ABSTRACT Compressive fluctuations are a minor yet significant component of astrophysical plasma turbulence. In the solar wind, long-wavelength compressive slow-mode fluctuations lead to changes in ...and in , where and are the perpendicular and parallel temperatures of the protons, B is the magnetic field strength, and is the proton density. If the amplitude of the compressive fluctuations is large enough, crosses one or more instability thresholds for anisotropy-driven microinstabilities. The enhanced field fluctuations from these microinstabilities scatter the protons so as to reduce the anisotropy of the pressure tensor. We propose that this scattering drives the average value of away from the marginal stability boundary until the fluctuating value of stops crossing the boundary. We model this "fluctuating-anisotropy effect" using linear Vlasov-Maxwell theory to describe the large-scale compressive fluctuations. We argue that this effect can explain why, in the nearly collisionless solar wind, the average value of is close to unity.
The rhodamine system is a flexible framework for building small‐molecule fluorescent probes. Changing N‐substitution patterns and replacing the xanthene oxygen with a dimethylsilicon moiety can shift ...the absorption and fluorescence emission maxima of rhodamine dyes to longer wavelengths. Acylation of the rhodamine nitrogen atoms forces the molecule to adopt a nonfluorescent lactone form, providing a convenient method to make fluorogenic compounds. Herein, we take advantage of all of these structural manipulations and describe a novel photoactivatable fluorophore based on a Si‐containing analogue of Q‐rhodamine. This probe is the first example of a “caged” Si‐rhodamine, exhibits higher photon counts compared to established localization microscopy dyes, and is sufficiently red‐shifted to allow multicolor imaging. The dye is a useful label for super‐resolution imaging and constitutes a new scaffold for far‐red fluorogenic molecules.
In the red zone: Caged Si‐Q‐rhodamine, a new red‐shifted label for localization microscopy, exhibits superior photon counts and is sufficiently red‐shifted to allow multicolor imaging. This dye is a useful label for advanced imaging experiments and constitutes a new scaffold for red fluorogenic molecules.
Abstract
In this work, we investigate how the complex structure found in solar wind proton velocity distribution functions (VDFs), rather than the commonly assumed two-component bi-Maxwellian ...structure, affects the onset and evolution of parallel-propagating microinstabilities. We use the
Arbitrary Linear Plasma Solver
, a numerical dispersion solver, to find the real frequencies and growth/damping rates of the Alfvén modes calculated for proton VDFs extracted from Wind spacecraft observations of the solar wind. We compare this wave behavior to that obtained by applying the same procedure to core-and-beam bi-Maxwellian fits of the Wind proton VDFs. We find several significant differences in the plasma waves obtained for the extracted data and bi-Maxwellian fits, including a strong dependence of the growth/damping rate on the shape of the VDF. By applying the quasilinear diffusion operator to these VDFs, we pinpoint resonantly interacting regions in velocity space where differences in VDF structure significantly affect the wave growth and damping rates. This demonstration of the sensitive dependence of Alfvén mode behavior on VDF structure may explain why the Alfvén ion-cyclotron instability thresholds predicted by linear theory for bi-Maxwellian models of solar wind proton background VDFs do not entirely constrain spacecraft observations of solar wind proton VDFs, such as those made by the Wind spacecraft.
ABSTRACT We investigate how the proton distribution function evolves when the protons undergo stochastic heating by strong, low-frequency, Alfvén-wave turbulence under the assumption that β is small. ...We apply our analysis to protons undergoing stochastic heating in the supersonic fast solar wind and obtain proton distributions at heliocentric distances ranging from 4 to 30 solar radii. We find that the proton distribution develops non-Gaussian structure with a flat core and steep tail. For , the proton distribution is well approximated by a modified Moyal distribution. Comparisons with future measurements from Solar Probe Plus could be used to test whether stochastic heating is occurring in the solar-wind acceleration region.
The dissipation of magnetized turbulence is an important paradigm for describing heating and energy transfer in astrophysical environments such as the solar corona and wind; however, the specific ...collisionless processes behind dissipation and heating remain relatively unconstrained by measurements. Remote sensing observations have suggested the presence of strong temperature anisotropy in the solar corona consistent with cyclotron resonant heating. In the solar wind, in situ magnetic field measurements reveal the presence of cyclotron waves, while measured ion velocity distribution functions have hinted at the active presence of cyclotron resonance. Here, we present Parker Solar Probe observations that connect the presence of ion-cyclotron waves directly to signatures of resonant damping in observed proton-velocity distributions using the framework of quasilinear theory. We show that the quasilinear evolution of the observed distribution functions should absorb the observed cyclotron wave population with a heating rate of 10^{-14} W/m^{3}, indicating significant heating of the solar wind.
