24 research outputs found
Individual and joint 2-D elastic full-waveform inversion of Rayleigh and Love waves
We investigate the performance of the individual 2-D elastic full-waveform inversion (FWI) of Rayleigh and Love waves as well as the feasibility of a simultaneous joint FWI of both wave types. The 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. 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 this case the joint FWI further improves the result. For an inaccurate initial P-wave velocity model, we observe artifacts in the results of the Rayleigh wave FWI and the joint FWI. Subsequently, we recorded a near-surface field dataset to verify the results by a realistic example. In the field data application the Love wave FWI is superior to the Rayleigh wave FWI, possibly due to the initial P-wave velocity model. Also in this case the joint FWI further improves the inversion result
Insights from 125Te and 57Fe nuclear resonance vibrational spectroscopy: a [4Fe-4Te] cluster from two points of view.
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
The Geometry of the Catalytic Active Site in [FeFe]-hydrogenases is Determined by Hydrogen Bonding and Proton Transfer
[FeFe]-hydrogenases are efficient metalloenzymes that catalyze the oxidation and evolution of molecular hydrogen, H2. They serve as a blueprint for the design of synthetic H2-forming catalysts. [FeFe]-hydrogenases harbor a six-iron cofactor that comprises a [4Fe-4S] cluster and a unique diiron site with cyanide, carbonyl, and hydride ligands. To address the ligand dynamics in catalytic turnover and upon carbon monoxide (CO) inhibition, we replaced the native aminodithiolate group of the diiron site by synthetic dithiolates, inserted into wild-type and amino acid variants of the [FeFe]-hydrogenase HYDA1 from Chlamydomonas reinhardtii. The reactivity with H2 and CO was characterized using in situ and transient infrared spectroscopy, protein crystallography, quantum chemical calculations, and kinetic simulations. All cofactor variants adopted characteristic populations of reduced species in the presence of H2 and showed significant changes in CO inhibition and reactivation kinetics. Differences were attributed to varying interactions between polar ligands and the dithiolate head group and/or the environment of the cofactor (i.e., amino acid residues and water molecules). The presented results show how catalytically relevant intermediates are stabilized by inner-sphere hydrogen bonding suggesting that the role of the aminodithiolate group must not be restricted to proton transfer. These concepts may inspire the design of improved enzymes and biomimetic H2-forming catalysts
Stepwise isotope editing of [FeFe]-hydrogenases exposes cofactor dynamics
The six-iron cofactor of [FeFe]-hydrogenases (H-cluster) is the most efficient
H2-forming catalyst in nature. It comprises a diiron active site with three
carbon monoxide (CO) and two cyanide (CN−) ligands in the active oxidized
state (Hox) and one additional CO ligand in the inhibited state (Hox-CO). The
diatomic ligands are sensitive reporter groups for structural changes of the
cofactor. Their vibrational dynamics were monitored by real-time attenuated
total reflection Fourier-transform infrared spectroscopy. Combination of 13CO
gas exposure, blue or red light irradiation, and controlled hydration of three
different [FeFe]-hydrogenase proteins produced 8 Hox and 16 Hox-CO species
with all possible isotopic exchange patterns. Extensive density functional
theory calculations revealed the vibrational mode couplings of the carbonyl
ligands and uniquely assigned each infrared spectrum to a specific labeling
pattern. For Hox-CO, agreement between experimental and calculated infrared
frequencies improved by up to one order of magnitude for an apical CN− at the
distal iron ion of the cofactor as opposed to an apical CO. For Hox, two
equally probable isomers with partially rotated ligands were suggested.
Interconversion between these structures implies dynamic ligand reorientation
at the H-cluster. Our experimental protocol for site-selective 13CO isotope
editing combined with computational species assignment opens new perspectives
for characterization of functional intermediates in the catalytic cycle
Accumulating the hydride state in the catalytic cycle of [FeFe]-hydrogenases
H2 turnover at the [FeFe]-hydrogenase cofactor (H-cluster) is assumed to
follow a reversible heterolytic mechanism, first yielding a proton and a
hydrido-species which again is double-oxidized to release another proton.
