491 research outputs found
[FeFe]-Hydrogenases: recent developments and future perspectives
[FeFe]-Hydrogenases are the most efficient enzymes for catalytic hydrogen turnover. Their H2 production efficiency is hitherto unrivalled. However, functional details of the catalytic machinery and possible modes of application are discussed controversially. The incorporation of synthetically modified cofactors and utilization of semi-artificial enzymes only recently allowed us to shed light on key steps of the catalytic cycle. Herein, we summarize the essential findings regarding the redox chemistry of [FeFe]-hydrogenases and discuss their catalytic hydrogen turnover. We furthermore will give an outlook on potential research activities and exploit the utilization of synthetic cofactor mimics
How high the temperature of a liquid be raised without boiling?
How high the temperature of a liquid be raised beyond its boiling point
without vaporizing (known as the limit of superheat) is an interesting subject
of investigation. A new method of finding the limit of superheat of liquids is
presented here. The superheated liquids are taken in the form of drops
suspended in visco elastic gel. The nucleation is detected acoustically by a
sensitive piezo-electric transducer, coupled to a multi channel scaler and the
nucleation is observed as a funtion of time and with increase of temperature.
The limit of superheat measured by the present method supersedes all other
measurements and theoretical predictions in reaching closest to the critical
temperature and warrants improved theoretical predictions.Comment: 10 pages, 1 fig. Phys, Rev. E. (2000) in pres
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
Superheated Microdrops as Cold Dark Matter Detectors
It is shown that under realistic background considerations, an improvement in
Cold Dark Matter sensitivity of several orders of magnitude is expected from a
detector based on superheated liquid droplets. Such devices are totally
insensitive to minimum ionizing radiation while responsive to nuclear recoils
of energies ~ few keV. They operate on the same principle as the bubble
chamber, but offer unattended, continuous, and safe operation at room
temperature and atmospheric pressure.Comment: 15 pgs, 4 figures include
A Corrected Mixture Law For B/A
A derivation is presented that corrects an expression for the effective acoustic nonlinearity parameter of a mixture of immiscible liquids. The derivation is based upon a mass fraction, rather than volume fraction, formulation
First Dark Matter Limits from a Large-Mass, Low-Background Superheated Droplet Detector
We report on the fabrication aspects and calibration of the first large
active mass ( g) modules of SIMPLE, a search for particle dark matter
using Superheated Droplet Detectors (SDDs). While still limited by the
statistical uncertainty of the small data sample on hand, the first weeks of
operation in the new underground laboratory of Rustrel-Pays d'Apt already
provide a sensitivity to axially-coupled Weakly Interacting Massive Particles
(WIMPs) competitive with leading experiments, confirming SDDs as a convenient,
low-cost alternative for WIMP detection.Comment: Final version, Phys. Rev. Lett. (in press
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 [4Feâ4S] cluster is linked to a unique diiron site with three carbon monoxide (CO) and two cyanide (CNâ) 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/CNâ 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 ÎŒH binding seems to exclude Hred and Hsred as catalytic intermediates. Only states with a conservative H-cluster geometry featuring a ÎŒCO 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 [4Feâ4S]-[FeFe] cofactor, the so-called H-cluster. The catalytic reaction occurs at the diiron site, while the [4Feâ4S] cluster functions as a redox shuttle. In the oxidized resting state (Hox), the iron ions of the diiron site bind one cyanide (CNâ) and carbon monoxide (CO) ligand each and a third carbonyl can be found in the FeâFe bridging position (”CO). 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 (HredÂŽ-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 [4Feâ4S] cluster and an apical CNâ ligand at the diiron site in HredÂŽ-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 [4Feâ4S] 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 Ăberpotential in der Wasserstoffabgabe-Reaktion
macht die [FeFe]-Hydrogenasen zu einem hervorragenden Beispiel fĂŒr effiziente
Biokatalyse. GegenwÀrtig sind die molekularen Details des Wasserstoffumsatzes
jedoch noch nicht vollstÀndig 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 HredâČ durch protonengekoppelten
Elektronentransport gebildet wird. Ladungskompensation bindet das
ĂŒberschĂŒssige Elektron am [4Fe-4S]-Zentrum und fĂŒhrt zu einer Stabilisierung
der konservativen Konfiguration des [FeFe]-Kofaktors. Die Rolle von HredâČ 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 fĂŒr
Multielektronenprozesse bildet
Independent Ion Migration in Suspensions of Strongly Interacting Charged Colloidal Spheres
We report on sytematic measurements of the low frequency conductivity in
aequous supensions of highly charged colloidal spheres. System preparation in a
closed tubing system results in precisely controlled number densities between
1E16/m3 and 1E19/m^3 (packing fractions between 1E-7 and 1E-2) and electrolyte
concentrations between 1E-7 and 1E-3 mol/l. Due to long ranged Coulomb
repulsion some of the systems show a pronounced fluid or crystalline order.
Under deionized conditions we find s to depend linearily on the packing
fraction with no detectable influence of the phase transitions. Further at
constant packing fraction s increases sublinearily with increasing number of
dissociable surface groups N. As a function of c the conductivity shows
pronounced differences depending on the kind of electrolyte used. We propose a
simple yet powerful model based on independent migration of all species present
and additivity of the respective conductivity contributions. It takes account
of small ion macro-ion interactions in terms of an effectivly transported
charge. The model successfully describes our qualitatively complex experimental
observations. It further facilitates quantitative estimates of conductivity
over a wide range of particle and experimental parameters.Comment: 32 pages, 17 figures, 2 tables, Accepted by Physical Review
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