Theoretical Study of the Electronic Spectrum of Plastocyanin

Abstract

The electronic spectrum of the blue copper protein plastocyanin has been studied by ab initio multiconfigurational second-order perturbation theory (the CASPT2 method). The six lowest electronic transitions have been calculated and assigned with an error of less than 2000 cm-1. The singly occupied orbital in the ground state is Cu 3d−SCys 3pπ antibonding with some NHis 2pσ character. The bright blue color originates from an electron transfer to this orbital from the corresponding Cu 3d−SCys 3pπ bonding orbital. The influence of different ligand models on the spectrum has been thoroughly studied; Cu(imidazole)2(SCH3)(S(CH3)2)+ as a model of CuHis2CysMet is the smallest system that gives converged results. The spectrum is surprisingly sensitive to changes in the geometry, especially in the Cu−S bond distances; a 5 pm change in the Cu−SCys bond length may change the excitation energies by as much as 2000 cm-1. The effect of the surrounding protein and solvent on the transition energies has been modeled by point charges and is found to be significant for some of the transitions (up to 2000 cm-1).

Theoretical Study of the Electronic Spectra of Small Molecules That Incorporate Analogues of the Copper–Cysteine Bond

“…The intense band arises from an excitation from the bonding combination of the Cu 3d x 2 y 2 orbital and the S cys 3p p orbital to the SOMO. [5][6][7] Closely related blue copper sites, such as cucumber basic protein, pseudoazurin and nitrite reductase, each have a slightly distorted active site compared to plastocyanin and exhibit remarkably different spectral features, in particular an increased intensity of the higher energy band. 4,8 Furthermore, recent studies have shown that a closely related protein, nitrosocyanin, adsorbs in the red.…”

“…7,[14][15][16][17][18][19][20] However, there has been less attention on small molecules that incorporate the copper-cysteine bond. Ryde et al have studied the structure and reorganization energy of small models of the copper cysteine bond, including CuSH 0/+ and [Cu(imidazole-CH 3 ) 2 (SC 2 H 5 (CH 3 SC 2 H 5 )] 0/+ , using a number of theoretical methods including density functional theory (DFT) with the B3LYP exchange-correlation functional, perturbation theory and coupled cluster theory.…”

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.…”

“…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.…”

“…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).…”