Three of the four presumed catalytic intermediates (Hox, Hred/Hred and Hsred)
were characterized, using various spectroscopic techniques. However, in
catalytically active enzyme, the state containing the hydrido-species, which
is eponymous for the proposed heterolytic mechanism, has yet only been
speculated about. We use different strategies to trap and spectroscopically
characterize this transient hydride state (Hhyd) for three wild-type
[FeFe]-hydrogenases. Applying a novel set-up for real-time attenuated total-
reflection Fourier-transform infrared spectroscopy, we monitor compositional
changes in the state-specific infrared signatures of [FeFe]-hydrogenases,
varying buffer pH and gas composition. We selectively enrich the equilibrium
concentration of Hhyd, applying Le Chatelier’s principle by simultaneously
increasing substrate and product concentrations (H2/H+). Site-directed
manipulation, targeting either the proton-transfer pathway or the adt ligand,
significantly enhances Hhyd accumulation independent of pH
Bridging hydride at reduced H-cluster species in [FeFe]-hydrogenases revealed by infrared spectroscopy, isotope editing, and quantum chemistry
[FeFe]-Hydrogenases contain a H2-converting cofactor (H-cluster) in which a canonical [4Fe–4S] cluster is linked to a unique diiron site with three carbon monoxide (CO) and two cyanide (CN–) ligands (e.g., in the oxidized state, Hox). There has been much debate whether reduction and hydrogen binding may result in alternative rotamer structures of the diiron site in a single (Hred) or double (Hsred) reduced H-cluster species. We employed infrared spectro-electrochemistry and site-selective isotope editing to monitor the CO/CN– stretching vibrations in [FeFe]-hydrogenase HYDA1 from Chlamydomonas reinhardtii. Density functional theory calculations yielded vibrational modes of the diatomic ligands for conceivable H-cluster structures. Correlation analysis of experimental and computational IR spectra has facilitated an assignment of Hred and Hsred to structures with a bridging hydride at the diiron site. Pronounced ligand rotation during μH binding seems to exclude Hred and Hsred as catalytic intermediates. Only states with a conservative H-cluster geometry featuring a μCO ligand are likely involved in rapid H2 turnover
Spectroscopical Investigations on the Redox Chemistry of [FeFe]-Hydrogenases in the Presence of Carbon Monoxide
[FeFe]-hydrogenases efficiently catalyzes hydrogen conversion at a unique [4Fe–4S]-[FeFe] cofactor, the so-called H-cluster. The catalytic reaction occurs at the diiron site, while the [4Fe–4S] cluster functions as a redox shuttle. In the oxidized resting state (Hox), the iron ions of the diiron site bind one cyanide (CN−) and carbon monoxide (CO) ligand each and a third carbonyl can be found in the Fe–Fe bridging position (µCO). In the presence of exogenous CO, A fourth CO ligand binds at the diiron site to form the oxidized, CO-inhibited H-cluster (Hox-CO). We investigated the reduced, CO-inhibited H-cluster (Hred´-CO) in this work. The stretching vibrations of the diatomic ligands were monitored by attenuated total reflection Fourier-transform infrared spectroscopy (ATR FTIR). Density functional theory (DFT) at the TPSSh/TZVP level was employed to analyze the cofactor geometry, as well as the redox and protonation state of the H-cluster. Selective 13CO isotope editing, spectro-electrochemistry, and correlation analysis of IR data identified a one-electron reduced, protonated [4Fe–4S] cluster and an apical CN− ligand at the diiron site in Hred´-CO. The reduced, CO-inhibited H-cluster forms independently of the sequence of CO binding and cofactor reduction, which implies that the ligand rearrangement at the diiron site upon CO inhibition is independent of the redox and protonation state of the [4Fe–4S] cluster. The relation of coordination dynamics to cofactor redox and protonation changes in hydrogen conversion catalysis and inhibition is discussed
Protonengekoppelte Reduktion des katalytischen [4Fe-4S]-Zentrums in [FeFe]-Hydrogenasen
In der Natur katalysieren [FeFe]-Hydrogenasen die Abgabe und Aufnahme von
molekularem Wasserstoff (H2) an einem einzigartigen Eisen-Schwefel-Kofaktor.
