344 research outputs found
Pyrimidine nucleotide starvation induces a decrease in the number of effector T cells but not memory T cells
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Linear stability of two-dimensional steady flow in wavy-walled channels
No full textLinear stability of fully developed two-dimensional periodic steady flows in sinusoidal wavy-walled channels is investigated numerically. Two types of channels are considered: the geometry of wavy walls is identical and the location of the crest of the lower and upper walls coincides (symmetric channel) or the crest of the lower wall corresponds to the furrow of the upper wall (sinuous channel). It is found that the critical Reynolds number is substantially lower than that for plane channel flow and that when the non-dimensionalized wall variation amplitude epsilon is smaller than a critical value (about 0.26 for symmetric channel, 0.28 for sinuous channel), critical modes are three-dimensional stationary and for larger epsilon, two-dimensional oscillatory instabilities set in. Critical Reynolds numbers of sinuous channel flows are smaller for three-dimensional disturbances and larger for two-dimensional disturbances than those of symmetric channel flows. The disturbance velocity distribution obtained by the linear stability analysis suggests that the three-dimensional stationary instability is mainly caused by local concavity of basic flows near the reattachment point, while the critical two-dimensional mode resembles closely the Tollmien-Schlichting wave for plane Poiseuille flow. (C) 1998 The Japan Society of Fluid Mechanics and Elsevier Science B.V. All rights reserved
Effects of host and pathogenicity on mutation rates in avian influenza A viruses
No full textMutation is the primary determinant of genetic diversity in influenza viruses. The rate of mutation, measured in an absolute time-scale, is likely to be dependent on the rate of errors in copying RNA sequences per replication and the number of replications per unit time. Conditions for viral replication are probably different among host taxa, potentially generating the host specificity of the viral mutation rate, and possibly between highly and low pathogenic (HP and LP) viruses. This study investigated whether mutation rates per year in avian influenza A viruses depend on host taxa and pathogenicity. We inferred mutation rates from the rates of synonymous substitutions, which are assumed to be neutral and thus equal to mutation rates, at four segments that code internal viral proteins (PB2, PB1, PA, NP). On the phylogeny of all avian viral sequences for each segment, multiple distinct subtrees (clades) were identified that represent viral subpopulations, which are likely to have evolved within particular host taxa. Using simple regression analysis, we found that mutation rates were significantly higher in viruses infecting chickens than domestic ducks and in those infecting wild shorebirds than wild ducks. Host dependency of the substitution rate was also confirmed by Bayesian phylogenetic analysis. However, we did not find evidence that the mutation rate is higher in HP than in LP viruses. We discuss these results considering viral replication rate as the major determinant of mutation rate per unit time.Y
Iridium-catalysed arylation of C-H bonds enabled by oxidatively induced reductive elimination
No full textDirect arylation of C-H bonds is in principle a powerful way of preparing value-added molecules that contain carbon-aryl fragments. Unfortunately, currently available synthetic methods are not sufficiently effective to be practical alternatives to conventional cross-coupling reactions. We propose that the main problem lies in the late portion of the catalytic cycle where reductive elimination gives the desired carbon-aryl bond. Accordingly, we have developed a strategy where the Ir(III) centre of the key intermediate is first oxidized to Ir(IV). Density functional theory calculations indicate that the barrier to reductive elimination is reduced by nearly 19 kcal mol-1 for this oxidized complex compared with that of its Ir(III) counterpart. Various experiments confirm this prediction, affording a new methodology capable of directly arylating C-H bonds at room temperature with a broad substrate scope and in good yields. This work highlights how the oxidation states of intermediates can be targeted deliberately to catalyse an otherwise impossible reaction. ©2018 Macmillan Publishers Limited, part of Springer Nature. (c) All rights reserve
Why are children less affected than adults by severe acute respiratory syndrome coronavirus 2 infection?
