385 research outputs found
Advances in Regiocontrol and Bench Stability in Nickel Catalyzed Reductive Couplings of Aldehydes and Alkynes.
Advances in nickel catalyzed reactions have been achieved that allow for alteration and control of the rate- and regioselectivity determining step in nickel-catalyzed aldehyde-alkyne reductive couplings for the synthesis of allylic alcohols. Combinations of ligand, reducing agent, and reaction conditions have been identified that allow access to highly regioselective outcomes that were not possible using previously developed protocols. Previous mechanistic studies have shown the rate determining step to be metallacycle-forming oxidative cyclization, however this study shows that reaction conditions can be changed so that the rate determining step can be altered for one of the two product pathways. These modified conditions render metallacycle formation reversible for one of the product pathways and sigma bond metathesis becomes rate determining. This improved mechanistic understanding has allowed access to highly regioselective outcomes for a variety of substrates that were not previously possible. The selectivity for a variety of biased alkynes has been increased to >98:2 in many cases that had previously only been ~4:1.
Methodology has been developed to allow bench stable and inexpensive nickel sources to be used in reductive couplings, eliminating the need for a glove box or air sensitive reducing agents. Previous protocols have typically employed air sensitive Ni0 sources or air-sensitive reducing agents, and this methodology eliminates the need for either of these. Two different protocols have been developed to efficiently couple a variety of aldehydes and alkynes using inexpensive and bench stable NiII pre-catalysts as well as bench-stable trialkylsilane reducing agents.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113425/1/jacksoev_1.pd
A Spitzer/IRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. I. WLM
We present Spitzer/IRAC observations at 3.6 and 4.5 microns along with
optical data from the Local Group Galaxies Survey to investigate the evolved
stellar population of the Local Group dwarf irregular galaxy WLM. These
observations provide a nearly complete census of the asymptotic giant branch
(AGB) stars. We find 39% of the infrared-detected AGB stars are not detected in
the optical data, even though our 50% completeness limit is three magnitudes
fainter than the red giant branch tip. An additional 4% of the
infrared-detected AGBs are misidentified in the optical, presumably due to
reddening by circumstellar dust. We also compare our results with those of a
narrow-band optical carbon star survey of WLM, and find the latter study
sensitive to only 18% of the total AGB population. We detect objects with
infrared fluxes consistent with them being mass-losing AGB stars, and derive a
present day total mass-loss rate from the AGB stars of 0.7-2.4 x 10^(-3) solar
masses per year. The distribution of mass-loss rates and bolometric
luminosities of AGBs and red supergiants are very similar to those in the LMC
and SMC and the empirical maximum mass-loss rate observed in the LMC and SMC is
in excellent agreement with our WLM data.Comment: Accepted by ApJ, 34 pages, 13 figures, version with high-resolution
figures available at: http://webusers.astro.umn.edu/~djackson
Galaxy Properties at the Faint End of the H I Mass Function
The Survey of H I in Extremely Low-mass Dwarfs (SHIELD) includes a volumetrically complete sample of 82 gas-rich dwarfs with selected from the ALFALFA survey. We are obtaining extensive follow-up observations of the SHIELD galaxies to study their gas, stellar, and chemical content, and to better understand galaxy evolution at the faint end of the H I mass function. Here, we investigate the properties of 30 SHIELD galaxies using Hubble Space Telescope imaging of their resolved stars and Westerbork Synthesis Radio Telescope observations of their neutral hydrogen. We measure tip of the red giant branch (TRGB) distances, star formation activity, and gas properties. The TRGB distances are up to 4Ă greater than estimates from flow models, highlighting the importance of velocity-independent distance indicators in the nearby universe. The SHIELD galaxies are in underdense regions, with 23% located in voids; one galaxy appears paired with a more massive dwarf. We quantify galaxy properties at low masses including stellar and H I masses, star formation rate (SFRs), specific SFRs, star formation efficiencies, birth-rate parameters, and gas fractions. The lowest-mass systems lie below the mass thresholds where stellar mass assembly is predicted to be impacted by reionization. Even so, we find the star formation properties follow the same trends as higher-mass gas-rich systems, albeit with a different normalization. The H I disks are small ( ), making it difficult to measure the H I rotation using standard techniques; we develop a new methodology and report the velocity extent, and its associated spatial extent, with robust uncertainties
On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
We report 4.5 micron luminosities for 27 nearby (D < 5 Mpc) dwarf irregular
galaxies measured with the Spitzer Infrared Array Camera. We have constructed
the 4.5 micron luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with
secure distance and interstellar medium oxygen abundance measurements. The 4.5
micron L-Z relation is 12+log(O/H) = (5.78 +/- 0.21) + (-0.122 +/- 0.012)
M_[4.5], where M_[4.5] is the absolute magnitude at 4.5 micron. The dispersion
in the near-infrared L-Z relation is smaller than the corresponding dispersion
in the optical L-Z relation. The subsequently derived stellar mass-metallicity
M-Z relation is 12+log(O/H) = (5.65 +/- 0.23) + (0.298 +/- 0.030) log Mstar.
