Relative Rates Of Formation Research Articles

Local stellar formation history from the 40 pc white dwarf sample

ABSTRACT We derive the local stellar formation history from the Gaia-defined 40 pc white dwarf sample. This is currently the largest volume-complete sample of white dwarfs for which spectroscopy is available, allowing for classification of the chemical abundances at the photosphere, and subsequently accurate determination of the atmospheric parameters. We create a population synthesis model and show that a uniform stellar formation history for the last ≈10.5 Gyr provides a satisfactory fit to the observed distribution of absolute Gaia G magnitudes. To test the robustness of our derivation, we vary various assumptions in the population synthesis model, including the initial mass function, initial-to-final mass relation, kinematic evolution, binary fraction, and white dwarf cooling time-scales. From these tests, we conclude that the assumptions in our model have an insignificant effect on the derived relative stellar formation rate as a function of look-back time. However, the onset of stellar formation (age of Galactic disc) is sensitive to a variety of input parameters including the white dwarf cooling models. Our derived stellar formation history gives a much better fit to the absolute Gaia G magnitudes than most previous studies.

The mechanism of cleavage of Evans chiral auxiliaries by LiOOH: origins of the different regioselectivities obtained with LiOH, LiOOH, LiOBn and LiSBn

Oxazolidinones are widely used as chiral auxiliaries in asymmetric synthesis. The chiral auxiliary can be selectively removed using LiOOH, LiOBn or LiSBn, but not LiOH, which instead favours endocyclic cleavage with opening of the oxazolidinone ring. Although Evans originally suggested that the regioselectivity of HO−-mediated hydrolysis may be governed by the relative rates of formation of the two tetrahedral intermediates, whereas the regioselectivity of HOO− cleavage might be determined by their relative breakdown rates, it was not clear why this should be the case. Since then, there appear to have been no mechanistic studies that account for the differing selectivities of LiOH v. LiOOH, and the reason for this difference in selectivity has remained somewhat of an enigma in organic chemistry for many years. Herein, we report DFT computations with M06-2X-D3//B3LYP-D3 to examine the origins of the regioselectivities. The computations predict that all four nucleophiles prefer to attack at the endocyclic carbonyl group, which is less hindered. The barrier for decomposition of the initially formed tetrahedral intermediate determines whether or not the reaction continues all the way to the endocyclic cleavage products. For LiOH, the barrier for decomposition of the intermediate is small and the endocyclic cleavage products are formed. For LiOOH, LiOBn and LiSBn, the decomposition barrier is large, and the exocyclic cleavage pathway instead becomes preferred. An alternative mechanism involving formation of a novel cyclic peroxide intermediate was found to involve a relatively high energy pathway and has been excluded.

Quantum interference effects elucidate triplet-pair formation dynamics in intramolecular singlet-fission molecules.

Quantum interference (QI)-the constructive or destructive interference of conduction pathways through molecular orbitals-plays a fundamental role in enhancing or suppressing charge and spin transport in organic molecular electronics. Graphical models were developed to predict constructive versus destructive interference in polyaromatic hydrocarbons and have successfully estimated the large conductivity differences observed in single-molecule transport measurements. A major challenge lies in extending these models to excitonic (photoexcited) processes, which typically involve distinct orbitals with different symmetries. Here we investigate how QI models can be applied as bridging moieties in intramolecular singlet-fission compounds to predict relative rates of triplet pair formation. In a series of bridged intramolecular singlet-fission dimers, we found that destructive QI always leads to a slower triplet pair formation across different bridge lengths and geometries. A combined experimental and theoretical approach reveals the critical considerations of bridge topology and frontier molecular orbital energies in applying QI conductance principles to predict rates of multiexciton generation.

