1,806 research outputs found
Emergence of negative viscosities and colored noise under current-driven Ehrenfest molecular dynamics
Molecules in molecular junctions are subject to current-induced forces that
can break chemical bonds, induce reactions, destabilize molecular geometry, and
halt the operation of the junction. Theories behind current-driven molecular
dynamics simulations rely on a perturbative time-scale separation within the
system with subsequent use of nonequilibrium Green's functions (NEGF) to
compute conservative, non-conservative, and stochastic forces exerted by
electrons on nuclear degrees of freedom. We analyze the effectiveness of this
approximation, paying particular attention to the phenomenon of negative
viscosities. The perturbative approximation is directly compared to the
nonequilibrium Ehrenfest approach. We introduce a novel time-stepping approach
to calculate the forces present in the Ehrenfest method via exact integration
of the equations of motion for the nonequilibrium Green's functions, which does
not necessitate a time-scale separation within the system and provides an exact
description for the corresponding classical dynamics. We observe that negative
viscosities are not artifacts of a perturbative treatment but also emerge in
Ehrenfest dynamics. However, the effects of negative viscosity have the
possibility of being overwhelmed by the predominantly positive dissipation due
to the higher-order forces unaccounted for by the perturbative approach.
Additionally, we assess the validity of the white-noise approximation for the
stochastic forces, finding that it is justifiable in the presence of a clear
time-scale separation and is more applicable when the current-carrying
molecular orbital is moved outside of the voltage window. Finally, we
demonstrate the method for molecular junction models consisting of one and two
classical degrees of freedom
First-passage time theory of activated rate chemical processes in electronic molecular junctions
Confined nanoscale spaces, electric fields and tunneling currents make the
molecular electronic junction an experimental device for the discovery of new,
out-of-equilibrium chemical reactions. Reaction-rate theory for
current-activated chemical reactions is developed by combining a Keldysh
nonequilibrium Green's functions treatment of electrons, Fokker-Planck
description of the reaction coordinate, and Kramers' first-passage time
calculations. The NEGF provide an adiabatic potential as well as a diffusion
coefficient and temperature with local dependence on the reaction coordinate.
Van Kampen's Fokker-Planck equation, which describes a Brownian particle moving
in an external potential in an inhomogeneous medium with a position-dependent
friction and diffusion coefficient, is used to obtain an analytic expression
for the first-passage time. The theory is applied to several transport
scenarios: a molecular junction with a single, reaction coordinate dependent
molecular orbital, and a model diatomic molecular junction. We demonstrate the
natural emergence of Landauer's blowtorch effect as a result of the interplay
between the configuration dependent viscosity and diffusion coefficients. The
resultant localized heating in conjunction with the bond-deformation due to
current-induced forces are shown to be the determining factors when considering
chemical reaction rates; each of which result from highly tunable parameters
within the system
Properties of High-Latitude CME-Driven Disturbances During Ulysses Second Northern Polar Passage
Ulysses observed five coronal mass ejections (CMEs) and their associated disturbances while the spacecraft was immersed in the polar coronal hole (CH) flow above 70° N in late 2001. Of these CMEs, two were very fast (\u3e850 km s−1) driving strong shocks in the wind ahead, and two others were over-expanding. The two fast CMEs were observed leaving the Sun by LASCO/SOHO, and were observed in the ecliptic by Genesis and ACE. These were large events, spanning at least from the northern heliospheric pole to the ecliptic. One-dimensional hydrodynamic simulations indicate that these could be described as overpressured CMEs launched from the Sun at speeds initially faster than ambient, but then decelerating to the ambient solar wind speed as they propagated outward. The two over-expanding CMEs mark their first occurrence since Ulysses’ first orbit when such CMEs were only observed in polar CH flow
Early social environment influences the behaviour of a family-living lizard
