Mechanism of Aqueous Carbon Dioxide Reduction by the Solvated Electron

Aqueous solvated electron, e_aq, a key species in radiation and plasma chemistry, can effciently reduce CO₂ in a potential green chemistry application. Here, the mechanism of this reaction is unravelled by condensed-phase Born-Oppenheimer molecular dynamics based on the correlated wave function and accurate DFT approximation. We introduce and apply the holistic protocol for solvated electron's reactions encompassing all relevant reaction stages starting from diffusion. The carbon dioxide reduction proceeds via a cavity intermediate, which is separated from the product, CO2^-, by an energy barrier due to the bending of CO₂ and the corresponding solvent reorganization energy. The formation of the intermediate is caused by solvated electron's diffusion, whereas the intermediate transformation to CO₂^- is triggered by solvent fluctuations. This picture of activation-controlled e_aq reaction is very different from both rapid barrierless electron transfer, and proton-coupled electron transfer, where key transformations are caused by proton migration.

Mechanism of Aqueous Carbon Dioxide Reduction by the Solvated Electron

Aqueous solvated electron, e_aq, a key species in radiation and plasma chemistry, can effciently reduce CO₂ in a potential green chemistry application. Here, the mechanism of this reaction is unravelled by condensed-phase Born-Oppenheimer molecular dynamics based on the correlated wave function and accurate DFT approximation. We introduce and apply the holistic protocol for solvated electron's reactions encompassing all relevant reaction stages starting from diffusion. The carbon dioxide reduction proceeds via a cavity intermediate, which is separated from the product, CO2^-, by an energy barrier due to the bending of CO₂ and the corresponding solvent reorganization energy. The formation of the intermediate is caused by solvated electron's diffusion, whereas the intermediate transformation to CO₂^- is triggered by solvent fluctuations. This picture of activation-controlled e_aq reaction is very different from both rapid barrierless electron transfer, and proton-coupled electron transfer, where key transformations are caused by proton migration.

Bacterial Transformation Buffers Environmental Fluctuations through the Reversible Integration of Mobile Genetic Elements

ABSTRACT Horizontal gene transfer (HGT) promotes the spread of genes within bacterial communities. Among the HGT mechanisms, natural transformation stands out as being encoded by the bacterial core genome. Natural transformation is often viewed as a way to acquire new genes and to generate genetic mixing within bacterial populations. Another recently proposed function is the curing of bacterial genomes of their infectious parasitic mobile genetic elements (MGEs). Here, we propose that these seemingly opposing theoretical points of view can be unified. Although costly for bacterial cells, MGEs can carry functions that are at points in time beneficial to bacteria under stressful conditions (e.g., antibiotic resistance genes). Using computational modeling, we show that, in stochastic environments, an intermediate transformation rate maximizes bacterial fitness by allowing the reversible integration of MGEs carrying resistance genes, although these MGEs are costly for host cell replication. Based on this dual function (MGE acquisition and removal), transformation would be a key mechanism for stabilizing the bacterial genome in the long term, and this would explain its striking conservation. IMPORTANCE Natural transformation is the acquisition, controlled by bacteria, of extracellular DNA and is one of the most common mechanisms of horizontal gene transfer, promoting the spread of resistance genes. However, its evolutionary function remains elusive, and two main roles have been proposed: (i) the new gene acquisition and genetic mixing within bacterial populations and (ii) the removal of infectious parasitic mobile genetic elements (MGEs). While the first one promotes genetic diversification, the other one promotes the removal of foreign DNA and thus genome stability, making these two functions apparently antagonistic. Using a computational model, we show that intermediate transformation rates, commonly observed in bacteria, allow the acquisition then removal of MGEs. The transient acquisition of costly MGEs with resistance genes maximizes bacterial fitness in environments with stochastic stress exposure. Thus, transformation would ensure both a strong dynamic of the bacterial genome in the short term and its long-term stabilization.

Bacterial transformation buffers environmental fluctuations through the reversible integration of mobile genetic elements

Horizontal gene transfer (HGT) is known to promote the spread of genes in bacterial communities, which is of primary importance to human health when these genes provide resistance to antibiotics. Among the main HGT mechanisms, natural transformation stands out as being widespread and encoded by the bacterial core genome. From an evolutionary perspective, transformation is often viewed as a mean to generate genetic diversity and mixing within bacterial populations. However, another recent paradigm proposes that its main evolutionary function would be to cure bacterial genomes from their parasitic mobile genetic elements (MGEs). Here, we propose to combine these two seemingly opposing points of view because MGEs, although costly for bacterial cells, can carry functions that are point-in-time beneficial to bacteria under stressful conditions (e.g. antibiotic resistance genes under antibiotic exposure). Using computational modeling, we show that, in stochastic environments (unpredictable stress exposure), an intermediate transformation rate maximizes bacterial fitness by allowing the reversible integration of MGEs carrying resistance genes but costly for the replication of host cells. By ensuring such reversible genetic diversification (acquisition then removal of MGEs), transformation would be a key mechanism for stabilizing the bacterial genome in the long term, which would explain its striking conservation.