Carbonylative Co- and Terpolymerizations of 10-Undecen-1-ol: A Route to Polyketoesters with Tunable Compositions

A strategy to synthesize branched polyketoesters from the carbonylative polymerization of bifunctional α,ω-alkenols such as 10-undecen-1-ol is presented. This strategy hinges on the competitive application of two related catalytic manifolds, alternating alkene/CO copolymerization, and alkene hydroesterification, which share a common metal acyl intermediate. Small molecule model studies of cationic Pd-catalyzed alkene carbonylation in the presence of alcohols demonstrate that the relative rates of ketone formation (through alternating alkene/CO insertion) and ester formation (through metal acyl alcoholysis) can be tuned across a wide range through judicious bis(phosphine) ligand design. Carbonylative polymerization of 10-undecen-1-ol with a (dppp(3,5-CF3)4)Pd(OTs)2 catalyst (dppp(3,5-CF3)4 = 1,3-bis[bis[3,5-bis(trifluoromethyl)phenyl]-phosphino]propane) led to the formation of high molecular weight polyketoesters with intermediate dispersity (Mn > 20,000 g/mol, D̵ = 2.6) and a ketone/ester microstructure ratio of approximately 1:2. In these polymerization reactions, deploying electron-deficient bis(phosphines) to suppress deleterious alkene isomerization was the key to accessing the high molecular weight polymer. Further, terpolymerization reactions of 1-hexene/10-undecen-1-ol/CO or 1-fluoro-10-undecene/10-undecen-1-ol/CO by (dppp(3,5-CF3)4)Pd(OTs)2 were also successful. This proof of concept polymerization unlocks access to tunable polymer microstructures without extensive postpolymerization treatment.

Stellar triples as a source for Ba stars

ABSTRACT Barium stars have been studied extensively over the past few decades, yet our current understanding of how these intriguing objects formed leaves much to be desired. Many trends observed in systems containing barium stars cannot be satisfactorily explained by classical binary evolution models, naturally raising the question of whether triples and other higher order multiples can give rise to such exotic objects. In this paper, we study the possibility that a Roche Lobe overflow from a tertiary in a hierarchical triple system can potentially lead to surface barium enrichment within the inner binary, while at the same time causing the inner binary to merge, thereby producing a barium star. This possibility has the potential to form a large proportion of Barium stars, as Roche Lobe overflow from a tertiary is typically much more stable for close orbits than that from a binary companion. Various formation channels and mechanisms by which this can be achieved are considered, and constraints on relative formation rates are placed on each scenario. Three recently discovered triple systems containing Ba stars further support our proposed formation mechanism. We conclude that a significant portion of barium stars may be formed from hierarchical triple systems, and that further studies are required in this area before a complete understanding of Barium star populations can be achieved.

Loading-induced bone formation is mediated by Wnt1 induction in osteoblast-lineage cells.

Mechanical loading on the skeleton stimulates bone formation. Although the exact mechanism underlying this process remains unknown, a growing body of evidence indicates that the Wnt signaling pathway is necessary for the skeletal response to loading. Recently, we showed that Wnts produced by osteoblast lineage cells mediate the osteo-anabolic response to tibial loading in adult mice. Here, we report that Wnt1 specifically plays a crucial role in mediating the mechano-adaptive response to loading. Independent of loading, short-term loss of Wnt1 in the Osx-lineage resulted in a decreased cortical bone area in the tibias of 5-month-old mice. In females, strain-matched loading enhanced periosteal bone formation in Wnt1F/F controls, but not in Wnt1F/F; OsxCreERT2 knockouts. In males, strain-matched loading increased periosteal bone formation in both control and knockout mice; however, the periosteal relative bone formation rate was 65% lower in Wnt1 knockouts versus controls. Together, these findings show that Wnt1 supports adult bone homeostasis and mediates the bone anabolic response to mechanical loading.

Autophagy Is Possibly Involved in Osteoblast Responses to Mechanical Loadings.

Both mechanical loading and autophagy play important roles in regulating bone growth and remodeling, but the relationship between the two remains unclear. In this study, we examined bone structure with micro-CT imaging and measured bone mechanical properties with three-point bending experiments using bones from wild-type (WT) mice and conditional knockout (cKO) mice with Atg7 deletion in their osteoblasts. We found that the knockout mice had significantly less bone volume, bone thickness, bone ultimate breaking force, and bone stiffness compared to wild-type mice. Additionally, bone marrow cells from knockout mice had reduced differentiation and mineralization capacities in terms of alkaline phosphatase and calcium secretion, as well as Runx2 and osteopontin expression. Knockout mice also had significantly less relative bone formation rate due to mechanical loading. Furthermore, we found that the osteoblasts from wild-type mice had stronger responses to mechanical stimulation compared to autophagy-deficient osteoblasts from knockout mice. When inhibiting autophagy with 3 MA in wild-type osteoblasts, we found similar results as we did in autophagy-deficient osteoblasts. We also found that mechanical loading-induced ATP release is able to regulate ERK1/2, Runx2, alkaline phosphatase, and osteopontin activities. These results suggest that the ATP pathway may play an important role in the possible involvement of autophagy in osteoblast mechanobiology.

