21 research outputs found
A one-pot, water compatible synthesis of pyrimidine nucleobases under plausible prebiotic conditions
Herein, we report a new prebiotically plausible pathway towards a pyrimidine nucleobase in continuous manner. The route involves simultaneous methylation and carbamoylation of cyanoacetylene-derived alpha,beta-unsaturated thioamide with N-methyl-N-nitrosourea (MNU) in aqueous media. This provides S-methylpyrimidinone in one-pot, which can be converted into a variety of 4-substituted pyrimidine nucleobases including cytosine and uracil
Retrieval of vegetative fluid resistance terms for rigid stems using airborne lidar.
Hydraulic resistance of riparian forests is an unknown but important term in flood conveyance modeling. Lidar has proven to be a very important new data source to physically characterize floodplain vegetation. This research outlines a recent campaign that aims to retrieve vegetation fluid resistance terms from airborne laser scanning to parameterize trunk roughness. Information on crown characteristics and vegetation spacing can be extracted for individual trees to aid in the determining of trunk stem morphology. Airborne lidar data were used to explore the potential to characterize some of the prominent tree morphometric properties from natural and planted riparian poplar zones such as tree position, tree height, trunk location, and tree spacing. Allometric equations of tree characteristics extrapolated from ground measurements were used to infer below-canopy morphometric variables. Results are presented from six riparian-forested zones on the Garonne and Allier rivers in southern and central France. The tree detection and crown segmentation (TDCS) method identified individual trees with 85% accuracy, and the TreeVaW method detected trees with 83% accuracy. Tree heights were overall estimated at both river locations with an RMSE error of around 19% for both methods, but crown diameter at the six sites produced large deviations from ground-measured values of above 40% for both methods. Total height-derived trunk diameters using the TDCS method produced the closest roughness coefficient values to the ground-derived roughness coefficients. The stem roughness values produced from this method fell within guideline values
Situational factors shape moral judgements in the trolley dilemma in Eastern, Southern and Western countries in a culturally diverse sample
Electron Paramagnetic Resonance Characterization of the Triheme Cytochrome from <i>Geobacter sulfurreducens</i>
Periplasmic cytochrome
A (PpcA) is a representative of a broad
class of multiheme cytochromes functioning as protein “nanowires”
for storage and extracellular transfer of multiple electrons in the
δ-proteobacterium <i>Geobacter sulfurreducens</i>.
PpcA contains three bis-His coordinated hemes held in a spatial arrangement
that is highly conserved among the multiheme cytochromes c<sub>3</sub> and c<sub>7</sub> families, carries low potential hemes, and is
notable for having one of the lowest number of amino acids utilized
to maintain a characteristic protein fold and site-specific heme function.
Low temperature X-band electron paramagnetic resonance (EPR) spectroscopy
has been used to characterize the electronic configuration of the
Fe(III) and the ligation mode for each heme. The three sets of EPR
signals are assigned to individual hemes in the three-dimensional
crystal structure. The relative energy levels of the Fe(III) 3d orbitals
for individual hemes were estimated from the principal <i>g</i>-values. The observed <i>g</i>-tensor anisotropy was used
as a probe of electronic structure of each heme, and differences were
determined by specifics of axial ligation. To ensure unambiguous assignment
of highly anisotropic low-spin (HALS) signal to individual hemes,
EPR analyses of iron atom electronic configurations have been supplemented
with investigation of porphyrin macrocycles by one-dimensional <sup>1</sup>H NMR chemical shift patterns for the methyl substituents.
