We investigate the emergence of space stems ahead of negative leaders. These are luminous spots that appear ahead of an advancing leader mediating the leader's stepped propagation. We show that space ...stems start as regions of locally depleted conductivity that form in the streamers of the corona around the leader. An attachment instability enhances the electric field leading to strongly inhomogeneous, bright, and locally warmer regions ahead of the leader that explain the existing observations. Since the attachment instability is only triggered by fields above 10 kV/cm and internal electric fields are lower in positive than in negative streamers, our results explain why, although common in negative leaders, space stems, and stepping are hardly observed if not absent in positive leaders. Further work is required to fully explain the streamer to leader transition, which requires an electric current persisting for timescales longer than the typical attachment time of electrons, around 100 ns.
Plain Language Summary
Long electrical discharges of negative polarity, such as most cloud‐to‐ground lightning flashes, propagate in a stepped manner, that is, alternating between standing and jumping suddenly. The underlying mechanism explaining this behavior is not well understood, although we know that space stems are a key element. These are bright and locally warmer segments that appear ahead of a discharge channel and apparently isolated from it. For the first time, we show how these space stems emerge spontaneously in our simulations from regions of locally lower conductivity that latter become bright and warm. Then on one hand, we propose a possible origin of the space stems and, on the other hand, we shed some light on possible mechanisms that grow these stems to longer times, beyond 100 ns.
Key Points
Space stems start as regions of locally lower conductivity in the streamer channels around the tip of negative leaders
An attachment instability enhances the electric field at the space stem decreasing further the conductivity and leading to bright and locally warmer regions
As the attachment instability is favoured by high electric fields inside negative streamer channels it explains why only negative leaders propagate in long steps
Over the past decade, organic solar cells (OSCs) have achieved a dramatic boost in their power conversion efficiencies from about 6 % to over 16 %. In addition to developments in device engineering, ...innovative photovoltaic materials, especially fluorinated donors and acceptors, have become the dominant factor for improved device performance. This minireview highlights fluorinated photovoltaic materials that enable efficient OSCs. Impressive OSCs have been obtained by developing some important molds of fluorinated donor and acceptor systems. The molecular design strategy and the matching principle of fluorinated donors and acceptors in OSCs are discussed. Finally, a concise summary and outlook are presented for advances in fluorinated materials to realize the practical application of OSCs.
The bacterial epigenome Sánchez-Romero, María A; Casadesús, Josep
Nature reviews. Microbiology,
01/2020, Letnik:
18, Številka:
1
Journal Article
Recenzirano
In all domains of life, genomes contain epigenetic information superimposed over the nucleotide sequence. Epigenetic signals control DNA-protein interactions and can cause phenotypic change in the ...absence of mutation. A nearly universal mechanism of epigenetic signalling is DNA methylation. In bacteria, DNA methylation has roles in genome defence, chromosome replication and segregation, nucleoid organization, cell cycle control, DNA repair and regulation of transcription. In many bacterial species, DNA methylation controls reversible switching (phase variation) of gene expression, a phenomenon that generates phenotypic cell variants. The formation of epigenetic lineages enables the adaptation of bacterial populations to harsh or changing environments and modulates the interaction of pathogens with their eukaryotic hosts.
Abstract
Paraquat (PQ, 1,1'-dimethyl-4-4'-bipyridinium dichloride) is a highly toxic quaternary ammonium herbicide widely used in agriculture, it exerts its toxic effects mainly because of its redox ...cycle through the production of superoxide anions in organisms, leading to an imbalance in the redox state of the cell causing oxidative damage and finally cell death. The contribution of mitochondrial dysfunction including increased production of reactive oxygen species besides the reduction in oxygen consumption as well as in the activity of some respiratory complexes has emerged as a key component in the mechanisms through which PQ induces cell death. Although several aspects of PQ-mitochondria interaction remain to be clarified, recent advances have been conducted with reproducible results. Currently, there is no treatment for PQ poisoning; however, several studies taking into account oxidative stress as the main mechanism of PQ-induced toxicity suggest an antioxidant therapy as a viable alternative. In fact, it has been shown that the antioxidants naringin, sylimarin, edaravone, Bathysa cuspidata extracts, alpha-lipoic acid, pirfenidone, lysine acetylsalicylate, selenium, quercetin, C-phycocyanin, bacosides, and vitamin C may be useful in the treatment against PQ toxicity. The main mechanisms involved in the protective effect of these antioxidants include the reduction of oxidative stress and inflammation and the induction of antioxidant defenses. Interestingly, recent findings suggest that the induction of nuclear factor erythroid like-2 (Nrf2), a major regulator of the antioxidant response, by some of the above-mentioned antioxidants, has been involved in the protective effect against PQ-induced toxicity.
Highlights • Bacterial genomes contain C5 -methyl-cytosine, N4 -methyl-cytosine, and N6 -methyl-adenine. • Base methylation controls DNA–protein interactions. • DNA methylation plays roles in ...bacterial virulence. • Methylome analysis may open a new era in bacterial epigenomics.
