Correlation between the Magnetic g Tensors and the Local Cysteine Geometries for a Series of Reduced [2Fe-2S] Protein Clusters. A Quantum Chemical Density Functional Theory and Structural Analysis

摘要

We relied on the density functional theory (DFT) to study the electronic structure of the [2Fe-2S](SH)_4 model of the active site of 2Fe ferredoxins and other proteins containing reduced [2Fe-2S] clusters. The two (Fe~(3+)-Fe~(2+)-S-H) dihedral angles OMEGA_1 and OMEGA_2 defined for the two ligands on the ferrous side were allowed to vary, while the two other (Fe~(2+)-Fe~(3+)-S-H) angles OMEGA_3 and OMEGA_4 on the ferric side were kept constant. The Lande (g), magnetic hyperfine, and quadrupole tensors for two geometries, C_2 (OMEGA_1 = OMEGA_2) and C_s (OMEGA_1 = -OMEGA_2), were calculated. To apply our model to the actual proteins, we listed all of the crystallographic structures available for the [2Fe-2S*] systems. A classification of these proteins, based on the four dihedral angles {Q/}/=i-4, separates them into three main classes. The main structural feature of the first class (OMEGA_1approx=OMEGA_2), with an average dihedral angle OMEGA_(av)= (OMEGA_1 + OMEGA_2)/2 comprised between 115deg and 150deg, corresponds to a local ferrous C_2 geometry (rather than C_(2v), as previously assumed by Bertrand and Gayda: Biochim. Biophys. Acta 1979, 579, 107). We then established a direct correlation between the three principal g values and OMEGA_(av). It is the first time that such a link has been made between the spectroscopic and structural parameters, a link, moreover, fully rationalized by our DFT calculations. We finally point out the basic differences between our C_2 results with those of the C_(2v) phenomenological model proposed in the late 1970s by Bertrand and Gayda.