181 research outputs found
Synthesis and post-polymerization reaction of end-clickable stereoregular polymethacrylates via termination of stereospecific living anionic polymerization
Poly(methyl methacrylate)s with high stereoregularity and clickable end-groups were synthesized via terminating reactions with α-(halomethyl)acrylates in stereospecific living anionic polymerization. The terminating reaction was efficient and tolerant to the reaction conditions to such an extent that almost quantitative end-functionalization was achieved in isotactic- and syndiotactic-specific polymerization systems. The terminating reactions were also achieved in polymerizations of vinyl methacrylate and trimethylsilyl methacrylate. For the polymerization of butyl acrylate, however, the termination efficiency was limited to less than 69%. Furthermore, the quantitative end-functionalization of the incorporated C[double bond, length as m-dash]C double bond at ω-end was achieved with various thiols catalyzed by Et3N. The base-catalysed thiol–ene reaction of the stereoregular poly(vinyl methacrylate) with a ω-end C[double bond, length as m-dash]C double bond selectively proceeded to retain vinyl ester functions, and the subsequent hydrolysis afforded ω-functional stereoregular poly(methacrylic acid). A combination of the terminating agent with a protected lithium amide afforded stereoregular poly(methyl methacrylates) with orthogonally clickable α- and ω-ends.ArticlePolymer Chemistry.6(7):1078-1087(2015)journal articl
Rapidly Accreting Supergiant Protostars: Embryos of Supermassive Black Holes?
Direct collapse of supermassive stars (SMSs) is a possible pathway for
generating supermassive black holes in the early universe. It is expected that
an SMS could form via very rapid mass accretion with Mdot ~ 0.1 - 1 Msun/yr
during the gravitational collapse of an atomic-cooling primordial gas cloud. In
this paper we study how stars would evolve under such extreme rapid mass
accretion, focusing on the early evolution until the stellar mass reaches 1000
Msun. To this end we numerically calculate the detailed interior structure of
accreting stars with primordial element abundances. Our results show that for
accretion rates higher than 0.01 Msun/yr, stellar evolution is qualitatively
different from that expected at lower rates. While accreting at these high
rates the star always has a radius exceeding 100 Rsun, which increases
monotonically with the stellar mass. The mass-radius relation for stellar
masses exceeding ~ 100 Msun follows the same track with R_* \propto M_*^0.5 in
all cases with accretion rates > 0.01 Msun/yr; at a stellar mass of 1000 Msun
the radius is about 7000 Rsun (~= 30 AU). With higher accretion rates the onset
of hydrogen burning is shifted towards higher stellar masses. In particular,
for accretion rates exceeding Mdot > 0.1 Msun/yr, there is no significant
hydrogen burning even after 1000 Msun have accreted onto the protostar. Such
"supergiant" protostars have effective temperatures as low as Teff ~= 5000 K
throughout their evolution and because they hardly emit ionizing photons, they
do not create an HII region or significantly heat their immediate surroundings.
Thus, radiative feedback is unable to hinder the growth of rapidly accreting
stars to masses in excess of 1000 Msun, as long as material is accreted at
rates Mdot > 0.01 Msun/yr.Comment: 11 pages, 10 figure
Targeting protein homodimerization: A novel drug discovery system
AbstractTo identify a novel class of antibiotics, we have developed a high-throughput genetic system for targeting the homodimerization (HD system) of histidine kinase (HK), which is essential for a bacterial signal transduction system (two-component system, TCS). By using the HD system, we screened a chemical library and identified a compound, I-8-15 (1-dodecyl-2-isopropylimidazole), that specifically inhibited the dimerization of HK encoded by the YycG gene of Staphylococcus aureus and induced concomitant bacterial cell death. I-8-15 also showed antibacterial activity against MRSA (methicillin-resistant S. aureus) and VRE (vancomycin-resistant Enterococcus faecalis) with MICs at 25 and 50μg/ml, respectively
Sperm allocation in relation to female size in a semelparous salmonid
To maximize reproductive success, males have to adaptively tailor their sperm expenditure in relation to the quality of potential mates because they require time to replenish their sperm supply for subsequent mating opportunities. Therefore, in mating contexts where males must choose among females in a short period of time, as is the case with semelparous species (which die after one intensely competitive short duration breeding season), selection on sperm allocation can be expected to be a powerful selective agent that shapes the male reproductive success. We quantitatively investigated sperm allocation patterns in chum salmon in relation to perceived female quality by developing a novel method for determining the amount of sperm allocated per ejaculate during spawning bouts. We examined the relationship between sperm expenditure and the body size of paired females (a proxy of egg number and egg quality) in the absence of male–male competition in an experimental channel. The estimated amount of sperm released per spawning event was positively correlated