Synergistic Mechanism of Platinum-GaOx Catalysts for Propane Dehydrogenation.

The synergy between metals and metal oxides can effectively improve the heterogeneous catalytic process. This paper describes the intrinsic effect of Pt modification over GaOx (Pt-GaOx ) on propane dehydrogenation. The presence of Pt promotes H2 dissociation and surface coverage of hydrogen species, which is beneficial for the activation of C-H in propane. With excessive Pt, Gaδ+ can be further reduced to form Pt-Ga alloy with less surface hydrogen species. Consequently, the relative propylene formation rate between Pt-GaOx and the summed contribution of individual Pt and GaOx increases linearly with the content of hydrogen species. Optimally, the relative propylene formation rate of Pt-GaOx with 0.03 wt % Pt exceeds 25 % of the summed contribution of individual components.

Synergistic Mechanism of Platinum‐GaOx Catalysts for Propane Dehydrogenation

AbstractThe synergy between metals and metal oxides can effectively improve the heterogeneous catalytic process. This paper describes the intrinsic effect of Pt modification over GaOx (Pt−GaOx) on propane dehydrogenation. The presence of Pt promotes H2 dissociation and surface coverage of hydrogen species, which is beneficial for the activation of C−H in propane. With excessive Pt, Gaδ+ can be further reduced to form Pt−Ga alloy with less surface hydrogen species. Consequently, the relative propylene formation rate between Pt−GaOx and the summed contribution of individual Pt and GaOx increases linearly with the content of hydrogen species. Optimally, the relative propylene formation rate of Pt−GaOx with 0.03 wt % Pt exceeds 25 % of the summed contribution of individual components.

A scanning pulse reaction technique for transient analysis of the methanol-to-hydrocarbons reaction

A method of scanning pulse gas chromatography (SP-GC) suitable for time-resolved analysis of transient response of catalysts to reactant pulses is introduced. Applied to the methanol-to-hydrocarbons reaction over several zeolite-based catalysts (HZSM-5, Ca/ZSM-5 and Ga/ZSM-5), SP-GC provided quantitative information about the transient reactivity and selectivity during methanol conversion. The SP-GC analysis demonstrates substantial differences in the relative rates of hydrocarbon formation between catalysts that favor aromatic or olefin cycles of the dual cycle hydrocarbon pool mechanism. The influence of metal promoters on the main reaction pathways was highlighted with an emphasis on mechanistic aspects.

The Photoinduced Electrocyclization Reaction of Triphenylamine (TPA) in Sustainable and Confined Micellar Solutions: A Steady‐State and Laser Flash Photolysis Approach

AbstractThe irradiation of triphenylamine (TPA) under homogeneous (cyclohexane, acetonitrile and methanol) and micellar [sodium dodecyl sulfate (SDS), cetyltrimethylammonium chloride (CTAC) and polyethylene glycol monododecyl ether (Brij‐P35)] conditions has been investigated through a combined steady‐state and time‐resolved spectroscopic approach. Photolysis of TPA at 254 nm in different media afforded N‐phenylcarbazole (N−PhCA) as the main photoproduct, and when the photoreaction of TPA was carried out in micellar solution the relative rate of formation of N−PhCA was found to be faster than in homogeneous media due to the environmental confined and hydrophobic micellar core. On the other hand, the transient N‐phenyl‐4a,4b‐dihydrocarbazole (DHC0) was detected and fully characterized via laser flash photolysis. Finally, the location of TPA within the hydrophobic core of the micelle was investigated by 1D and 2D NMR spectroscopic analyses.

Performance of Olive-Pomace Oils in Discontinuous and Continuous Frying. Comparative Behavior with Sunflower Oils and High-Oleic Sunflower Oils.