Within optimized geometry of hemes in PpcA, the magnetic interactions
between hemes were found to be minimal, similar to the c<sub>3</sub> family of tetraheme cytochromes
Artificial Hydrogenases Based on Cobaloximes and Heme Oxygenase
International audienceThe insertion of cobaloxime catalysts in the heme-binding pocket of heme oxygenase (HO) yields artificial hydrogenases active for H-2 evolution in neutral aqueous solutions. These novel biohybrids have been purified and characterized by using UV/visible and EPR spectroscopy. These analyses revealed the presence of two distinct binding conformations, thereby providing the cobaloxime with hydrophobic and hydrophilic environments, respectively. Quantum chemical/molecular mechanical docking calculations found open and closed conformations of the binding pocket owing to mobile amino acid residues. HO-based biohybrids incorporating a {Co(dmgH)(2)} (dmgH(2)=dimethylglyoxime) catalytic center displayed up to threefold increased turnover numbers with respect to the cobaloxime alone or to analogous sperm whale myoglobin adducts. This study thus provides a strong basis for further improvement of such biohybrids, using well-designed modifications of the second and outer coordination spheres, through site-directed mutagenesis of the host protein
Protein Delivery of a Ni Catalyst to Photosystem I for Light-Driven Hydrogen Production
The
direct conversion of sunlight into fuel is a promising means
for the production of storable renewable energy. Herein, we use Nature’s
specialized photosynthetic machinery found in the Photosystem I (PSI)
protein to drive solar fuel production from a nickel diphosphine molecular
catalyst. Upon exposure to visible light, a self-assembled PSI-[Ni(P<sub>2</sub><sup>Ph</sup>N<sub>2</sub><sup>Ph</sup>)<sub>2</sub>](BF<sub>4</sub>)<sub>2</sub> hybrid generates H<sub>2</sub> at a rate 2 orders
of magnitude greater than rates reported for photosensitizer/[Ni(P<sub>2</sub><sup>Ph</sup>N<sub>2</sub><sup>Ph</sup>)<sub>2</sub>](BF<sub>4</sub>)<sub>2</sub> systems. The protein environment enables photocatalysis
at pH 6.3 in completely aqueous conditions. In addition, we have developed
a strategy for incorporating the Ni molecular catalyst with the native
acceptor protein of PSI, flavodoxin. Photocatalysis experiments with
this modified flavodoxin demonstrate a new mechanism for biohybrid
creation that involves protein-directed delivery of a molecular catalyst
to the reducing side of Photosystem I for light-driven catalysis.
This work further establishes strategies for constructing functional,
inexpensive, earth-abundant solar fuel-producing PSI hybrids that
use light to rapidly produce hydrogen directly from water
The Hydrogen Catalyst Cobaloxime: A Multifrequency EPR and DFT Study of Cobaloxime’s Electronic Structure
Solar fuels research aims to mimic photosynthesis and
devise integrated
systems that can capture, convert, and store solar energy in the form
of high-energy molecular bonds. Molecular hydrogen is generally considered
an ideal solar fuel because its combustion is essentially pollution-free.
Cobaloximes rank among the most promising earth-abundant catalysts
for the reduction of protons to molecular hydrogen. We have used multifrequency
EPR spectroscopy at X-band, Q-band, and D-band combined with DFT calculations
to reveal electronic structure and establish correlations among the
structure, surroundings, and catalytic activity of these complexes.
To assess the strength and nature of ligand cobalt interactions, the
BF<sub>2</sub>-capped cobaloxime, Co(dmgBF<sub>2</sub>)<sub>2</sub>, was studied in a variety of different solvents with a range of
polarities and stoichiometric amounts of potential ligands to the
cobalt ion. This allows the differentiation of labile and strongly
coordinating axial ligands for the Co(II) complex. Labile, or weakly
coordinating, ligands such as methanol result in larger <i>g</i>-tensor anisotropy than strongly coordinating ligands such as pyridine.
In addition, a coordination number effect is seen for the strongly
coordinating ligands with both singly ligated LCo(dmgBF<sub>2</sub>)<sub>2</sub> and doubly ligated L<sub>2</sub>Co(dmgBF<sub>2</sub>)<sub>2</sub> . The presence of two strongly coordinating axial ligands
leads to the smallest <i>g</i>-tensor anisotropy. The relevance
of the strength of the axial ligand(s) to the catalytic efficiency
of Co(dmgBF<sub>2</sub>)<sub>2</sub> is discussed. Finally, the influence
of molecular oxygen and formation of Co(III) superoxide radicals LCo(dmgBF<sub>2</sub>)<sub>2</sub>O<sub>2</sub><sup>•</sup> is studied.
The experimental results are compared with a comprehensive set of
DFT calculations on Co(dmgBF<sub>2</sub>)<sub>2</sub> model systems
with various axial ligands. Comparison with experimental values for
the “key” magnetic parameters such as <i>g</i>-tensor and <sup>59</sup>Co hyperfine coupling tensor allows the
determination of the conformation of the axially ligated Co(dmgBF<sub>2</sub>)<sub>2</sub> complexes. The data presented here are vital
for understanding the influence of solvent and ligand coordination
on the catalytic efficiency of cobaloximes
The Hydrogen Catalyst Cobaloxime: A Multifrequency EPR and DFT Study of Cobaloxime’s Electronic Structure
Solar fuels research aims to mimic photosynthesis and
devise integrated
systems that can capture, convert, and store solar energy in the form
of high-energy molecular bonds. Molecular hydrogen is generally considered
an ideal solar fuel because its combustion is essentially pollution-free.