The M2 proton channel from influenza A virus is an essential protein that mediates transport of protons across the viral envelope. This protein has a single transmembrane helix, which tetramerizes ...into the active channel. At the heart of the conduction mechanism is the exchange of protons between the His37 imidazole moieties of M2 and waters confined to the M2 bundle interior. Protons are conducted as the total charge of the four His37 side chains passes through 2 + and 3 + with a pK a near 6. A 1.65 Å resolution X-ray structure of the transmembrane protein (residues 25–46), crystallized at pH 6.5, reveals a pore that is lined by alternating layers of sidechains and well-ordered water clusters, which offer a pathway for proton conduction. The His37 residues form a box-like structure, bounded on either side by water clusters with well-ordered oxygen atoms at close distance. The conformation of the protein, which is intermediate between structures previously solved at higher and lower pH, suggests a mechanism by which conformational changes might facilitate asymmetric diffusion through the channel in the presence of a proton gradient. Moreover, protons diffusing through the channel need not be localized to a single His37 imidazole, but instead may be delocalized over the entire His-box and associated water clusters. Thus, the new crystal structure provides a possible unification of the discrete site versus continuum conduction models.
Performance feedback (PF) has become popular in the school-based literature as a method to increase the treatment integrity of prescribed interventions. The current study extracted and aggregated ...data from single-case studies that used PF in school settings to increase teachers' use of classroom-based interventions. Results suggest PF is moderately effective at increasing integrity after a new strategy or intervention has been introduced, curbing a general tendency for declining integrity following skill training. The type of skill for which teachers received PF and setting moderated the effect. Important questions remain, however, regarding the use of PF in schools and the most effective method for its implementation.
The computational design of advanced materials based on surfactant self-assembly without ever stepping foot in the laboratory is an important goal, but there are significant barriers to this ...approach, because of the limited spatial and temporal scales accessible by computer simulations. In this paper, we report our work to bridge the gap between laboratory and computational time scales by implementing the coarse-grained (CG) force field previously reported by Shinoda et al. Shinoda, W.; DeVane, R.; Klein, M. L. Mol. Simul. 2007, 33, 27–36 into the HOOMD-Blue graphical processing unit (GPU)-accelerated molecular dynamics (MD) software package previously reported by Anderson et al. Anderson, J. A.; Lorenz, C. D.; Travesset, A. J. Comput. Phys. 2008, 227, 5342–5359. For a system of 25 750 particles, this implementation provides performance on a single GPU, which is superior to that of a widely used parallel MD simulation code running on an optimally sized CPU-based cluster. Using our GPU setup, we have collected 0.6 ms of MD trajectory data for aqueous solutions of 7 different nonionic polyethylene glycol (PEG) surfactants, with most of the systems studied representing ∼1 000 000 atoms. From this data, we calculated various properties as a function of the length of the hydrophobic tails and PEG head groups. Specifically, we determined critical micelle concentrations (CMCs), which are in good agreement with experimental data, and characterized the size and shape of micelles. However, even with the microsecond trajectories employed in this study, we observed that the micelles composed of relatively hydrophobic surfactants are continuing to grow at the end of our simulations. This suggests that the final micelle size distributions of these systems are strongly dependent on initial conditions and that either longer simulations or advanced sampling techniques are needed to properly sample their equilibrium distributions. Nonetheless, the combination of coarse-grained modeling and GPU acceleration marks a significant step toward the computational prediction of the thermodynamic properties of slowly evolving surfactant systems.
The A/M2 protein of influenza A virus forms a tetrameric proton-selective pH-gated ion channel. The H37xxxW41 motif located in the channel pore is responsible for its gating and proton selectivity. ...Channel activation most likely involves protonation of the H37 residues, while the conductive state of the channel is characterized by two or three charged His residues in a tetrad. A/M2 channel activity is inhibited by the antiviral drug amantadine. Although a large number of functional amantadine-resistant mutants of A/M2 have been observed in vitro, only a few are observed in highly transmissible viruses in the presence or absence of amantadine. We therefore examined 49 point mutants of the pore-lining residues, representing both natural and nonnatural variants. Their ion selectivity, amantadine sensitivity, specific activity, and pH-dependent conductance were measured in Xenopus oocytes. These measurements showed how variations in the sequence lead to variations in the proton conduction. The results are consistent with a multistep mechanism that allows the protein to fine-tune its pH−rate profile over a wide range of proton concentrations, hypothesized to arise from different protonation states of the H37 tetrad. Mutations that give native-like conductance at low pH as well as minimal leakage current at pH 7.0 were surprisingly rare. Moreover, the results are consistent with a location of the amantadine-binding site inside the channel pore. These findings have helped to define the set of functionally fit mutants that should be targeted when considering the design of novel drugs that inhibit amantadine-resistant strains of influenza A virus.
Prostate cancer is a highly heterogeneous disease and mortality is mainly due to metastases but the initial steps of metastasis have not been well characterized. We have performed integrative whole ...exome sequencing and transcriptome analysis of primary prostate tumor foci and corresponding lymph node metastases (LNM) from 43 patients enrolled in clinical trial. We present evidence that, while there are some cases of clonally independent primary tumor foci, 87% of primary tumor foci and metastases are descended from a common ancestor. We demonstrate that genes related to oxidative phosphorylation are upregulated in LNM and in African-American patients relative to White patients. We further show that mutations in
,
,
,
,
, and
are enriched in prostate cancer metastases. These findings were validated in a meta-analysis of 3929 primary tumors and 2721 metastases and reveal a pattern of molecular alterations underlying the pathology of metastatic prostate cancer. We show that LNM contain multiple subclones that are already present in primary tumor foci. We observed enrichment of mutations in several genes including understudied genes such as
,
,
,
, and
and found that mutations in
and
are associated with a poor outcome in prostate cancer.
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