979 research outputs found
Does slow and steady win the race? Investigating feedback processes in giant molecular clouds
We investigate the effects of gradual heating on the evolution of turbulent
molecular clouds of mass M and virial parameters ranging
between . This gradual heating represents the energy output from
processes such as winds from massive stars or feedback from High Mass X-ray
binaries (HMXBs), contrasting the impulsive energy injection from supernovae
(SNe). For stars with a mass high enough that their lifetime is shorter than
the life of the cloud, we include a SN feedback prescription. Including both
effects, we investigate the interplay between slow and fast forms of feedback
and their effectiveness at triggering/suppressing star formation. We find that
SN feedback can carve low density chimneys in the gas, offering a path of least
resistance for the energy to escape. Once this occurs the more stable, but less
energetic, gradual feedback is able to keep the chimneys open. By funneling the
hot destructive gas away from the centre of the cloud, chimneys can have a
positive effect on both the efficiency and duration of star formation.
Moreover, the critical factor is the number of high mass stars and SNe (and any
subsequent HMXBs) active within the free-fall time of each cloud. This can vary
from cloud to cloud due to the stochasticity of SN delay times and in HMXB
formation. However, the defining factor in our simulations is the efficiency of
the cooling, which can alter the Jeans mass required for sink particle
formation, along with the number of massive stars in the cloud.Comment: 35 pages, 46 figures, accepted for publication in MNRA
The allosteric transition of glucosamine-6-phosphate deaminase: the structure of the T state at 2.3 Ã… resolution
AbstractBackground: The allosteric hexameric enzyme glucosamine-6-phosphate deaminase from Escherichia coli catalyses the regulatory step of N-acetylglucosamine catabolism, which consists of the isomerisation and deamination of glucosamine 6-phosphate (GlcN6P) to form fructose 6-phosphate (Fru6P) and ammonia. The reversibility of the catalysis and its rapid-equilibrium random kinetic mechanism, among other properties, make this enzyme a good model for studying allosteric processes.Results: Here we present the structure of P6322 crystals, obtained in sodium acetate, of GlcN6P deaminase in its ligand-free T state. These crystals are very sensitive to X-ray radiation and have a high (78%) solvent content. The active-site lid (residues 162–185) is highly disordered in the T conformer; this may contribute significantly to the free-energy change of the whole allosteric transition. Comparison of the structure with the crystallographic coordinates of the R conformer (Brookhaven Protein Data Bank entry 1dea) allows us to describe the geometrical changes associated with the allosteric transition as the movement of two rigid entities within each monomer. The active site, located in a deep cleft between these two rigid entities, presents a more open geometry in the T conformer than in the R conformer.Conclusions: The differences in active-site geometry are related to alterations in the substrate-binding properties associated with the allosteric transition. The rigid nature of the two mobile structural units of each monomer seems to be essential in order to explain the observed kinetics of the deaminase hexamer. The triggers for both the homotropic and heterotropic allosteric transitions are discussed and particular residues are assigned to these functions. A structural basis for an entropic term in the allosteric transition is an interesting new feature that emerges from this study
Dietary restriction in ILSXISS mice is associated with widespread changes in splicing regulatory factor expression levels.
This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record Dietary restriction (DR) represents one of the most reproducible interventions to extend lifespan and improve health outcomes in a wide range of species, but substantial variability in DR response has been observed, both between and within species. The mechanisms underlying this variation in effect are still not well characterised. Splicing regulatory factors have been implicated in the pathways linked with DR-induced longevity in C. elegans and are associated with lifespan itself in mice and humans. We used qRT-PCR to measure the expression levels of a panel of 20 age- and lifespan-associated splicing regulatory factors in brain, heart and kidney derived from three recombinant inbred strains of mice with variable lifespan responses to short-term (2 months) or long-term (10 months) 40% DR to determine their relationship to DR-induced longevity. We identified 3 patterns of association; i) splicing factors associated with DR alone, ii) splicing factors associated with strain alone or iii) splicing factors associated with both DR and strain. Tissue specific variation was noted in response to short term or long-term DR, with the majority of effects noted in brain following long term DR in the positive responder strain TejJ89. Association in heart and kidney were less evident, and occurred following short term DR. Splicing factors associated with both DR and strain may be mechanistically involved in strain-specific differences in response to DR. We provide here evidence concordant with a role for some splicing factors in the lifespan modulatory effects of DR across different mouse strains and in different tissues
Crystallization and preliminary structural analysis of the giant haemoglobin from Glossoscolex paulistus at 3.2 Å
Diffraction data to 3.2 Å from crystals of the 3.6 MDa erythrocruorin from a Brazilian earthworm represent the highest resolution reported to date for similar complexes. An unambiguous molecular replacement solution shows the particle to belong to the type I class
Preliminary crystallographic studies of EcTI, a serine proteinase inhibitor from Enterolobium contortisiliquum seeds
Enterolobium contortisiliquum trypsin inhibitor (EcTI) belongs to the Kunitz family of plant inhibitors, which are widely distributed in nature, especially in plant seeds. EcTI is composed of two polypeptide chains with a total of 174 residues, homologous to other inhibitors from the same family. EcTI crystals, which were obtained with the acupuncture-gel technique, diffract to 2.0 Angstrom resolution and belong to space group P2(1), with unit-cell parameters a = 37.12, b = 38.42, c = 54.08 Angstrom, beta = 98.08 degrees. Molecular-replacement techniques using Erythrina caffra trypsin inhibitor (PDB code 1tie) as the search model indicate one monomer in the asymmetric unit. the secondary-structure content of EcTI was determined by circular dichroism spectroscopy, yielding values compatible with the expected topology.Universidade Federal de São Paulo, EPM, Dept Bioquim, BR-04044020 São Paulo, BrazilUniv São Paulo, IFSC, Lab Cristalog Prot & Biol Mol Estructural, San Carlos, SP, BrazilUniv São Paulo, IFSC, Dept Biofis, San Carlos, SP, BrazilUniversidade Federal de São Paulo, EPM, Dept Bioquim, BR-04044020 São Paulo, BrazilWeb of Scienc
Domestic ventilation rates, indoor humidity and dust mite allergens : are our homes causing the asthma pandemic?