Das geringe elektrochemische Ãœberpotential in der Wasserstoffabgabe-Reaktion
macht die [FeFe]-Hydrogenasen zu einem hervorragenden Beispiel für effiziente
Biokatalyse. Gegenwärtig sind die molekularen Details des Wasserstoffumsatzes
jedoch noch nicht vollständig verstanden. Daher adressieren wir in dieser
Untersuchung die initiale Reduktion des katalytischen Zentrums der
[FeFe]-Hydrogenasen mittels Infrarotspektroskopie und Elektrochemie und
zeigen, dass der reduzierte Zustand Hred′ durch protonengekoppelten
Elektronentransport gebildet wird. Ladungskompensation bindet das
überschüssige Elektron am [4Fe-4S]-Zentrum und führt zu einer Stabilisierung
der konservativen Konfiguration des [FeFe]-Kofaktors. Die Rolle von Hred′ beim
Wasserstoffumsatz und mögliche Auswirkungen auf den katalytischen Mechanismus
werden diskutiert. Es liegt nahe, dass die Regulation elektronischer
Eigenschaften in der Umgebung von metallischen Kofaktoren die Grundlage für
Multielektronenprozesse bildet
Hydrogen and oxygen trapping at the H-cluster of [FeFe]-hydrogenase revealed by site-selective spectroscopy and QM/MM calculations
[FeFe]-hydrogenases are superior hydrogen conversion catalysts. They bind a
cofactor (H-cluster) comprising a four-iron and a diiron unit with three
carbon monoxide (CO) and two cyanide (CN−) ligands. Hydrogen (H2) and oxygen
(O2) binding at the H-cluster was studied in the C169A variant of
[FeFe]-hydrogenase HYDA1, in comparison to the active oxidized (Hox) and CO-
inhibited (Hox-CO) species in wildtype enzyme. 57Fe labeling of the diiron
site was achieved by in vitro maturation with a synthetic cofactor analogue.
Site-selective X-ray absorption, emission, and nuclear inelastic/forward
scattering methods and infrared spectroscopy were combined with quantum
chemical calculations to determine the molecular and electronic structure and
vibrational dynamics of detected cofactor species. Hox reveals an apical
vacancy at Fed in a [4Fe4S-2Fe]3 − complex with the net spin on Fed whereas
Hox-CO shows an apical CN− at Fed in a [4Fe4S-2Fe(CO)]3 − complex with net
spin sharing among Fep and Fed (proximal or distal iron ions in [2Fe]). At
ambient O2 pressure, a novel H-cluster species (Hox-O2) accumulated in C169A,
assigned to a [4Fe4S-2Fe(O2)]3 − complex with an apical superoxide (O2−)
carrying the net spin bound at Fed. H2 exposure populated the two-electron
reduced Hhyd species in C169A, assigned as a [(H)4Fe4S-2Fe(H)]3 − complex with
the net spin on the reduced cubane, an apical hydride at Fed, and a proton at
a cysteine ligand. Hox-O2 and Hhyd are stabilized by impaired O2– protonation
or proton release after H2 cleavage due to interruption of the proton path
towards and out of the active site
Protonation/reduction dynamics at the [4Fe–4S] cluster of the hydrogen-forming cofactor in [FeFe]-hydrogenases
The [FeFe]-hydrogenases of bacteria and algae are the most efficient hydrogen
conversion catalysts in nature. Their active-site cofactor (H-cluster)
comprises a [4Fe–4S] cluster linked to a unique diiron site that binds three
carbon monoxide (CO) and two cyanide (CN−) ligands. Understanding microbial
hydrogen conversion requires elucidation of the interplay of proton and
electron transfer events at the H-cluster. We performed real-time spectroscopy
on [FeFe]-hydrogenase protein films under controlled variation of atmospheric
gas composition, sample pH, and reductant concentration. Attenuated total
reflection Fourier-transform infrared spectroscopy was used to monitor shifts
of the CO/CN− vibrational bands in response to redox and protonation changes.
Three different [FeFe]-hydrogenases and several protein and cofactor variants
were compared, including element and isotopic exchange studies. A protonated
equivalent (HoxH) of the oxidized state (Hox) was found, which preferentially
accumulated at acidic pH and under reducing conditions. We show that the one-
electron reduced state Hred′ represents an intrinsically protonated species.
Interestingly, the formation of HoxH and Hred′ was independent of the
established proton pathway to the diiron site. Quantum chemical calculations
of the respective CO/CN− infrared band patterns favored a cysteine ligand of
the [4Fe–4S] cluster as the protonation site in HoxH and Hred′. We propose
that proton-coupled electron transfer facilitates reduction of the [4Fe–4S]
cluster and prevents premature formation of a hydride at the catalytic diiron
site. Our findings imply that protonation events both at the [4Fe–4S] cluster
and at the diiron site of the H-cluster are important in the hydrogen
conversion reaction of [FeFe]-hydrogenases