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Regioselective Growth of Colloidal Crystals Induced by Depletion Attraction
No full textColloidal crystals display photonic stopbands that generate reflective structural colors. While micropatterning offers significant value for various applications, the resolution is somewhat limited for conventional top-down approaches. In this work, a simple, single-step bottom-up approach is introduced to produce photonic micropatterns through depletion-mediated regioselective growth of colloidal crystals. Lithographically-featured micropatterns with planar surfaces and nano-needle arrays as substrates are employed. Heterogeneous nucleation is drastically suppressed on nano-needle arrays due to minimal particle-to-needles overlap of excluded volumes, while it is promoted on planar surfaces with large particle-to-plane volume overlap, enabling regioselective growth of colloidal crystals. This strategy allows high-resolution micropatterning of colloidal photonic crystals, with a minimum feature size as small as 10 mu m. Stopband positions, or structural colors, are controllable through concentration and depletant and salt, as well as particle size. Notably, secondary colors can be created through structural color mixing by simultaneously crystallizing two different particle sizes into their own crystal grains, resulting in two distinct reflectance peaks at controlled wavelengths. The simple and highly reproducible method for regioselective colloidal crystallization provides a general route for designing elaborate photonic micropatterns suitable for various applications. Photonic micropatterns are created through single-step growth of colloidal crystals on lithographically prepatterned substrates with planar surfaces and nano-needle arrays. When the colloidal assembly is mediated by depletion attraction, the planar surfaces effectively cause heterogeneous nucleation and crystal growth, whereas the nano-needle arrays inhibit it. This results in regioselective crystallization on the planar areas, yielding high-resolution structural-color graphics.image
An experimental and computational analysis of the formation of the terminal nitrido complex (eta(3)-Cp*)(2)Mo(N)(N-3) by elimination of N-2 from CP*Mo-2(N-3)(2): The barrier to elimination is strongly influenced by the exo versus endo configuration of the azide ligand
No full textHyperglycemia-induced oxidative stress promotes tumor metastasis by upregulating vWF expression in endothelial cells through the transcription factor GATA1
No full textDiabetes mellitus (DM) characterized by hyperglycemia is a chronic metabolic disorder that leads to many symptoms and vascular complications. Despite the close association between DM and cancer progression, the response and role of endothelial cells (ECs) under diabetic conditions in tumor metastasis remain to be elucidated. In this study, we sought to determine whether and how ECs under diabetic conditions contribute to tumor metastasis. We have taken advantage of syngeneic mouse tumor models of Lewis lung carcinoma (LLC) and melanoma (B16F10) cells and a streptozotocin (STZ)-induced hyperglycemia model. We demonstrated that hyperglycemia increased the metastasis of LLC and B16F10 cells in an experimental metastasis model with an intravenous injection of the tumor cells. We also found that hyperglycemia promoted lung metastasis of tumor cells by increasing the adhesiveness of ECs to facilitate the adhesion of tumor cells to ECs rather than affecting the metastatic behavior of tumor cells themselves. From the analysis of gene expression in primary lung ECs from STZ-treated mice, we identified that vWF promoted the adhesion of tumor cells to ECs and the transendothelial migration of tumor cells. Mechanistically, hyperglycemia-induced oxidative stress in ECs, and increased oxidative stress enhanced vWF expression in ECs through an increase in the transcription factor GATA1. These results provide evidence for the role of vWF in ECs in promoting hyperglycemia-induced tumor metastasis and potential therapeutic targets for the regulation of vWF expression in ECs and hyperglycemia-induced tumor metastasis.N
Copper-Catalyzed Enantiotopic-Group-Selective Allylation of gem-Diborylalkanes
No full textWe report a copper-catalyzed enatiotopic-group-selective allylation of gem-diborylalkanes with allyl bromides. The combination of copper(I) bromide and H-8-BINOL derived phosphoramidite ligand proved to be the most effective catalytic system to provide various enantioenriched homoallylic boronate esters, containing a boron-substituted stereogenic center that is solely derived from gem-diborylalkanes, in good yields with high enantiomeric ratios under mild conditions. Experimental and theoretical studies have been conducted to elucidate the reaction mechanism, revealing how the enatiotopic-group-selective trans-metalation of gem-diborylalkanes with chiral copper complex occurs to generate chiral alpha-borylalkyl-copper species for the first time. Additional synthetic applications to the synthesis of various chiral building blocks are also included.
A non-classical hydrogen bond in the molybdenum arene complex [eta(6)-C6H5C6H3(Ph)OH]Mo(PMe3)(3): evidence that hydrogen bonding facilitates oxidative addition of the O-H bond
No full textMo(PMe3)(6) reacts with 2,6-Ph2C6H3OH to give the eta(6)-arene complex [eta(6)-C6H5C6H3(Ph)OH]Mo(PMe3)(3) which exhibits a non-classical Mo...H-OAr hydrogen bond; DFT calculations indicate that the hydrogen bonding interaction facilitates oxidative addition of the O-H bond to give [eta(6),eta(1)- C6H5C6H3(Ph)O]Mo(PMe3)(2)H
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