Financial support for this research was provided by the Australian Research Council (DP130102998; grant to M.J.W. and R.B.W.), Natural Sciences and Engineering Research Council of Canada (scholarship to J.L.R.), the Australasian Society for the Study of Animal Behaviour, and Macquarie University. D.W.A.N. was supported by an ARC Discovery Early Career Research Award (DE150101774) and UNSW Vice Chancellors Fellowship.Early social environment can play a significant role in shaping behavioural development. For instance, in many social mammals and birds, isolation rearing results in individuals that are less exploratory, shyer, less social and more aggressive than individuals raised in groups. Moreover, dynamic aspects of social environments, such as the nature of relationships between individuals, can also impact the trajectory of development. We tested if being raised alone or socially affects behavioural development in the family-living tree skink, Egernia striolata. Juveniles were raised in two treatments: alone or in a pair. We assayed exploration, boldness, sociability and aggression repeatedly throughout each juvenile's first year of life, and also assessed social interactions between pairs to determine if juveniles formed dominant–subordinate relationships. We found that male and/or the larger skinks within social pairs were dominant. Developing within this social environment reduced skink growth, and subordinate skinks were more prone to tail loss. Thus, living with a conspecific was costly for E. striolata. The predicted negative effects of isolation failed to materialize. Nevertheless, there were significant differences in behavioural traits depending on the social environment (isolated, dominant or subordinate member of a pair). Isolated skinks were more social than subordinate skinks. Subordinate skinks also became more aggressive over time, whereas isolated and dominant skinks showed invariable aggression. Dominant skinks became bolder over time, whereas isolated and subordinate skinks were relatively stable in their boldness. In summary, our study is evidence that isolation rearing does not consistently affect behaviour across all social taxa. Our study also demonstrates that the social environment plays an important role in behavioural development of a family-living lizard.Publisher PDFPeer reviewe
Expression of Nek1 during kidney development and cyst formation in multiple nephron segments in the Nek1-deficient kat2J mouse model of polycystic kidney disease
BACKGROUND: Neks, mammalian orthologs of the fungal protein kinase never-in-mitosis A, have been implicated in the pathogenesis of polycystic kidney disease. Among them, Nek1 is the primary protein inactivated in kat2J mouse models of PKD. RESULT: We report the expression pattern of Nek1 and characterize the renal cysts that develop in kat2J mice. Nek1 is detectable in all murine tissues but its expression in wild type and kat2J heterozygous kidneys decrease as the kidneys mature, especially in tubular epithelial cells. In the embryonic kidney, Nek1 expression is most prominent in cells that will become podocytes and proximal tubules. Kidney development in kat2J homozygous mice is aberrant early, before the appearance of gross cysts: developing cortical zones are thin, populated by immature glomeruli, and characterized by excessive apoptosis of several cell types. Cysts in kat2J homozygous mice form postnatally in Bowman’s space as well as different tubular subtypes. Late in life, kat2J heterozygous mice form renal cysts and the cells lining these cysts lack staining for Nek1. The primary cilia of cells lining cysts in kat2J homozygous mice are morphologically diverse: in some cells they are unusually long and in others there are multiple cilia of varying lengths. CONCLUSION: Our studies indicate that Nek1 deficiency leads to disordered kidney maturation, and cysts throughout the nephron
The Principal Pipeline Initiative in Action
Strong principals are central to improving schools—indeed, leadership is second only to teaching among school-related factors that influence student achievement. Districts struggle, however, to develop a sufficient pool of highly capable principals. While research has identified strategies that are effective in preparing and supporting school leaders, few districts have pulled together a coherent set of strategies to form a pipeline to the principalship.Recognizing the need to improve the supply of high-quality principals, The Wallace Foundation launched the Principal Pipeline Initiative (PPI) in 2011. The goal was to test the proposition that districts could produce a large cadre of strong novice principals by making a concerted effort to implement a set of interrelated policies and practices, and that doing so would positively affect school outcomes. Participating districts focused on implementing four key components:? Adopting standards of practice and performance to guide principal preparation, hiring, evaluation, and support? Improving the quality of preservice preparation for principals? Using selective hiring and placement practices to match principal candidates with schools? Implementing on-the-job evaluation and support for novice principals (those in their first three years on the job)Six large districts received multi-year grants of 13.25 million from the foundation to cover a portion of the start-up costs of developing a principal pipeline. The districts were selected partly because they had already adopted some policies and practices consistent with the PPI components. The foundation also provided technical assistance. The PPI sought not only to institute, grow, and sustain the key components within each participating district, but also to generate examples that other districts might follow
IFN-γ-producing CD4+ T cells promote experimental cerebral malaria by modulating CD8+ T cell accumulation within the brain.
