25)(26)(27)30), whereas the electron spin density at the Met S ␦ atom and the Cys H ␤ is small (26)(27)(28)38). Yet, it was found recently (31) that also the latter orbitals must be included in the description of the unpaired electron spin density to obtain a reliable prediction of the paramagnetic relaxation of the protons close to the copper site.…”

“…In particular, the metal sites of blue copper proteins are characterized by a short coppersulfur bond. This unusual geometry is believed to be the main reason for the strong covalency of the metal site (10,16,17) and, thus, responsible for the rapid and long-range electron transfer reactivity (18-22) that characterizes the blue copper proteins. Detailed knowledge of the geometric and electronic metal site structures of the blue copper proteins is, therefore, imperative for understanding the function of the proteins at the molecular level.…”

“…So far, the geometric structure of the metal site in blue copper proteins (12)(13)(14) has been determined primarily by x-ray crystallography and extended x-ray absorption fine structure (3,23,24), whereas the electronic structure of the blue copper site has been determined theoretically from quantum chemical calculations (5,(25)(26)(27) and experimentally by x-ray absorption spectroscopy (16,17), and more recently by nuclear paramagnetic relaxation (28)(29)(30)(31). These structures have formed the basis for a detailed understanding of the biological function of the proteins by elucidating the interplay between the electronic and geometric structure of the metal site (8,15,(32)(33)(34).…”

“…Because the paramagnetic restraints are derived from the relaxation rates of nuclei close to the metal site, the point dipole approximation of the electron spin fails and a description of the paramagnetic relaxation that takes the electron delocalization into account must be used (37). Therefore, the approach requires knowledge of the distribution of the unpaired electron spin, which can be obtained experimentally by x-ray absorption spectroscopy (16,17), electron nuclear double resonance (38), paramagnetic NMR (28-31) spectroscopy, or theoretically by quantum chemical calculations (5,(25)(26)(27). The blue copper protein, plastocyanin from Anabaena variabilis (A.v.…”