Frying performance of olive-pomace oils (OPOs) as compared to sunflower oils (SOs) and high-oleic sunflower oils (HOSOs) was studied in discontinuous frying (DF) and continuous frying (CF) for the first time. DF is used in household, restaurants and frying outlets, while CF is used in the food industry. Oil alteration during frying was determined by measurements of polar compounds (PC) and polymers. Fried potatoes were analyzed for oil absorption and alteration, color, and evaluated in an acceptability test. Results for DF showed that all SOs reached 25% PC at the 9th frying operation (FO), whereas HOSOs did between the 17–18th FO and variable results were found for OPOs since initial levels of diacylglycerols were different. Rates of formation of PC or polymers were the lowest for OPOs, thus showing the best performance in DF. Specifically for PC, relative rates of formation were 1.00–1.11, 2.46–2.71 and 1.37–1.41 for OPOs, SOs and HOSOs respectively. In CF, OPOs and HOSOs behaved similarly and better than SOs, although none reached 25% PC after 40 FO. The good performance of OPOs can be attributed to the high monounsaturated-to-polyunsaturated ratio, in common with HOSOs, and the additional positive effect of minor compounds, especially β-sitosterol and squalene.

Modeling the outcome of supernova explosions in binary population synthesis using the stellar compactness

Following the collapse of their cores, some of the massive binary stars that populate our Universe are expected to form merging binaries composed of black holes and neutron stars. Gravitational-wave observations of the resulting compact binaries can reveal precious details on the inner workings of the supernova mechanism and the subsequent formation of compact objects. Within the framework of the population-synthesis code mobse, we present the implementation of a new supernova model that relies on the compactness of the collapsing star. The model has two free parameters, namely the compactness threshold that separates the formation of black holes and that of neutron stars, and the fraction of the envelope that falls back onto the newly formed black holes. We compare this model extensively against other prescriptions that are commonly used in binary population synthesis. We find that the cleanest signatures of the role of the pre-supernova stellar compactness are (1) the relative formation rates of the different kinds of compact binaries, which mainly depend on the compactness threshold parameter, and (2) the location of the upper edge of the mass gap between the lightest black holes and the heaviest neutron stars, which mainly depends on the fallback fraction.

Quantitative Description of the Lateral Growth of Two-Dimensional Covalent Organic Frameworks Reveals Self-Templation Effects

Quantitative mechanistic insight into polymerizations that produce highly crystalline two-dimensional covalent organic frameworks (2D COFs) is a prerequisite for their controlled synthesis. However, developing quantitative models for COF formation is challenging since a plethora of molecular species and reactive pathways must be considered simultaneously. Here, we demonstrate an approach to develop an analytical relationship between 2D COF lateral (in-plane) growth rates and fundamental microscopic processes, based on a previously reported kinetic Monte Carlo model. The bond formation rate, the relative bond formation rates at crystal faces and edges, and the lattice size are all found to be critical parameters for the determination of the lateral growth rates. Our analysis also reveals the importance of self-templation in 2D COF growth, which provides a unified understanding of supramolecular interactions in 2D polymerization. Our analytical model represents an important step toward a generalized mechanistic understanding of 2D COF formation. Also, the quantitative description provided here opens the way to monomer design and polymerization conditions that favor lateral growth.

The TrkA agonist gambogic amide augments skeletal adaptation to mechanical loading.

The periosteal and endosteal surfaces of mature bone are densely innervated by sensory nerves expressing TrkA, the high-affinity receptor for nerve growth factor (NGF). In previous work, we demonstrated that administration of exogenous NGF significantly increased load-induced bone formation through the activation of Wnt signaling. However, the translational potential of NGF is limited by the induction of substantial mechanical and thermal hyperalgesia in mice and humans. Here, we tested the effect of gambogic amide (GA), a recently identified robust small molecule agonist for TrkA, on hyperalgesia and load-induced bone formation. Behavioral analysis was used to assess pain up to one week after axial forelimb compression. Contrary to our expectations, GA treatment was not associated with diminished use of the loaded forelimb or sensitivity to thermal stimulus. Furthermore, dynamic histomorphometry revealed a significant increase in relative periosteal bone formation rate as compared to vehicle treatment. Additionally, we found that GA treatment was associated with an increase in the number of osteoblasts per bone surface in loaded limbs as well as a significant increase in the fold change of Ngf, Wnt7b, and Axin2 mRNA expression as compared to vehicle (control). To test the effect of GA on osteoblasts directly, we cultured MC3T3-E1 cells for up to 21days in osteogenic differentiation media containing NGF, GA, or vehicle (control). Media containing GA induced the significant upregulation of the osteoblastic differentiation markers Runx2, Bglap2, and Sp7 in a dose-dependent manner, whereas treatment with NGF was not associated with any significant increases in these markers. Furthermore, consistent with our in vivo findings, we observed that administration of 50nM of GA upregulated expression of Ngf at both Day 3 and Day 7. However, cells treated with the highest dose of GA (500nM) had significantly increased apoptosis and impaired cell proliferation. In conclusion, our study indicates GA may be useful for augmenting skeletal adaptation to mechanical forces without inducing hyperalgesia.

