714 research outputs found
Diffusion-desorption ratio of adsorbed CO and CO on water ice
Diffusion of atoms and molecules is a key process for the chemical evolution
in the star forming regions of the interstellar medium. Accurate data on the
mobility of many important interstellar species is however often not available
and this provides a serious limitation for the reliability of models describing
the physical and chemical processes in molecular clouds. Here we aim to provide
the astrochemical modeling community with reliable data on the ratio between
the energy barriers for diffusion and desorption for adsorbed CO and CO on
water ices. To this end, we use a fully atomistic, off-lattice kinetic Monte
Carlo technique to generate dynamical trajectories of CO and CO molecules
on the surface of crystalline ice at temperatures relevant for the interstellar
medium. The diffusion to desorption barrier ratios are determined to be 0.31
for CO and 0.39 for CO . These ratios can be directly used to improve the
accuracy of current gas-grain chemical models.Comment: 4 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Interactions of adsorbed CO on water ice at low temperatures
We present a computational study into the adsorption properties of CO on
amorphous and crystalline water surfaces under astrophysically relevant
conditions. Water and carbon dioxide are two of the most dominant species in
the icy mantles of interstellar dust grains and a thorough understanding of
their solid phase interactions at low temperatures is crucial for understanding
the structural evolution of the ices due to thermal segregation. In this paper,
a new HO-CO interaction potential is proposed and used to model the
ballistic deposition of CO layers on water ice surfaces, and to study the
individual binding sites at low coverages. Contrary to recent experimental
results, we do not observe CO island formation on any type of water
substrate. Additionally, density functional theory calculations are performed
to assess the importance of induced electrostatic interactions.Comment: Accepted for publication in Physical Chemistry Chemical Physic
Identification of genetic modifiers of behavioral phenotypes in serotonin transporter knockout rats
BACKGROUND: Genetic variation in the regulatory region of the human serotonin transporter gene (SLC6A4) has been shown to affect brain functionality and personality. However, large heterogeneity in its biological effects is observed, which is at least partially due to genetic modifiers. To gain insight into serotonin transporter (SERT)-specific genetic modifiers, we studied an intercross between the Wistar SERT-/- rat and the behaviorally and genetically divergent Brown Norway rat, and performed a QTL analysis. RESULTS: In a cohort of >150 intercross SERT-/- and control (SERT+/+) rats we characterized 12 traits that were previously associated with SERT deficiency, including activity, exploratory pattern, cocaine-induced locomotor activity, and abdominal and subcutaneous fat. Using 325 genetic markers, 10 SERT-/--specific quantitative trait loci (QTLs) for parameters related to activity and exploratory pattern (Chr.1,9,11,14), and cocaine-induced anxiety and locomotor activity (Chr.5,8) were identified. No significant QTLs were found for fat parameters. Using in silico approaches we explored potential causal genes within modifier QTL regions and found interesting candidates, amongst others, the 5-HT1D receptor (Chr. 5), dopamine D2 receptor (Chr. 8), cannabinoid receptor 2 (Chr. 5), and genes involved in fetal development and plasticity (across chromosomes). CONCLUSIONS: We anticipate that the SERT-/--specific QTLs may lead to the identification of new modulators of serotonergic signaling, which may be targets for pharmacogenetic and therapeutic approaches.
Reaction Networks For Interstellar Chemical Modelling: Improvements and Challenges
We survey the current situation regarding chemical modelling of the synthesis
of molecules in the interstellar medium. The present state of knowledge
concerning the rate coefficients and their uncertainties for the major
gas-phase processes -- ion-neutral reactions, neutral-neutral reactions,
radiative association, and dissociative recombination -- is reviewed. Emphasis
is placed on those reactions that have been identified, by sensitivity
analyses, as 'crucial' in determining the predicted abundances of the species
observed in the interstellar medium. These sensitivity analyses have been
carried out for gas-phase models of three representative, molecule-rich,
astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the
expanding circumstellar envelope IRC +10216. Our review has led to the proposal
of new values and uncertainties for the rate coefficients of many of the key
reactions. The impact of these new data on the predicted abundances in TMC-1
and L134N is reported. Interstellar dust particles also influence the observed
abundances of molecules in the interstellar medium. Their role is included in
gas-grain, as distinct from gas-phase only, models. We review the methods for
incorporating both accretion onto, and reactions on, the surfaces of grains in
such models, as well as describing some recent experimental efforts to simulate
and examine relevant processes in the laboratory. These efforts include
experiments on the surface-catalysed recombination of hydrogen atoms, on
chemical processing on and in the ices that are known to exist on the surface
of interstellar grains, and on desorption processes, which may enable species
formed on grains to return to the gas-phase.Comment: Accepted for publication in Space Science Review
Incorporation of stochastic chemistry on dust grains in the PDR code using moment equations
Unlike gas-phase reactions, chemical reactions taking place on interstellar
dust grain surfaces cannot always be modeled by rate equations. Due to the
small grain sizes and low flux,these reactions may exhibit large fluctuations
and thus require stochastic methods such as the moment equations.