This paper is concerned with historical changes in domestic ventilation rates, relative humidity and the associated risk of house dust mite colonization. A controlled trial evaluated allergen and water vapour control measures on the level of house dust mite (HDM) Der p1 allergen and indoor humidity, concurrently with changes in lung function in 54 subjects who completed the protocol. Mechanical heat recovery ventilation units significantly reduced moisture content in the active group, while HDM allergen reservoirs in carpets and beds were reduced by circa 96%. Self reported health status confirmed a significant clinical improvement in the active group. The study can form the basis for assessing minimum winter ventilation rates that can suppress RH below the critical ambient equilibrium humidity of 60% and thus inhibit dust mite colonization and activity in temperate and maritime in' uenced climatic regions
A comprehensive study on the relation between the metal enrichment of ionised and atomic gas in star-forming galaxies
We study the relation between the metallicities of ionised and neutral gas in
star-forming galaxies at z=0-3 using the EAGLE cosmological, hydrodynamical
simulations. This is done by constructing a dense grid of sightlines through
the simulated galaxies and obtaining the star formation rate- and HI column
density-weighted metallicities, Z_{SFR} and Z_{HI}, for each sightline as
proxies for the metallicities of ionised and neutral gas, respectively. We find
Z_{SFR} > Z_{HI} for almost all sightlines, with their difference generally
increasing with decreasing metallicity. The stellar masses of galaxies do not
have a significant effect on this trend, but the positions of the sightlines
with respect to the galaxy centres play an important role: the difference
between the two metallicities decreases when moving towards the galaxy centres,
and saturates to a minimum value in the central regions of galaxies,
irrespective of redshift and stellar mass. This implies that the mixing of the
two gas phases is most efficient in the central regions of galaxies where
sightlines generally have high column densities of HI. However, a high HI
column density alone does not guarantee a small difference between the two
metallicities. In galaxy outskirts, the inefficiency of the mixing of
star-forming gas with HI seems to dominate over the dilution of heavy elements
in HI through mixing with the pristine gas. We find good agreement between the
available observational data and the ZSFR-ZHI relation predicted by the EAGLE
simulations. Though, observed regions with a nuclear starburst mode of star
formation appear not to follow the same relation.Comment: Under review with Ap
Tropospheric Phase Calibration in Millimeter Interferometry
We review millimeter interferometric phase variations caused by variations in
the precipitable water vapor content of the troposphere, and we discuss
techniques proposed to correct for these variations. We present observations
with the Very Large Array at 22 GHz and 43 GHz designed to test these
techniques. We find that both the Fast Switching and Paired Array calibration
techniques are effective at reducing tropospheric phase noise for radio
interferometers. In both cases, the residual rms phase fluctuations after
correction are independent of baseline length for b > b_{eff}. These techniques
allow for diffraction limited imaging of faint sources on arbitrarily long
baselines at mm wavelengths. We consider the technique of tropospheric phase
correction using a measurement of the precipitable water vapor content of the
troposphere via a radiometric measurement of the brightness temperature of the
atmosphere. Required sensitivities range from 20 mK at 90 GHz to 1 K at 185 GHz
for the MMA, and 120 mK for the VLA at 22 GHz. The minimum gain stability
requirement is 200 at 185 GHz at the MMA assuming that the astronomical
receivers are used for radiometry. This increases to 2000 for an uncooled
system. The stability requirement is 450 for the cooled system at the VLA at 22
GHz. To perform absolute radiometric phase corrections also requires knowledge
of the tropospheric parameters and models to an accuracy of a few percent. It
may be possible to perform an `empirically calibrated' radiometric phase
correction, in which the relationship between fluctuations in brightness
temperature differences with fluctuations in interferometric phases is
calibrated by observing a celestial calibrator at regular intervals.Comment: AAS LATEX preprint format. to appear in Radio Science 199
Direct observation of ultrafast exciton localization in an organic semiconductor with soft X-ray transient absorption spectroscopy
The localization dynamics of excitons in organic semiconductors influence the efficiency of charge transfer and separation in these materials. Here we apply time-resolved X-ray absorption spectroscopy to track photoinduced dynamics of a paradigmatic crystalline conjugated polymer: poly(3-hexylthiophene) (P3HT) commonly used in solar cell devices. The π→π* transition, the first step of solar energy conversion, is pumped with a 15 fs optical pulse and the dynamics are probed by an attosecond soft X-ray pulse at the carbon K-edge. We observe X-ray spectroscopic signatures of the initially hot excitonic state, indicating that it is delocalized over multiple polymer chains. This undergoes a rapid evolution on a sub 50 fs timescale which can be directly associated with cooling and localization to form either a localized exciton or polaron pair
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