Noninvasive In Situ NMR Study of "Dead Lithium" Formation and Lithium Corrosion in Full-Cell Lithium Metal Batteries.

Capacity retention in lithium metal batteries needs to be improved if they are to be commercially viable, the low cycling stability and Li corrosion during storage of lithium metal batteries being even more problematic when there is no excess lithium in the cell. Herein, we develop in situ NMR metrology to study “anode-free” lithium metal batteries where lithium is plated directly onto a bare copper current collector from a LiFePO4 cathode. The methodology allows inactive or “dead lithium” formation during plating and stripping of lithium in a full-cell lithium metal battery to be tracked: dead lithium and SEI formation can be quantified by NMR and their relative rates of formation are here compared in carbonate and ether-electrolytes. Little-to-no dead Li was observed when FEC is used as an additive. The bulk magnetic susceptibility effects arising from the paramagnetic lithium metal were used to distinguish between different surface coverages of lithium deposits. The amount of lithium metal was monitored during rest periods, and lithium metal dissolution (corrosion) was observed in all electrolytes, even during the periods when the battery is not in use, i.e., when no current is flowing, demonstrating that dissolution of lithium remains a critical issue for lithium metal batteries. The high rate of corrosion is attributed to SEI formation on both lithium metal and copper (and Cu+, Cu2+ reduction). Strategies to mitigate the corrosion are explored, the work demonstrating that both polymer coatings and the modification of the copper surface chemistry help to stabilize the lithium metal surface.

CO as a substrate and inhibitor of H+ reduction for the Mo-, V-, and Fe-nitrogenase isozymes

Three known nitrogenase isozymes, Mo-, V-, and Fe-, catalyze biological reduction of dinitrogen (N2) to ammonia (NH3). All three utilize the same reductive elimination mechanism: an intermediate with two metal-bound hydrides reductively-eliminates hydrogen gas (H2) in a reaction coupled to binding and activation of N2. Nonetheless, the three isozymes show dramatically different relative rates of H2 formation and N2 reduction, revealing important differences in reactivity with substrates. Carbon monoxide (CO) has been characterized as both an inhibitor and substrate for Mo- and V‑nitrogenases, but not for the Fe‑nitrogenase. Here, we present a comparative study of the reactivity of the three isozymes with CO, examining CO both as a substrate and as an inhibitor of proton (H+) reduction under steady-state conditions. For Mo‑nitrogenase, there is neither detectable reduction of CO nor inhibition of H+ reduction. Fe- and V‑nitrogenase show CO reduction and inhibition of H+ reduction that depends on the CO partial pressure. For V‑nitrogenase, ethylene (C2H4) is the major reduction product with a maximum specific activity of ~7.5 nmol C2H4/nmol VFe protein/min at 1 atm CO. The major product of CO reduction for Fe‑nitrogenase is methane (CH4) with a maximum specific activity of ~4.8 nmol CH4/nmol FeFe protein/min at 0.05 atm CO. The rate of CH4 production by Fe‑nitrogenase progressively increases to a maximum at 0.05 atm CO and then declines by ~90% with increasing CO partial pressure up to 1 atm. CO does not inhibit proton reduction in Mo‑nitrogenase but shows 16% inhibition for V‑nitrogenase and 35% for Fe‑nitrogenase.

Complementary Role of Co- and Post-Translational Events in De Novo Protein Biogenesis.