We evaluate the formation rates of H2, HD and D2 molecules on dust grain
surfaces and their abundances in the gas phase under interstellar conditions.
We incorporate the moment equations into the Meudon PDR code and compare the
results with those obtained from the rate equations. We find that within the
experimental constraints on the energy barriers for diffusion and desorption
and for the density of adsorption sites on the grain surface, H2, HD and D2
molecules can be formed efficiently on dust grains.
Under a broad range of conditions, the moment equation results coincide with
those obtained from the rate equations. However, in a range of relatively high
grain temperatures, there are significant deviations. In this range, the rate
equations fail while the moment equations provide accurate results. The
incorporation of the moment equations into the PDR code can be extended to
other reactions taking place on grain surfaces
Relevance of the H_2 + O reaction pathway for the surface formation of interstellar water. Combined experimental and modeling study
The formation of interstellar water is commonly accepted to occur on the surfaces of icy dust grains in dark molecular clouds at low temperatures (10–20 K), involving hydrogenation reactions of oxygen allotropes. As a result of the large abundances of molecular hydrogen and atomic oxygen in these regions, the reaction H_2 + O has been proposed to contribute significantly to the formation of water as well. However, gas-phase experiments and calculations, as well as solid-phase experimental work contradict this hypothesis. Here, we use precisely executed temperature-programmed desorption (TPD) experiments in an ultra-high vacuum setup combined with kinetic Monte Carlo simulations to establish an upper limit of the water production starting from H_2 and O. These reactants were brought together in a matrix of CO_2 in a series of (control) experiments at different temperatures and with different isotopological compositions. The water detected with the quadrupole mass spectrometer upon TPD was found to originate mainly from contamination in the chamber itself. However, if water is produced in small quantities on the surface through H_2 + O, this can only be explained by a combined classical and tunneled reaction mechanism. An absolutely conservative upper limit for the reaction rate was derived with a microscopic kinetic Monte Carlo model that converts the upper limit into the highest possible reaction rate. Incorporating this rate into simulation runs for astrochemically relevant parameters shows that the upper limit to the contribution of the reaction H_2 + O in OH, and hence water formation, is 11% in dense interstellar clouds. Our combined experimental and theoretical results indicate, however, that this contribution is most likely much lower
Haplotype block structure is conserved across mammals
Genetic variation in genomes is organized in haplotype blocks, and species-specific block structure is defined by differential contribution of population history effects in combination with mutation and recombination events. Haplotype maps characterize the common patterns of linkage disequilibrium in populations and have important applications in the design and interpretation of genetic experiments. Although evolutionary processes are known to drive the selection of individual polymorphisms, their effect on haplotype block structure dynamics has not been shown. Here, we present a high-resolution haplotype map for a 5-megabase genomic region in the rat and compare it with the orthologous human and mouse segments. Although the size and fine structure of haplotype blocks are species dependent, there is a significant interspecies overlap in structure and a tendency for blocks to encompass complete genes. Extending these findings to the complete human genome using haplotype map phase I data reveals that linkage disequilibrium values are significantly higher for equally spaced positions in genic regions, including promoters, as compared to intergenic regions, indicating that a selective mechanism exists to maintain combinations of alleles within potentially interacting coding and regulatory regions. Although this characteristic may complicate the identification of causal polymorphisms underlying phenotypic traits, conservation of haplotype structure may be employed for the identification and characterization of functionally important genomic regions
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