and extends the SDSS M-Z relation to lower mass by about 2.5 dex. We find that
the dispersion in the M-Z relation is similar over five orders of magnitude in
stellar mass, and that the relationship between stellar mass and interstellar
medium metallicity is similarly tight from high-mass to low-mass systems. We
find a larger scatter at low mass in the relation between effective yield and
total baryonic mass. In fact, there are a few dwarf galaxies with large yields,
which is difficult to explain if galactic winds are ubiquitous in dwarf
galaxies. The low scatter in the L-Z and M-Z relationships are difficult to
understand if galactic superwinds or blowout are responsible for the low
metallicities at low mass or luminosity. Naively, one would expect an ever
increasing scatter at lower masses, which is not observed.Comment: Accepted, Ap.J.; 18 pages (AASTeX 5.2; emulateapj.cls) with 12
figures. Full paper with figures at http://www.astro.umn.edu/~hlee/papers
Hydrophilic microporous membranes for selective ion separation and flow-battery energy storage
Membranes with fast and selective ion transport are widely used for water purification and devices for energy conversion and storage including fuel cells, redox flow batteries and electrochemical reactors. However, it remains challenging to design cost-effective, easily processed ion-conductive membranes with well-defined pore architectures. Here, we report a new approach to designing membranes with narrow molecular-sized channels and hydrophilic functionality that enable fast transport of salt ions and high size-exclusion selectivity towards small organic molecules. These membranes, based on polymers of intrinsic microporosity containing Trögerâs base or amidoxime groups, demonstrate that exquisite control over subnanometre pore structure, the introduction of hydrophilic functional groups and thickness control all play important roles in achieving fast ion transport combined with high molecular selectivity. These membranes enable aqueous organic flow batteries with high energy efficiency and high capacity retention, suggesting their utility for a variety of energy-related devices and water purification processes
Hot Dust and PAH Emission at Low Metallicity: A Spitzer Survey of Local Group and Other Nearby Dwarf Galaxies
We present Spitzer Space Telescope 4.5 and 8.0 micron imaging of 15 Local
Group and nearby dwarf galaxies. Our sample spans a range of more than one dex
in nebular metallicity and over three orders of magnitude in current star
formation rate, allowing us to examine the dependence of the diffuse 8 micron
emission, originating from hot dust and PAHs, on these parameters. We detect
prominent diffuse 8 micron emission in four of the most luminous galaxies in
the sample (IC 1613, IC 5152, NGC 55, and NGC 3109), low surface brightness
emission from four others (DDO 216, Sextans A, Sextans B, WLM), and no diffuse
emission from the remaining objects. We observe general correlations of the
diffuse 8 micron emission with both the current star formation rate and the
nebular metallicity of the galaxies in our sample. However, we also see
exceptions to these correlations that suggest other processes may also have a
significant effect on the generation of hot dust/PAH emission. These systems
all have evidence for old and intermediate age star formation, thus the lack of
diffuse 8 micron emission cannot be attributed to young galaxy ages. Also, we
find that winds are unlikely to explain the paucity of diffuse 8 micron
emission, since high resolution imaging of the neutral gas in these objects
show no evidence of blowout. Additionally, we propose the lack of diffuse 8
micron emission in low-metallicity systems may be due to the destruction of
dust grains by supernova shocks, assuming the timescale to regrow dust grains
and PAH molecules is long compared to the destruction timescale. The most
likely explanation for the observed weak diffuse 8 micron emission is at least
partly due to a general absence of dust (including PAHs), in agreement with
their low metallicities.Comment: Accepted by ApJ, 26 pages, 8 figures, version with high-resolution
figures available at: http://webusers.astro.umn.edu/~djackson
A Student\u27s Guide to giant Viruses Infecting Small Eukaryotes: From Acanthamoeba to Zooxanthellae
The discovery of infectious particles that challenge conventional thoughts concerning âwhat is a virusâ has led to the evolution a new field of study in the past decade. Here, we review knowledge and information concerning âgiant virusesâ, with a focus not only on some of the best studied systems, but also provide an effort to illuminate systems yet to be better resolved. We conclude by demonstrating that there is an abundance of new hostâvirus systems that fall into this âgiantâ category, demonstrating that this field of inquiry presents great opportunities for future research
Dysfunctional BMPR2 signaling drives an abnormal endothelial requirement for glutamine in pulmonary arterial hypertension
