817 research outputs found
Versuch einer methodologischen Annäherung
Elektronische Version der gedr. Ausg. 199
Emigration and settlement patterns of German communities in North America
The liberal-national movement of the 1830s and 1840s attempted to concentrate
the mass emigration in colonies so to preserve "Germandorn" abroad. Many
settlement projects were made, only few executed. Largest among them those of
the Gießen-Society and the Texas Association.Elektronische Version der gedr. Ausg. 199
Management of severe intraoperative hemorrhage during intraventricular neuroendoscopic procedures: the dry field technique
Objective Neuroendoscopic procedures inside the ventricular system always bear the risk for an unexpected intraoperative
hemorrhage with potentially devastating consequences. The authors present here their experience, and a stage-to-stage guide
for the endoscopic management of intraoperative hemorrhages.
Methods A step-by-step guide for the management to gain control of and stop the bleeding is described including a grading
system. More advanced techniques are presented in cases examples.
Conclusion Most of intraoperative hemorrhages can be controlled by constant irrigation and coagulation. More advanced
techniques can be applied quickly and easily to ensure control of the hemorrhages and avoid the need for a microsurgical
conversion
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
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
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
Study of High Energy Heavy-Ion Collisions in a Relativistic BUU-Approach with Momentum-Dependent Mean-Fields
We introduce momentum-dependent scalar and vector fields into the Lorentz
covariant relativistic BUU- (RBUU-) approach employing a polynomial ansatz for
the relativistic nucleon-nucleon interaction. The momentum-dependent
parametrizations are shown to be valid up to about 1 GeV/u for the empirical
proton-nucleus optical potential. We perform numerical simulations for
heavy-ion collisions within the RBUU-approach adopting momentum-dependent and
momentum-independent mean-fields and calculate the transverse flow in and
perpendicular to the reaction plane, the directivity distribution as well as
subthreshold K+-production. By means of these observables we discuss the
particular role of the momentum-dependent forces and their implications on the
nuclear equation of state. We find that only a momentum-dependent parameter-set
can explain the experimental data on the transverse flow in the reaction plane
from 150 - 1000 MeV/u and the differential K+-production cross sections at 1
GeV/u at the same time.Comment: 27 pages, figures can be obtained from the authors, UGI-93-0
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