EPR analysis of multiple forms of [4Fe–4S] 3+ clusters in HiPIPs

The electron paramagnetic resonance (EPR) spectrum from the [4Fe–4S] 3+ cluster in several high-potential iron–sulfur proteins (HiPIPs) is complex: it is not the pattern of a single, isolated S =1/2 system. Multifrequency EPR from 9 to 130 GHz reveals that the apparent peak positions ( g values) are frequency-independent: the spectrum is dominated by the Zeeman interaction plus g -strain broadening. The spectra taken at frequencies above the X-band are increasingly sensitive to rapid-passage effects; therefore, the X-band data, which are slightly additionally broadened by dipolar interaction, were used for quantitative spectral analysis. For a single geometrical [4Fe–4S] 3+ structure the (Fe–Fe) 5+ mixed-valence dimer can be assigned in six different ways to a pair of iron ions, and this defines six valence isomers. Systematic multicomponent g -strain simulation shows that the [4Fe–4S] 3+ paramagnets in seven HiPIPs from different bacteria each consist of three to four discernible species, and these are assigned to valence isomers of the clusters. This interpretation builds on previous EPR analyzes of [4Fe–4S] 3+ model compounds, and it constitutes a high-resolution extension of the current literature model, proposed from paramagnetic NMR studies.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47867/1/775_2005_Article_656.pd

Hydrogen Bond Geometries from Electron Paramagnetic Resonance and Electron-Nuclear Double Resonance Parameters: Density Functional Study of Quinone Radical Anion-Solvent Interactions

Density functional theory was used to study the impact of hydrogen bonding on the p-benzosemiquinone radical anion BQ•- in coordination with water or alcohol molecules. After complete geometry optimizations, 1H, 13C, and 17O hyperfine as well as 2H nuclear quadrupole coupling constants and the g-tensor were computed. The suitability of different model systems with one, two, four, and 20 water molecules was tested; best agreement between theory and experiment could be obtained for the largest model system. Q-band pulse 2H electron−nuclear double resonance (ENDOR) experiments were performed on BQ•- in D2O. They compare very well with the spectra simulated by use of the theoretical values from density functional theory. For BQ•- in coordination with four water or alcohol molecules, rather similar hydrogen-bond lengths between 1.75 and 1.78 Å were calculated. Thus, the computed electron paramagnetic resonance (EPR) parameters are hardly distinguishable for the different solvents, in agreement with experimental findings. Furthermore, the distance dependence of the EPR parameters on the hydrogen-bond length was studied. The nuclear quadrupole and the dipolar hyperfine coupling constants of the bridging hydrogens show the expected 1/R3O···H dependencies on the H-bond length RO···H. A 1/R2O···H correlation was obtained for the g-tensor. It is shown that the point-dipole model is suitable for the estimation of hydrogen-bond lengths from anisotropic hyperfine coupling constants of the bridging 1H nuclei for H-bond lengths larger than approximately 1.7 Å. Furthermore, the estimation of H-bond lengths from 2H nuclear quadrupole coupling constants of bridging deuterium nuclei by empirical relations is discussed

An EPR/ENDOR, Mössbauer, and quantum chemical investigation of diiron complexes mimicking the active oxidized state of [FeFe]hydrogenase

We present a study of two compounds closely resembling the active site of [FeFe]hydrogenase, which catalyzes the reversible heterolytic splitting of molecular hydrogen. Utilizing Mössbauer and advanced electron paramagnetic resonance techniques, we were able to resolve the electronic structure of these model compounds in great detail. The experimental results are also compared with quantum-chemical calculations in order to gain more insight into their electronic properties. The obtained data allow us to better understand the function of the native hydrogen catalyst

EPR/ENDOR, Mössbauer, and Quantum-Chemical Investigations of Diiron Complexes Mimicking the Active Oxidized State of [FeFe]Hydrogenase

Understanding the catalytic process of the heterolytic splitting and formation of molecular hydrogen is one of the key topics for the development of a future hydrogen economy. With an interest in elucidating the enzymatic mechanism of the [Fe₂(S₂C₂H₄NH)­(CN)₂(CO)₂(μ-CO)] active center uniquely found in [FeFe]­hydrogenases, we present a detailed spectroscopic and theoretical analysis of its inorganic model [Fe₂(S₂X)­(CO)₃(dppv)­(PMe₃)]⁺ [dppv = <i>cis</i>-1,2-bis­(diphenylphosphino)­ethylene] in two forms with S₂X = ethanedithiolate (<b>1edt</b>) and azadithiolate (<b>1adt</b>). These complexes represent models for the oxidized mixed-valent Fe^IFe^II state analogous to the active oxidized “H_ox” state of the native H-cluster. For both complexes, the ³¹P hyperfine interactions were determined by pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) methods. For <b>1edt</b>, the ⁵⁷Fe parameters were measured by electron spin-echo envelope modulation and Mössbauer spectroscopy, while for <b>1adt</b>, ¹⁴N and selected ¹H couplings could be obtained by ENDOR and hyperfine sublevel correlation spectroscopy. The spin density was found to be predominantly localized on the Fe­(dppv) site. This spin distribution is different from that of the H-cluster, where both the spin and charge densities are delocalized over the two Fe centers. This difference is attributed to the influence of the “native” cubane subcluster that is lacking in the inorganic models. The degree and character of the unpaired spin delocalization was found to vary from <b>1edt</b>, with an abiological dithiolate, to <b>1adt</b>, which features the authentic cofactor. For <b>1adt</b>, we find two ¹⁴N signals, which are indicative for two possible isomers of the azadithiolate, demonstrating its high flexibility. All interaction parameters were also evaluated through density functional theory calculations at various levels