The relation between co- and post-translational protein folding and aggregation in the cell is poorly understood. Here, we employ a combination of fluorescence anisotropy decays in the frequency domain, fluorescence-detected solubility assays, and NMR spectroscopy to explore the role of the ribosome in protein folding within a biologically relevant context. First, we find that a primary function of the ribosome is to promote cotranslational nascent-protein solubility, thus supporting cotranslational folding even in the absence of molecular chaperones. Under these conditions, however, only a fraction of the soluble expressed protein is folded and freely tumbling in solution. Hence, the ribosome alone is insufficient to guarantee quantitative formation of the native state of the apomyoglobin (apoMb) model protein. Right after biosynthesis, nascent chains encoding apoMb emerge from the ribosomal exit tunnel and undergo a crucial irreversible post-translational kinetic partitioning between further folding and aggregation. Mutational analysis in combination with protein-expression kinetics and NMR show that nascent proteins can attain their native state only when the relative rates of soluble and insoluble product formation immediately upon release from the ribosome are tilted in favor of soluble species. Finally, the outcome of the above immediately post-translational kinetic partitioning is much more sensitive to amino acid sequence perturbations than the native fold, which is rather mutation-insensitive. Hence, kinetic channeling of nascent-protein conformation upon release from the ribosome may be a major determinant of evolutionary pressure.

Prx1-Expressing Progenitor Primary Cilia Mediate Bone Formation in response to Mechanical Loading in Mice.

Increases in mechanical loading can enhance the addition of new bone, altering geometry and density such that bones better withstand higher forces. Bone-forming osteoblasts have long been thought to originate from progenitors, but the exact source is yet to be identified. Previous studies indicate osteogenic precursors arise from Prx1-expressing progenitors during embryonic development and adult fracture repair. However, it is unknown whether this cell population is also a source for mechanically induced active osteoblasts. We first identified that Prx1 is expressed in skeletally mature mouse periosteum, a thin tissue covering the surface of the bone that is rich in osteoprogenitors. We then traced Prx1 progenitor lineage using a transgenic mouse model carrying both a Prx1-driven tamoxifen-inducible Cre and a ROSA-driven lacZ reporter gene. Cells that expressed Prx1 when compressive axial loading was applied were detected within the cortical bone days after stimulation, indicating osteocytes are of Prx1-expressing cell origin. In addition, we evaluated how these cells sense and respond to physical stimulation in vivo by disrupting their primary cilia, which are antenna-like sensory organelles known to enhance mechanical and chemical signaling kinetics. Although Prx1-driven primary cilium disruption did not affect osteoblast recruitment to the bone surface, the relative mineral apposition and bone formation rates were decreased by 53% and 34%, respectively. Thus, this cell population contributes to load-induced bone formation, and primary cilia are needed for a complete response. Interestingly, Prx1-expressing progenitors are easily extracted from periosteum and are perhaps an attractive alternative to marrow stem cells for bone tissue regeneration strategies.

Identification and characterization of a cyclic lipopeptide iturin A from a marine-derived Bacillus velezensis 11-5 as a fungicidal agent to Magnaporthe oryzae in rice

Magnaporthe oryzae is an important rice pathogen globally. However, plant beneficial microbes and their secondary metabolites to control blast disease in rice are poorly understood. In the present study, a marine-derived Bacillus velezensis 11-5 has been characterized as an antagonist against an isolate of Magnaporthe oryzae, the causal agent of rice blast disease. A cyclic lipopeptide (CLP) iturin A has been identified from the fermentation broth of B. velezensis 11-5 by nuclear magnetic resonance spectroscopy and mass spectrometry. In addition, the in vitro and in planta biocontrol activities of CLP iturin A were evaluated in the further study, respectively. The results revealed that iturin A shows significant activity on the conidia germination and the relative appressoria formation rate of M. oryzae at the concentrations of 10 and 50 µM for 12 h and for 24 h, respectively. In addition, CLP iturin A shows the significant activity to control M. oryzae in rice plants when the concentration of the compound is higher than 10 µM. Taken all results together, this study shows that B. velezensis 11-5 has the possibility to be developed as a biopesticide to control rice blast disease in rice plants, and for the first time, this study shows a CLP iturin A produced by B. velezensis 11-5 is an agrochemical to control rice blast disease both in vitro and in rice. And therefore, the results provide information on the application of B. velezensis 11-5 and its CLP iturin A as potential biopesticides to control blast disease in rice for agricultural practice.