DFT Virtual Screening Identifies Rhodium–Amidinate Complexes As Potential Homogeneous Catalysts for Methane-to-Methanol Oxidation

In the search for new organometallic catalysts for low-temperature selective conversion of CH_4 to CH_3OH, we apply quantum mechanical virtual screening to select the optimum combination of ligand and solvent on rhodium to achieve low barriers for CH_4 activation and functionalization to recommend for experimental validation. Here, we considered Rh because its lower electronegativity compared with Pt and Pd may allow it to avoid poisoning by coordinating media. We report quantum mechanical predictions (including implicit and explicit solvation) of the mechanisms for Rh^(III)(NN) and Rh^(III)(NN^F) complexes [where (NN) = bis(N-phenyl)benzylamidinate and (NN^F) = bis(N-pentafluorophenyl)pentafluorobenzylamidinate] to catalytically activate and functionalize methane using trifluoroacetic acid (TFAH) or water as a solvent. In particular, we designed the (NN^F) ligand as a more electrophilic analogue to the (NN) ligand, and our results predict the lowest transition state barrier (ΔG‡ = 27.6 kcal/mol) for methane activation in TFAH from a pool of four different classes of ligands. To close the catalytic cycle, the functionalization of methylrhodium intermediates was also investigated, involving carbon–oxygen bond formation via S_N2 attack by solvent, or S_R2 attack by a vanadium oxo. Activation barriers for the functionalization of methylrhodium intermediates via nucleophilic attack are lower when the solvent is water, but CH_4 activation barriers are higher. In addition, we have found a correlation between CH_4 activation barriers and rhodium–methyl bond energies that allow us to predict the activation transition state energies for future ligands, as well

Arene C–H activation using Rh(I) catalysts supported by bidentate nitrogen chelates

The Rh(I) complexes [(^(Fl)DAB)Rh(coe)(TFA)] (1) and [(BOZO)Rh(coe)(TFA)] (2) [^(Fl)DAB = N,N-bis-(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene, coe = cyclooctene, TFA = trifluoroacetate, BOZO = bis(2-oxazolin-2-yl)] are efficient catalyst precursors for H/D exchange between arenes and DTFA. Catalyst precursor 1 exhibits a TOF of 0.06 s^(−1) at 150 °C for benzene H/D exchange. DFT calculations revealed that H/D exchange through reversible oxidative addition or internal electrophilic substitution of benzene is a viable pathway

Functional expression and characterization of five wax ester synthases in Saccharomyces cerevisiae and their utility for biodiesel production

<p>Abstract</p> <p>Background</p> <p>Wax ester synthases (WSs) can synthesize wax esters from alcohols and fatty acyl coenzyme A thioesters. The knowledge of the preferred substrates for each WS allows the use of yeast cells for the production of wax esters that are high-value materials and can be used in a variety of industrial applications. The products of WSs include fatty acid ethyl esters, which can be directly used as biodiesel.</p> <p>Results</p> <p>Here, heterologous WSs derived from five different organisms were successfully expressed and evaluated for their substrate preference in <it>Saccharomyces cerevisiae</it>. We investigated the potential of the different WSs for biodiesel (that is, fatty acid ethyl esters) production in <it>S. cerevisiae</it>. All investigated WSs, from <it>Acinetobacter baylyi </it>ADP1, <it>Marinobacter hydrocarbonoclasticus </it>DSM 8798, <it>Rhodococcus opacus </it>PD630, <it>Mus musculus </it>C57BL/6 and <it>Psychrobacter arcticus </it>273-4, have different substrate specificities, but they can all lead to the formation of biodiesel. The best biodiesel producing strain was found to be the one expressing WS from <it>M. hydrocarbonoclasticus </it>DSM 8798 that resulted in a biodiesel titer of 6.3 mg/L. To further enhance biodiesel production, acetyl coenzyme A carboxylase was up-regulated, which resulted in a 30% increase in biodiesel production.</p> <p>Conclusions</p> <p>Five WSs from different species were functionally expressed and their substrate preference characterized in <it>S. cerevisiae</it>, thus constructing cell factories for the production of specific kinds of wax ester. WS from <it>M. hydrocarbonoclasticus </it>showed the highest preference for ethanol compared to the other WSs, and could permit the engineered <it>S. cerevisiae </it>to produce biodiesel.</p

COSPAR Sample Safety Assessment Framework (SSAF).

The Committee on Space Research (COSPAR) Sample Safety Assessment Framework (SSAF) has been developed by a COSPAR appointed Working Group. The objective of the sample safety assessment would be to evaluate whether samples returned from Mars could be harmful for Earth's systems (e.g., environment, biosphere, geochemical cycles). During the Working Group's deliberations, it became clear that a comprehensive assessment to predict the effects of introducing life in new environments or ecologies is difficult and practically impossible, even for terrestrial life and certainly more so for unknown extraterrestrial life. To manage expectations, the scope of the SSAF was adjusted to evaluate only whether the presence of martian life can be excluded in samples returned from Mars. If the presence of martian life cannot be excluded, a Hold & Critical Review must be established to evaluate the risk management measures and decide on the next steps. The SSAF starts from a positive hypothesis (there is martian life in the samples), which is complementary to the null-hypothesis (there is no martian life in the samples) typically used for science. Testing the positive hypothesis includes four elements: (1) Bayesian statistics, (2) subsampling strategy, (3) test sequence, and (4) decision criteria. The test sequence capability covers self-replicating and non-self-replicating biology and biologically active molecules. Most of the investigations associated with the SSAF would need to be carried out within biological containment. The SSAF is described in sufficient detail to support planning activities for a Sample Receiving Facility (SRF) and for preparing science announcements, while at the same time acknowledging that further work is required before a detailed Sample Safety Assessment Protocol (SSAP) can be developed. The three major open issues to be addressed to optimize and implement the SSAF are (1) setting a value for the level of assurance to effectively exclude the presence of martian life in the samples, (2) carrying out an analogue test program, and (3) acquiring relevant contamination knowledge from all Mars Sample Return (MSR) flight and ground elements. Although the SSAF was developed specifically for assessing samples from Mars in the context of the currently planned NASA-ESA MSR Campaign, this framework and the basic safety approach are applicable to any other Mars sample return mission concept, with minor adjustments in the execution part related to the specific nature of the samples to be returned. The SSAF is also considered a sound basis for other COSPAR Planetary Protection Category V, restricted Earth return missions beyond Mars. It is anticipated that the SSAF will be subject to future review by the various MSR stakeholders