We report the discovery of 6576 new spectroscopically confirmed white dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 12. We obtain T
eff, log g and mass for hydrogen atmosphere ...white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon-dominated spectra (DQs). We found one central star of a planetary nebula, one ultracompact helium binary (AM CVn), one oxygen line-dominated white dwarf, 15 hot DO/PG1159s, 12 new cataclysmic variables, 36 magnetic white dwarf stars, 54 DQs, 115 helium-dominated white dwarfs, 148 white dwarf + main-sequence star binaries, 236 metal-polluted white dwarfs, 300 continuum spectra DCs, 230 hot subdwarfs, 2936 new hydrogen-dominated white dwarf stars, and 2675 cool hydrogen-dominated subdwarf stars. We calculate the mass distribution of all 5883 DAs with S/N ≥ 15 in DR12, including the ones in DR7 and DR10, with an average S/N = 26, corrected to the 3D convection scale, and also the distribution after correcting for the observed volume, using 1/V
max.
Over the past several decades, organometallic cross-coupling chemistry has developed into one of the most reliable approaches to assemble complex aromatic compounds from preoxidized starting ...materials. More recently, transition metal–catalyzed carbon-hydrogen activation has circumvented the need for preoxidized starting materials, but this approach is limited by a lack of practical amination protocols. Here, we present a blueprint for aromatic carbon-hydrogen functionalization via photoredox catalysis and describe the utility of this strategy for arene amination. An organic photoredox-based catalyst system, consisting of an acridinium photooxidant and a nitroxyl radical, promotes site-selective amination of a variety of simple and complex aromatics with heteroaromatic azoles of interest in pharmaceutical research. We also describe the atom-economical use of ammonia to form anilines, without the need for prefunctionalization of the aromatic component.
Directed evolution circumvents our profound ignorance of how a protein's sequence encodes its function by using iterative rounds of random mutation and artificial selection to discover new and useful ...proteins. Proteins can be tuned to adapt to new functions or environments by simple adaptive walks involving small numbers of mutations. Directed evolution studies have shown how rapidly some proteins can evolve under strong selection pressures and, because the entire 'fossil record' of evolutionary intermediates is available for detailed study, they have provided new insight into the relationship between sequence and function. Directed evolution has also shown how mutations that are functionally neutral can set the stage for further adaptation.
•Oxidation of Orange G (azo dye) with iron activated persulfate was studied.•An oxidation pathway was proposed for OG oxidation by activated persulfate.•The influence of iron valence (0, II and III) ...on the activity was studied.•Product distribution, mineralization and detoxification were also studied.
Persulfate (PS) was employed in the oxidation of Orange G (OG), an azo dye commonly found in textile wastewaters. Activation of PS was conducted with iron to generate sulfate free radicals (SO4-) with high redox potential capable to oxidize most of the organics in water. Identification of oxidation intermediates was carried out by analyzing at different times organic by-products generated from treatment of a concentrate dye solution (11.6mM) with 153mM of PS and 20mM of Fe(II) at 20°C. Intermediate reaction products (mainly phenol (PH) and benzoquinone (BQ), and in less extent aniline, phenolic compounds and naphthalene type compounds with quinone groups) were identified by GC/MS and HPLC, and an oxidation pathway was proposed for the oxidation of OG with iron activated PS. The effect of iron valence (0, II and III) in the oxidation of an aqueous solution of OG (containing 0.1mM) was studied in a 0.5L batch reactor at 20°C. Initial activator and PS concentrations employed were both 1mM. Complete pollutant removal was achieved within the first 30min when iron II or III were employed as activators. Quinone intermediates generated during pollutant oxidation may act as electron shuttles, allowing the reduction of Fe(III) into Fe(II) in the redox cycling of iron. Therefore, activation of PS by Fe(III) allowed complete OG removal. When zero valent iron (ZVI) was employed (particle diameter size 0.74mm) the limiting step in SO4- generation was the surface reaction between ZVI and the oxidant yielding a lower oxidation rate of the dye. An increase in the oxidant dosage (0.2mM OG, 2mM Fe(III) and 6mM PS) allowed complete pollutant and ecotoxicity removal, as well as mineralization close to 75%.
A call to increase diversity among academic surgery faculty (ASF) was made in 2008, after recognizing the lack of surgeons considered underrepresented in medicine (URM). We aimed to quantify and ...assess trends among URM ASF in the interval since that call to action.
Publicly available data on ASF were reviewed. We calculated the percentage of ASF in 2018 by URM group, then stratified by academic rank of assistant professor, associate professor, and full professor. We compared 2005-2018 ASF of Hispanic or Latino (HL) and African American (AA) background; 2005 data were unavailable for other URM groups.
In 2018, URM surgeons accounted for 7.06% (n = 1013/14,340) of ASF (AA: n = 492, 3.43%; HL: n = 485, 3.38%; American Indian or Alaskan Native: n = 23, 0.16%; and Native Hawaiian/Pacific Islander: n = 13, 0.09%). When comparing 2005-2018, AA ASF remained stable across ranks (total: n = 298, 3.12% versus n = 492, 3.43%; P = 0.09), whereas HL ASF decreased across ranks (total: n = 415, 4.35% versus n = 485, 3.38%; P = 0.00007).
Surgeons from URM backgrounds account for 7% of ASF. No increase in AA and a decrease in HL ASF occurred from 2005 to 2018. There is a paucity of data for other URM groups. Active strategies to increase diversity and inclusion in academic surgery are necessary.
•Approximately 7% of all academic surgeons in the United States come from backgrounds considered underrepresented in medicine (African American, Hispanic/Latino, American Indian/Alaskan Native, and Native Hawaiian/Pacific Islander).•The percentage of African American academic surgery faculty has not significantly increased when comparing 2005-2018 URM ASF data.•The percentage of Hispanic/Latino academic surgery faculty decreased when comparing 2005-2018 URM ASF data.
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