Geometrical influence on the non-biomimetic heterolytic splitting of H2 by bio-inspired [FeFe]-hydrogenase complexes: a rare example of inverted frustrated Lewis pair based reactivity

Despite the high levels of interest in the synthesis of bio-inspired [FeFe]-hydrogenase complexes, H₂ oxidation, which is one specific aspect of hydrogenase enzymatic activity, is not observed for most reported complexes. To attempt H–H bond cleavage, two disubstituted diiron dithiolate complexes in the form of [Fe₂(μ-pdt)L₂(CO)₄] (L: PMe₃, dmpe) have been used to play the non-biomimetic role of a Lewis base, with frustrated Lewis pairs (FLPs) formed in the presence of B(C₆F₅)₃ Lewis acid. These unprecedented FLPs, based on the bimetallic Lewis base partner, allow the heterolytic splitting of the H₂ molecule, forming a protonated diiron cation and hydrido-borate anion. The substitution, symmetrical or asymmetrical, of two phosphine ligands at the diiron dithiolate core induces a strong difference in the H₂ bond cleavage abilities, with the FLP based on the first complex being more efficient than the second. DFT investigations examined the different mechanistic pathways involving each accessible isomer and rationalized the experimental findings. One of the main DFT results highlights that the iron site acting as a Lewis base for the asymmetrical complex is the {Fe(CO)₃} subunit, which is less electron-rich than the {FeL(CO)₂} site of the symmetrical complex, diminishing the reactivity towards H₂. Calculations relating to the different mechanistic pathways revealed the presence of a terminal hydride intermediate at the apical site of a rotated {Fe(CO)₃} site, which is experimentally observed, and a semi-bridging hydride intermediate from H₂ activation at the Fe–Fe site; these are responsible for a favourable back-reaction, reducing the conversion yield observed in the case of the asymmetrical complex. The use of two equivalents of Lewis acid allows for more complete and faster H₂ bond cleavage due to the encapsulation of the hydrido-borate species by a second borane, favouring the reactivity of each FLP, in agreement with DFT calculations.

Bio-inspired [FeFe]-hydrogenase complexes and B(C₆F₅)₃ form FLPs that are able to activate H₂, providing rare examples of inverted enzymatic reactivity. The influence of the symmetry/asymmetry of coordination is studied via DFT.     

Combustion of silane-nitrous oxide-argon mixtures: Analysis of laminar flame propagation and condensed products

The laminar burning rate, the explosion peak pressure, and the pressure rise coefficient have been measured for the first time for silane-nitrous oxide-argon mixtures using the spherically expanding flame technique in a constant volume combustion chamber. For these three parameters, the values obtained were higher than for hydrogen-nitrous oxide-argon and typical hydrocarbon-based mixtures. A maximum burning rate of 1800 g/m² s was measured at 101 kPa, whereas under similar conditions, a maximum burning rate around 950 g/m² s has been reported for hydrogen-nitrous oxide-argon mixtures. To better understand the chemical dynamics of flames propagating in SiH₄–N₂O–Ar mixtures, a detailed reaction model from the literature was improved using collision limit violation analysis and updated thermodynamic properties calculated with a high-level ab initio approach. The reaction model predicts the burning rate within 14% on average but demonstrates error close to 50% for the richest mixtures. The chemistry of the H–O–N system is important under all the conditions presently studied. The chemistry of the Si–H–O–N system demonstrates an increasing importance under rich conditions. In particular, the reactions (i) forming SiO_x(s); (ii) describing the interaction of Si-species with N₂O; and (iii) involving silicon hydrides, have an important role for the heat release dynamics. The condensed combustion products formed in the silane-nitrous oxide-argon flames were sampled and characterized using electron micrograph, electronic diffraction, energy-dispersive spectroscopy, and X-ray powder diffraction. For all equivalence ratios, silica spherical particles with a mean diameter in the range 200–300 nm were observed. In addition, for mixtures with Φ ≥ 2.2, silicon nanowires were formed. X-ray diffraction experiments showed that the silicon nanowires are composed of metal silicon characterized by a cubic structure (lattice parameter: a=5.425Å) with the Fm-3m space group.     

