Herein, a systematic
study of [L2Fe2S2]n model complexes (where L = bis(benzimidazolato) and n = 2-, 3-, 4-) has been carried out using iron and sulfur
K-edge X-ray absorption (XAS) and iron Kβ and valence-to-core
X-ray emission spectroscopies (XES). These data are used as a test
set to evaluate the relative strengths and weaknesses of X-ray core
level spectroscopies in assessing redox changes in iron–sulfur
clusters. The results are correlated to density functional theory
(DFT) calculations of the spectra in order to further support the
quantitative information that can be extracted from the experimental
data. It is demonstrated that due to canceling effects of covalency
and spin state, the information that can be extracted from Fe Kβ
XES mainlines is limited. However, a careful analysis of the Fe K-edge
XAS data shows that localized valence vs delocalized valence species
may be differentiated on the basis of the pre-edge and K-edge energies.
These findings are then applied to existing literature Fe K-edge XAS
data on the iron protein, P-cluster, and FeMoco sites of nitrogenase.
The ability to assess the extent of delocalization in the iron protein
vs the P-cluster is highlighted. In addition, possible charge states
for FeMoco on the basis of Fe K-edge XAS data are discussed. This
study provides an important reference for future X-ray spectroscopic
studies of iron–sulfur clusters.
A biomimetic [2Fe-2S] cluster has been isolated in the fully reduced diferrous form and characterized by X-ray diffraction. This completes a consistent series of synthetic analogues of protein-bound [2Fe-2S](z) redox centers (z = 2+, 1+, 0) with identical capping ligands. (57)Fe Mössbauer data of the extremely oxidation-sensitive complex compare well with those of the very few reports of all-ferrous ferredoxins and Rieske centers; they confirm the S(T) = 0 ground state and establish a lower limit for the exchange coupling, -J ≥ 30 cm(-1).
Cores and effect: A biomimetic [2 Fe‐2 S] cluster is characterized crystallographically in both the [FeIIIFeIII] and the mixed‐valent [FeIIIFeII] forms—the [2 Fe‐2 S] cores show only minor geometric differences. The reduced form has an S=1/2 ground state and the unpaired electron is partially delocalized over the cluster core. The experimental effective coupling constant predicts the position of the intervalence charge transfer band in the IR regime.
Give and take both: A bis(benzimidazolate)-capped biomimetic [2Fe-2S] cluster has been characterized in different protonation states, both in the diferric and mixed-valent forms. Protonation does not lead to structural changes of the [2Fe-2S] core, but facilitates its reduction and causes pronounced valence localization in the mixed-valent state. Concerted proton and electron transfer back to the diferric cluster emulates a key step in the function of mitochondrial Rieske proteins (see scheme).
Copycat: An accurate synthetic model for Rieske type [2Fe‐2S] cluster has been prepared that emulates structural and spectroscopic features of the natural protein sites, including the characteristic low gav value in the EPR spectra of the reduced [2Fe‐2S]+ species. The picture shows the crystal structure of the molecule (C gray, Fe red, N blue, S yellow), its EPR spectrum after reduction (bottom) and its Mössbauer spectrum (top).
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