Remoción de Semillas en Hábitats Transformados: Pogonomyrmex barbatus (Hymenoptera: Formicidae) y Cinco Especies de Cactáceas del Centro de México

One of the main consequences of human activities in semiarid zones is the transformation of habitats. In this work we studied the effect of this transformation on seed removal of five cacti species by the harvester ant Pogonomyrmex barbatus in the Tehuacán-Cuicatlán valley, a semiarid zone in central Mexico. Seed removal was quantified at three sites which have been under the effect of human activities: an abandoned crop field (CCA), a site with evidence of current human activities (TAH), and a site inside a botanic garden ( JB). We hypothesized that sites which have been under intense human activities would have low rates of seed removal because they offer harsh conditions for harvester ants, reducing their foraging activity. Results showed that vegetation and surface soil characteristics of the sites studied are affecting the rates of seed removal of the five cacti species studied. The lowest seed removal rate was found at CCA, the most transformed site. In contrast with our hypothesis the highest seed removal was found at TAH, the site which represents the intermediate transformation condition, because this site still conserves some characteristics which permit intense foraging activity by harvester ants. We also found that the seed removal rate varied among the different cacti species studied. Seed of E. chiotilla had the highest removal rate, whereas O. decumbens had the lowest. Differences in seed removal rate could be associated with the high heterogeneity found in sites with intermediate levels of transformation. Another factor that must be considered is the external morphology of seeds since smaller seeds presented highest removal rates.

Analysis and Optimization of Starting Characteristics of Tubular Linear Induction Motor

Traditional mechanical punching machines are mainly driven by rotary motors and a complete set of intermediate transformation mechanism is needed, which results in a reduction of the system efficiency. This paper focuses on the tubular linear induction motor (TLIM) with low cost and simple control structure which directly drives the punching machines. In order to provide a theoretical basis for its further control system, the finite element model of this TLIM is established. The starting performances, such as the trust force, phase current, velocity, displacement and the load force, are analyzed by the finite element analysis (FEA). Moreover, the optimization of the starting trust force is investigated.

The Influence of Subgrain Size on the Bainite Refinement for Steels

The relaxation-precipitation-controlling phase transformation (RPC) technique after deformation at non-recrystallization zone to refine the intermediate transformation microstructure has been simulated on a Gleeble-1500 thermo-simulator. The optical microscope, SEMTEMPTA(particle tracking autoradiography) technique to reveal the boron distribution were employed to study the variation of austenite grain size and subgrain size, the features of microstructure after RPC, precipitation and the evolution of dislocation configuration during the relaxation and the boron distribution. The results show that after relaxation at non-recrystallization zone, the subgrain formed inside an original austenite grain. With the relaxation time increasing, the size of the subgrains increased and the misorientation also increased. During the cooling after the relaxation the boron can also segregate at the boundaries of subgrains and the boron segregation can reveal the subgrains forming in deformed austenite before phase transformation. It has been found that during the relaxation strain induced precipitates occurs and these precipitates can pin the subgrain boundary and make it more stable. Comparing the subgrain size demonstrated by PTA with the optical microstructure a conclusion can be drawn that the packet of bainite generally cannot break through the boundaries of subgrains, so the subgrain appearing at the relaxation stage can confine the growth of the microstructure during the transformation in succeeding and the final bainite is refined.

Influence of Cooling Rate on the Microstructure of Bainitic Steel

The microstructure of a bainitic steel after different cooling rates has been investigated by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The effect of cooling rate on the intermediate transformation microstructure was studied. The results showed that the final microstructure contained complex mixture of bainitic ferrite, granular bainite and polygonal ferrite. There was mainly lath-like bainitic ferrite at fast cooling rate (20Ks-1), while microstructure in samples cooled with intermediate rates (8~15 Ks-1) contained bainitic ferrite and granular bainite. When cooling rate decreased to less than 5Ks-1, polygonal ferrite occurred.