Electronic sorting and recovery of single live cells from microlitre sized samples

Sorting and recovering specific live cells from samples containing less than a few thousand cells have become major hurdles in rare cell exploration such as stem cell research, cell therapy and cell based diagnostics. We describe here a new technology based on a microelectronic chip integrating an array of over 100,000 independent electrodes and sensors which allow individual and parallel single cell manipulation of up to 10,000 cells while maintaining viability and proliferation capabilities. Manipulation is carried out using dynamic dielectrophoretic traps controlled by an electronic interface. We also demonstrate the capabilities of the chip by sorting and recovering individual live fluorescent cells from an unlabeled population.     

Quantitative determination of cetirizine enantiomers in guinea pig plasma, brain tissue and microdialysis samples using liquid chromatography/tandem mass spectrometry

Sensitive enantioselective liquid chromatographic assays using tandem mass spectrometric detection were developed and validated for the determination of S-cetirizine (S-CZE) and R-cetirizine (R-CZE) in guinea pig plasma, brain tissue, and microdialysis samples. Enantioselective separation was achieved on an alpha1-acid glycoprotein column within 14 min for all methods. A cetirizine analog, ucb 20028, was used as internal standard. Cetirizine and the internal standard were detected by multiple reaction monitoring using transitions m/z 389.1 --> 200.9 and 396.1 --> 276.1, respectively. The samples were prepared using protein precipitation with acetonitrile. For guinea pig plasma, the assay was linear over the range 0.25-5000 ng/mL for both S-CZE and R-CZE, with a lower limit of quantification (LLOQ) of 0.25 ng/mL. For the brain tissue and microdialysis samples, the assays were linear over the range 2.5-250 ng/g and 0.25-50 ng/mL, respectively, and the LLOQ values were 2.5 ng/g and 0.25 ng/mL, respectively. The intra- and inter-day precision values were < or =7.1% and < or =12.6%, respectively, and the intra- and inter-day accuracy varied by less than +/-8.0% and +/-6.0% of the nominal value, respectively, for both enantiomers in all the matrices investigated.     

Photostabilization of butyl methoxydibenzoylmethane (Avobenzone) and ethylhexyl methoxycinnamate by bis-ethylhexyloxyphenol methoxyphenyl triazine (Tinosorb S), a new UV broadband filter

It is now well documented that chronic UVA exposure induces damage to human skin. Therefore, modern sunscreens should not only provide protection from both UVB and UVA radiation but also maintain this protection during the entire period of exposure to the sun. UVA filters, however, are rare and not sufficiently photostable. We investigated the effect of the introduction of a new UV filter, bis-ethylhexyloxyphenol methoxyphenyl triazine (Tinosorb S), in oil in water sunscreen formulations on the photostability of butyl methoxydibenzoylmethane (Avobenzone [AVB]) after irradiation with an optically filtered Xenon arc source (UV irradiance adjusted at 1 mean effective dose [MED]/min). With spectrophotometrical methods to assess the sun protection factor (SPF) and UVA ratio and chromatographical methods to determine the amount of UV filters recovered after irradiation we showed that Tinosorb S prevented the photodegradation of AVB in a concentration-dependent way, leading to a sustained SPF and UVA ratio even after irradiation with doses of up to 30 MED. Since AVB was shown to destabilize ethylhexyl methoxycinnamate (EHM) we tested the effect of Tinosorb S in sunscreens containing this UV filter combination. Here too Tinosorb S showed photoprotective properties toward both UV filters. Thus, Tinosorb S can be used successfully to improve the photostability and efficiency of sunscreens containing AVB and EHM.     

Multiresolution stochastic simulations of reaction-diffusion processes

Stochastic simulations of reaction-diffusion processes are used extensively for the modeling of complex systems in areas ranging from biology and social sciences to ecosystems and materials processing. These processes often exhibit disparate scales that render their simulation prohibitive even for massive computational resources. The problem is resolved by introducing a novel stochastic multiresolution method that enables the efficient simulation of reaction-diffusion processes as modeled by many-particle systems. The proposed method quantifies and efficiently handles the associated stiffness in simulating the system dynamics and its computational efficiency and accuracy are demonstrated in simulations of a model problem described by the Fisher-Kolmogorov equation. The method is general and can be applied to other many-particle models of physical processes.     

An immersed lifting and dragging line model for the vortex particle-mesh method

Theoretical and Computational Fluid Dynamics - The numerical study of the wake of full-scale slender devices such as aircraft wings and wind turbine blades requires high-fidelity large eddy...