Cobaloximes rank among the most promising earth-abundant catalysts
for the reduction of protons to molecular hydrogen. We have used multifrequency
EPR spectroscopy at X-band, Q-band, and D-band combined with DFT calculations
to reveal electronic structure and establish correlations among the
structure, surroundings, and catalytic activity of these complexes.
To assess the strength and nature of ligand cobalt interactions, the
BF<sub>2</sub>-capped cobaloxime, Co(dmgBF<sub>2</sub>)<sub>2</sub>, was studied in a variety of different solvents with a range of
polarities and stoichiometric amounts of potential ligands to the
cobalt ion. This allows the differentiation of labile and strongly
coordinating axial ligands for the Co(II) complex. Labile, or weakly
coordinating, ligands such as methanol result in larger <i>g</i>-tensor anisotropy than strongly coordinating ligands such as pyridine.
In addition, a coordination number effect is seen for the strongly
coordinating ligands with both singly ligated LCo(dmgBF<sub>2</sub>)<sub>2</sub> and doubly ligated L<sub>2</sub>Co(dmgBF<sub>2</sub>)<sub>2</sub> . The presence of two strongly coordinating axial ligands
leads to the smallest <i>g</i>-tensor anisotropy. The relevance
of the strength of the axial ligand(s) to the catalytic efficiency
of Co(dmgBF<sub>2</sub>)<sub>2</sub> is discussed. Finally, the influence
of molecular oxygen and formation of Co(III) superoxide radicals LCo(dmgBF<sub>2</sub>)<sub>2</sub>O<sub>2</sub><sup>•</sup> is studied.
The experimental results are compared with a comprehensive set of
DFT calculations on Co(dmgBF<sub>2</sub>)<sub>2</sub> model systems
with various axial ligands. Comparison with experimental values for
the “key” magnetic parameters such as <i>g</i>-tensor and <sup>59</sup>Co hyperfine coupling tensor allows the
determination of the conformation of the axially ligated Co(dmgBF<sub>2</sub>)<sub>2</sub> complexes. The data presented here are vital
for understanding the influence of solvent and ligand coordination
on the catalytic efficiency of cobaloximes
Cobaloxime-based artificial hydrogenases.
International audienceCobaloximes are popular H2 evolution molecular catalysts but have so far mainly been studied in nonaqueous conditions. We show here that they are also valuable for the design of artificial hydrogenases for application in neutral aqueous solutions and report on the preparation of two well-defined biohybrid species via the binding of two cobaloxime moieties, {Co(dmgH)2} and {Co(dmgBF2)2} (dmgH2 = dimethylglyoxime), to apo Sperm-whale myoglobin (SwMb). All spectroscopic data confirm that the cobaloxime moieties are inserted within the binding pocket of the SwMb protein and are coordinated to a histidine residue in the axial position of the cobalt complex, resulting in thermodynamically stable complexes. Quantum chemical/molecular mechanical docking calculations indicated a coordination preference for His93 over the other histidine residue (His64) present in the vicinity. Interestingly, the redox activity of the cobalt centers is retained in both biohybrids, which provides them with the catalytic activity for H2 evolution in near-neutral aqueous conditions
Cobaloxime-Based Artificial Hydrogenases
Cobaloximes are popular H<sub>2</sub> evolution molecular catalysts but have so far mainly been studied
in nonaqueous conditions. We show here that they are also valuable
for the design of artificial hydrogenases for application in neutral
aqueous solutions and report on the preparation of two well-defined
biohybrid species via the binding of two cobaloxime moieties, {Co(dmgH)<sub>2</sub>} and {Co(dmgBF<sub>2</sub>)<sub>2</sub>} (dmgH<sub>2</sub> = dimethylglyoxime), to apo <i>Sperm-whale</i> myoglobin
(<i>Sw</i>Mb). All spectroscopic data confirm that the cobaloxime
moieties are inserted within the binding pocket of the <i>Sw</i>Mb protein and are coordinated to a histidine residue in the axial
position of the cobalt complex, resulting in thermodynamically stable
complexes. Quantum chemical/molecular mechanical docking calculations
indicated a coordination preference for His93 over the other histidine
residue (His64) present in the vicinity. Interestingly, the redox
activity of the cobalt centers is retained in both biohybrids, which
provides them with the catalytic activity for H<sub>2</sub> evolution
in near-neutral aqueous conditions