It is well established that IFN-γ is required for the development of experimental cerebral malaria (ECM) during Plasmodium berghei ANKA infection of C57BL/6 mice. However, the temporal and tissue-specific cellular sources of IFN-γ during P. berghei ANKA infection have not been investigated, and it is not known whether IFN-γ production by a single cell type in isolation can induce cerebral pathology. In this study, using IFN-γ reporter mice, we show that NK cells dominate the IFN-γ response during the early stages of infection in the brain, but not in the spleen, before being replaced by CD4(+) and CD8(+) T cells. Importantly, we demonstrate that IFN-γ-producing CD4(+) T cells, but not innate or CD8(+) T cells, can promote the development of ECM in normally resistant IFN-γ(-/-) mice infected with P. berghei ANKA. Adoptively transferred wild-type CD4(+) T cells accumulate within the spleen, lung, and brain of IFN-γ(-/-) mice and induce ECM through active IFN-γ secretion, which increases the accumulation of endogenous IFN-γ(-/-) CD8(+) T cells within the brain. Depletion of endogenous IFN-γ(-/-) CD8(+) T cells abrogates the ability of wild-type CD4(+) T cells to promote ECM. Finally, we show that IFN-γ production, specifically by CD4(+) T cells, is sufficient to induce expression of CXCL9 and CXCL10 within the brain, providing a mechanistic basis for the enhanced CD8(+) T cell accumulation. To our knowledge, these observations demonstrate, for the first time, the importance of and pathways by which IFN-γ-producing CD4(+) T cells promote the development of ECM during P. berghei ANKA infection
Esculin hydrolysis negative and TcdA-only producing strains of Clostridium (Clostridioides) difficile from the environment in Western Australia
Background and Aims: Clostridium (Clostridiodes) difficile clade 3 ribotype (RT) 023 strains that fail to produce black colonies on bioMérieux ChromID agar have been reported, as well as variant strains of C. difficile that produce only toxin A. We have recently isolated strains of C. difficile from the environment in Western Australia (WA) with similar characteristics. The objective of this study was to characterize these strains. It was hypothesized that a putative β-glucosidase gene was lacking in these strains of C. difficile, including RT 023, leading to white colonies. Methods and Results: A total of 17 environmental isolates of C. difficile from garden soil and compost, and gardening shoe soles in Perth, WA, failed to produce black colonies on ChromID agar. MALDI-TOF MS analysis confirmed these strains as C. difficile. Four strains contained only a tcdA gene (A+B−CDT−) by PCR and were a novel RT (QX 597). All isolates were susceptible to all antimicrobials tested except one with low-level resistance to clindamycin (MIC = 8 mg/L). The four tcdA-positive strains were motile. All isolates contained neither bgl locus but only bgl K or a putative β-glucosidase gene by PCR. Whole-genome sequencing showed the 17 strains belonged to novel multi-locus sequence types 632, 848, 849, 850, 851, 852 and 853, part of the evolutionarily divergent clade C-III. Four isolates carried a full-length tcdA but not tcdB nor binary toxin genes. Conclusions: ChromID C. difficile agar is used for the specific detection of C. difficile in the samples. To date, all strains except RT 023 strains from clinical samples hydrolyse esculin. This is the first report to provide insights into the identification of esculin hydrolysis negative and TcdA-only producing (A+B−CDT−) strains of C. difficile from environmental samples. Significance and Impact of the Study: White colonies of C. difficile from environmental samples could be overlooked when using ChromID C. difficile agar, leading to false-negative results, however, whether these strains are truly pathogenic remains to be proven
Complete genome sequences of evolutionary clade C-III strains of Clostridioides (Clostridium) difficile isolated from the environment in Western Australia
Clostridioides (Clostridium) difficile in the environment is thought to contribute to C. difficile infection in community settings. Here, we provide complete genome assemblies for two esculin hydrolysis-negative strains of C. difficile that were isolated from soils in Western Australia; the strains produce white colonies on chromogenic media and belong to evolutionarily divergent clade C-III
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