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Iron-sulfur clusters are common building blocks for electron transport and active sites of metalloproteins. Their comprehensive investigation is crucial for understanding these enzymes, which play important roles in modern biomimetic catalysis and biotechnology applications. We address this issue by utilizing (Et4N)3[Fe4Te4(SPh)4], a tellurium modified version of a conventional reduced [4Fe-4S]+ cluster, and performed both 57Fe- and 125Te-NRVS to reveal its characteristic vibrational features. Our analysis exposed major differences in the resulting 57Fe spectrum profile as compared to that of the respective [4Fe-4S] cluster, and between the 57Fe and 125Te profiles. DFT calculations are applied to rationalize structural, electronic, vibrational, and redox-dependent properties of the [4Fe-4Te]+ core. We herein highlight the potential of sulfur/tellurium exchange as a method to isolate the iron-only motion in enzymatic systems.

Iron-sulfur clusters are common building blocks for electron transport and active sites of metalloproteins. Their comprehensive investigation is crucial for understanding these enzymes, which play important roles in modern biomimetic catalysis and biotechnology applications. We address this issue by utilizing (Et4N)3[Fe4Te4(SPh)4], a tellurium modified version of a conventional reduced [4Fe-4S]+ cluster, and performed both 57Fe- and 125Te-NRVS to reveal its characteristic vibrational features. Our analysis exposed major differences in the resulting 57Fe spectrum profile as compared to that of the respective [4Fe-4S] cluster, and between the 57Fe and 125Te profiles. DFT calculations are applied to rationalize structural, electronic, vibrational, and redox-dependent properties of the [4Fe-4Te]+ core. We herein highlight the potential of sulfur/tellurium exchange as a method to isolate the iron-only motion in enzymatic systems.

Full-waveform inversion (FWI) of surface waves can provide a valuable contribution to near-surface investigations, since they are mainly sensitive to the S -wave velocity and hold a high signal-to-noise ratio. We investigate the performance of the individual 2-D elastic FWI of Rayleigh and Love waves as well as the feasibility of a simultaneous joint FWI of both wave types. We apply these three methods to synthetic data as well as to field data. In synthetic reconstruction tests we compare the performance of the individual wave type inversions and explore the benefits of a simultaneous joint inversion. In these tests both individual wave type inversions perform similarly well, given that the initial P -wave velocity model is accurate enough. In the case of an inaccurate initial P -wave velocity model, we observe artefacts in the reconstructed S -wave velocity models of the Rayleigh wave FWI and the joint FWI. If the P -wave velocity is sufficiently known the joint FWI can further improve the model of the S -wave velocity. In the field data application the Love wave FWI is superior to the Rayleigh wave FWI, possibly due to the insufficient initial P -wave velocity model, while the joint FWI further improves the inversion result also in this case. [ABSTRACT FROM AUTHOR]

[FeFe]-hydrogenases are efficient biological hydrogen conversion catalysts and blueprints for technological fuel production. The relations between substrate interactions and electron/proton transfer events at their unique six-iron cofactor (H-cluster) need to be elucidated. The H-cluster comprises a four-iron cluster, [4Fe4S], linked to a diiron complex, [FeFe]. We combined 57 Fe-specific X-ray nuclear resonance scattering experiments (NFS, nuclear forward scattering; NRVS, nuclear resonance vibrational spectroscopy) with quantum-mechanics/molecular-mechanics computations to study the [FeFe]-hydrogenase HYDA1 from a green alga. Selective 57 Fe labeling at only [4Fe4S] or [FeFe], or at both subcomplexes was achieved by protein expression with a 57 Fe salt and in vitro maturation with a synthetic diiron site precursor containing 57 Fe. H-cluster states were populated under infrared spectroscopy control. NRVS spectral analyses facilitated assignment of the vibrational modes of the cofactor species. This approach revealed the H-cluster structure of the oxidized state (Hox) with a bridging carbon monoxide at [FeFe] and ligand rearrangement in the CO-inhibited state (Hox-CO). Protonation at a cysteine ligand of [4Fe4S] in the oxidized state occurring at low pH (HoxH) was indicated, in contrast to bridging hydride binding at [FeFe] in a one-electron reduced state (Hred). These findings are direct evidence for differential protonation either at the four-iron or diiron subcomplex of the H-cluster. NFS time-traces provided Mössbauer parameters such as the quadrupole splitting energy, which differ among cofactor states, thereby supporting selective protonation at either subcomplex. In combination with data for reduced states showing similar [4Fe4S] protonation as HoxH without (Hred') or with (Hhyd) a terminal hydride at [FeFe], our results imply that coordination geometry dynamics at the diiron site and proton-coupled electron transfer to either the four-iron or the diiron subcomplex discriminate catalytic and regulatory functions of [FeFe]-hydrogenases. We support a reaction cycle avoiding diiron site geometry changes during rapid H 2 turnover.

In nature, [FeFe]-hydrogenases catalyze the uptake and release of molecular hydrogen (H 2 ) at a unique iron-sulfur cofactor. The absence of an electrochemical overpotential in the H 2 release reaction makes [FeFe]-hydrogenases a prime example of efficient biocatalysis. However, the molecular details of hydrogen turnover are not yet fully understood. Herein, we characterize the initial one-electron reduction of [FeFe]-hydrogenases by infrared spectroscopy and electrochemistry and present evidence for proton-coupled electron transport during the formation of the reduced state Hred'. Charge compensation stabilizes the excess electron at the [4Fe-4S] cluster and maintains a conservative configuration of the diiron site. The role of Hred' in hydrogen turnover and possible implications on the catalytic mechanism are discussed. We propose that regulation of the electronic properties in the periphery of metal cofactors is key to orchestrating multielectron processes., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The compounds [Fe(CO) 3 (dRpf)] n+ , n=0, 1, 2 and dRpf=1,1'-bis(dicyclohexylphosphino)ferrocene ([1] n+ ) or 1,1'-bis(diisopropylphosphino)ferrocene ([2] n+ ), were obtained as two-step reversible redox systems by photolytic and redox reactions. The iron-iron distance decreases from about 4 Å to about 3 Å on oxidation, which takes place primarily at the tricarbonyliron moiety. Whereas ferrocene oxidation is calculated to occur only in excited states, the near infrared absorptions of the mixed-valent monocations are due to an unprecedented "inverse" inter-valence charge transfer from the electron-rich iron(II) in the ferrocene backbone to the electron-deficient tricarbonyliron(I). Protonation of complex 1 results in the formation of the structurally characterized hydride [1H]BF 4 , which reacts with acetone to form the dication, 1 2+ , and isopropanol. While the hydride [2H]BF 4 was found to be unstable, protonation of 2 in acetone resulted in the clean formation of 2 2+ , formally a hydrogen transfer., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)