A model for improving microbial biofuel production using a synthetic feedback loop

Cells use feedback to implement a diverse range of regulatory functions. Building synthetic feedback control systems may yield insight into the roles that feedback can play in regulation since it can be introduced independently of native regulation, and alternative control architectures can be compared. We propose a model for microbial biofuel production where a synthetic control system is used to increase cell viability and biofuel yields. Although microbes can be engineered to produce biofuels, the fuels are often toxic to cell growth, creating a negative feedback loop that limits biofuel production. These toxic effects may be mitigated by expressing efflux pumps that export biofuel from the cell. We developed a model for cell growth and biofuel production and used it to compare several genetic control strategies for their ability to improve biofuel yields. We show that controlling efflux pump expression directly with a biofuel-responsive promoter is a straightforward way of improving biofuel production. In addition, a feed forward loop controller is shown to be versatile at dealing with uncertainty in biofuel production rates

Development and implementation of rapid metabolic engineering tools for chemical and fuel production in Geobacillus thermoglucosidasius NCIMB 11955

Background The thermophile Geobacillus thermoglucosidasius has considerable attraction as a chassis for the production of chemicals and fuels. It utilises a wide range of sugars and oligosaccharides typical of those derived from lignocellulose and grows at elevated temperatures. The latter improves the rate of feed conversion, reduces fermentation cooling costs and minimises the risks of contamination. Full exploitation of its potential has been hindered by a dearth of effective gene tools. Results Here we designed and tested a collection of vectors (pMTL60000 series) in G. thermoglucosidasius NCIMB 11955 equivalent to the widely used clostridial pMTL80000 modular plasmid series. By combining a temperature-sensitive replicon and a heterologous pyrE gene from Geobacillus kaustophilus as a counter-selection marker, a highly effective and rapid gene knock-out/knock-in system was established. Its use required the initial creation of uracil auxotroph through deletion of pyrE using allele-coupled exchange (ACE) and selection for resistance to 5-fluoroorotic acid. The turnaround time for the construction of further mutants in this pyrE minus strain was typically 5 days. Following the creation of the desired mutant, the pyrE allele was restored to wild type, within 3 days, using ACE and selection for uracil prototrophy. Concomitant with this process, cargo DNA (pheB) could be readily integrated at the pyrE locus. The system’s utility was demonstrated through the generation in just 30 days of three independently engineered strains equivalent to a previously constructed ethanol production strain, TM242. This involved the creation of two in-frame deletions (ldh and pfl) and the replacement of a promoter region of a third gene (pdh) with an up-regulated variant. In no case did the production of ethanol match that of TM242. Genome sequencing of the parental strain, TM242, and constructed mutant derivatives suggested that NCIMB 11955 is prone to the emergence of random mutations which can dramatically affect phenotype. Conclusions The procedures and principles developed for clostridia, based on the use of pyrE alleles and ACE, may be readily deployed in G. thermoglucosidasius. Marker-less, in-frame deletion mutants can be rapidly generated in 5 days. However, ancillary mutations frequently arise, which can influence phenotype. This observation emphasises the need for improved screening and selection procedures at each step of the engineering processes, based on the generation of multiple, independent strains and whole-genome sequencing

Future research prioritization in cardiac resynchronization therapy

Background: Although cardiac resynchronization therapy (CRT) is effective for some patients with heart failure and a reduced left ventricular ejection fraction (HFrEF), evidence gaps remain for key clinical and policy areas. The objective of the study was to review the data on the effects of CRT for patients with HFrEF receiving pharmacological therapy alone or pharmacological therapy and an implantable cardioverter-defibrillator (ICD) and then, informed by a diverse group of stakeholders, to identify evidence gaps, prioritize them, and develop a research plan. Methods: Relevant studies were identified using PubMed and EMBASE and ongoing trials using clinicaltrials.gov. Forced-ranking prioritization method was applied by stakeholders to reach a consensus on the most important questions. Twenty-six stakeholders contributed to the expanded list of evidence gaps, including key investigators from existing randomized controlled trials and others representing different perspectives, including patients, the public, device manufacturers, and policymakers. Results: Of the 18 top-tier evidence gaps, 8 were related to specific populations or subgroups of interest. Seven were related to the comparative effectiveness and safety of CRT interventions or comparators, and 3 were related to the association of CRT treatment with specific outcomes. The association of comorbidities with CRT effectiveness ranked highest, followed by questions about the effectiveness of CRT among patients with atrial fibrillation and the relationship between gender, QRS morphology and duration, and outcomes for patients either with CRT plus ICD or with ICD. Conclusions: Evidence gaps presented in this article highlight numerous, important clinical and policy questions for which there is inconclusive evidence on the role of CRT and provide a framework for future collaborative research