Pulmonary arterial hypertension (PAH) is increasingly recognized as a systemic disease driven by alteration in the normal functioning of multiple metabolic pathways affecting all of the major carbon substrates, including amino acids. We found that human pulmonary hypertension patients (WHO Group I, PAH) exhibit systemic and pulmonary-specific alterations in glutamine metabolism, with the diseased pulmonary vasculature taking up significantly more glutamine than that of controls. Using cell culture models and transgenic mice expressing PAH-causing BMPR2 mutations, we found that the pulmonary endothelium in PAH shunts significantly more glutamine carbon into the tricarboxylic acid (TCA) cycle than wild-type endothelium. Increased glutamine metabolism through the TCA cycle is required by the endothelium in PAH to survive, to sustain normal energetics, and to manifest the hyperproliferative phenotype characteristic of disease. The strict requirement for glutamine is driven by loss of sirtuin-3 (SIRT3) activity through covalent modification by reactive products of lipid peroxidation. Using 2-hydroxybenzylamine, a scavenger of reactive lipid peroxidation products, we were able to preserve SIRT3 function, to normalize glutamine metabolism, and to prevent the development of PAH in BMPR2 mutant mice. In PAH, targeting glutamine metabolism and the mechanisms that underlie glutamine-driven metabolic reprogramming represent a viable novel avenue for the development of potentially disease-modifying therapeutics that could be rapidly translated to human studies
Richness and resilience in the Pacific: DNA metabarcoding enables parallelized evaluation of biogeographic patterns
Islands make up a large proportion of Earth\u27s biodiversity, yet are also some of the most sensitive systems to environmental perturbation. Biogeographic theory predicts that geologic age, area, and isolation typically drive islands\u27 diversity patterns, and thus potentially impact non-native spread and community homogenization across island systems. One limitation in testing such predictions has been the difficulty of performing comprehensive inventories of island biotas and distinguishing native from introduced taxa. Here, we use DNA metabarcoding and statistical modelling as a high throughput method to survey community-wide arthropod richness, the proportion of native and non-native species, and the incursion of non-natives into primary habitats on three archipelagos in the Pacific â the Ryukyus, the Marianas and Hawaii â which vary in age, isolation and area. Diversity patterns largely match expectations based on island biogeography theory, with the oldest and most geographically connected archipelago, the Ryukyus, showing the highest taxonomic richness and lowest proportion of introduced species. Moreover, we find evidence that forest habitats are more resilient to incursions of non-natives in the Ryukyus than in the less taxonomically rich archipelagos. Surprisingly, we do not find evidence for biotic homogenization across these three archipelagos: the assemblage of non-native species on each island is highly distinct. Our study demonstrates the potential of DNA metabarcoding to facilitate rapid estimation of biogeographic patterns, the spread of non-native species, and the resilience of ecosystems
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Songbirds as sentinels of mercury in terrestrial habitats of eastern North America
Mercury (Hg) is a globally distributed environmental
contaminant with a variety of deleterious effects
in fish, wildlife, and humans. Breeding songbirds may be
useful sentinels for Hg across diverse habitats because they
can be effectively sampled, have well-defined and small
territories, and can integrate pollutant exposure over time
and space. We analyzed blood total Hg concentrations from
8,446 individuals of 102 species of songbirds, sampled on
their breeding territories across 161 sites in eastern North
America [geometric mean Hg concentration = 0.25 ÎŒg/g
wet weight (ww), range < 0.01â14.60 ÎŒg/g ww]. Our
records span an important time periodâthe decade leading
up to implementation of the USEPA Mercury and Air
Toxics Standards, which will reduce Hg emissions from
coal-fired power plants by over 90%. Mixed-effects
modeling indicated that habitat, foraging guild, and age
were important predictors of blood Hg concentrations
across species and sites. Blood Hg concentrations in adult
invertebrate-eating songbirds were consistently higher in
wetland habitats (freshwater or estuarine) than upland
forests. Generally, adults exhibited higher blood Hg concentrations
than juveniles within each habitat type. We
used model results to examine species-specific differences
in blood Hg concentrations during this time period, identifying
potential Hg sentinels in each region and habitat
type. Our results present the most comprehensive assessment
of blood Hg concentrations in eastern songbirds to
date, and thereby provide a valuable framework for
designing and evaluating risk assessment schemes using
sentinel songbird species in the time after implementation
of the new atmospheric Hg standards.Keywords: Songbird, Sentinel, Bioaccumulation, Mercury, Passeriforme
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