with the size of paired females. However, the number of spawning events a female participated in, which reduces the number of eggs she spawns in each subsequent bout, did not affect this relationship. These results provide support for predictions arising from the sperm allocation hypothesis, male salmon do economize their sperm expenditure in accordance with paired female body size as predicted for their first spawning event, but males overestimate or are unable to assess the quality of females beyond size and provide more sperm than they should in theory when paired with a female that spawned previously. Overall, the observed sperm allocation pattern in chum salmon appears to be adapted to maximize reproductive success assuming female size is an honest indicator of quality, although temporal changes in a female\u27s quality during a reproductive season should be considered when examining sperm allocation strategies
Constraints on the Intracluster Dust Emission in the Coma Cluster of Galaxies
We have undertaken a search for the infrared emission from the intracluster
dust in the Coma cluster of galaxies by the Multiband Imaging Photometer for
Spitzer. Our observations yield the deepest mid and far-infrared images of a
galaxy cluster ever achieved. In each of the three bands, we have not detected
a signature of the central excess component in contrast to the previous report
on the detection by Infrared Space Observatory (ISO). We still find that the
brightness ratio between 70 and 160 microns shows a marginal sign of the
central excess, in qualitative agreement with the ISO result. Our analysis
suggests that the excess ratio is more likely due to faint infrared sources
lying on fluctuating cirrus foreground. Our observations yield the 2 sigma
upper limits on the excess emission within 100 kpc of the cluster center as 5 x
10^-3 MJy/sr, 6 x 10^-2 MJy/sr, and 7 x 10^-2 MJy/sr, at 24, 70, and 160
microns, respectively. These values are in agreement with those found in other
galaxy clusters and suggest that dust is deficient near the cluster center by
more than 3 orders of magnitude compared to the interstellar medium.Comment: 10 pages, 9 figures, minor changes to match version published in Ap
Regression of Necrotic Lesions after Methotrexate Withdrawal in Patients with Methotrexate-Associated Lymphoproliferative Disorders: A Retrospective CT Study
This retrospective study investigated whether necrotic lesions detected on a computed tomography (CT) scan are more regressive than non-necrotic lesions after methotrexate withdrawal in patients pathologically diagnosed with methotrexate-associated lymphoproliferative disorders (MTX-LPD). In total, 89 lesions extracted from 24 patients on CT scans were included in the analysis. All patients had been evaluated for the presence of necrosis within lesions via CT scan upon first suspicion of MTX-LPD (baseline CT scan). The percentage lesion size reduction between the baseline and initial follow-up CT scan was calculated. The association between necrosis within lesions and size changes was estimated via linear regression analyses using both crude and adjusted models. Necrosis was significantly more common in extranodal lesions (27 out of 30 lesions, 90%) than in nodal lesions (9 out of 59 lesions, 15%, p<0.001). In the crude model, the regression of necrotic lesions was 58.5% greater than that of non-necrotic lesions; the difference was statistically significant (p<0.001). Additionally, the longest diameter of necrotic lesions at the baseline CT scan was significantly greater than that of non-necrotic lesions (p<0.001). Based on the adjusted model, necrotic lesions showed 49.3% greater regression than non-necrotic lesions (p=0.017). Necrosis detected on a CT scan was found to be an independent predictor of regression after MTX withdrawal in patients with MTX-LPD
Effects of Low-Dose Non-Caloric Sweetener Consumption on Gut Microbiota in Mice
Non-caloric artificial sweeteners (NASs) provide sweet tastes to food without adding calories or glucose. NASs can be used as alternative sweeteners for controlling blood glucose levels and weight gain. Although the consumption of NASs has increased over the past decade in Japan and other countries, whether these sweeteners affect the composition of the gut microbiome is unclear. In the present study, we examined the effects of sucralose or acesulfame-K ingestion (at most the maximum acceptable daily intake (ADI) levels, 15 mg/kg body weight) on the gut microbiome in mice. Consumption of sucralose, but not acesulfame-K, for 8 weeks reduced the relative amount of Clostridium cluster XIVa in feces. Meanwhile, sucralose and acesulfame-K did not increase food intake, body weight gain or liver weight, or fat in the epididymis or cecum. Only sucralose intake increased the concentration of hepatic cholesterol and cholic acid. Moreover, the relative concentration of butyrate and the ratio of secondary/primary bile acids in luminal metabolites increased with sucralose consumption in a dose-dependent manner. These results suggest that daily intake of maximum ADI levels of sucralose, but not acesulfame-K, affected the relative amount of the Clostridium cluster XIVa in fecal microbiome and cholesterol bile acid metabolism in mice
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