Problems and Solutions Klein, Benjamin G.; Bivens, Irl C.; King, L. R.
The College mathematics journal,
01/2003, Letnik:
34, Številka:
1
Journal Article
Problems and Solutions Klein, Benjamin G.; Bivens, Irl C.; King, L. R.
The College mathematics journal,
01/2003, Letnik:
34, Številka:
1
Journal Article
Stochastic heating refers to an increase in the average magnetic moment of
charged particles interacting with electromagnetic fluctuations whose
frequencies are smaller than the particles' cyclotron ...frequencies. This type of
heating arises when the amplitude of the gyroscale fluctuations exceeds a
certain threshold, causing particle orbits in the plane perpendicular to the
magnetic field to become stochastic rather than nearly periodic. We consider
the stochastic heating of protons by Alfv\'en-wave (AW) and
kinetic-Alfv\'en-wave (KAW) turbulence, which may make an important
contribution to the heating of the solar wind. Using phenomenological
arguments, we derive the stochastic-proton-heating rate in plasmas in which
$\beta_{\rm p} \sim 1-30$, where $\beta_{\rm p}$ is the ratio of the proton
pressure to the magnetic pressure. (We do not consider the $\beta_{\rm p}
\gtrsim 30$ regime, in which KAWs at the proton gyroscale become
non-propagating.) We test our formula for the stochastic-heating rate by
numerically tracking test-particle protons interacting with a spectrum of
randomly phased AWs and KAWs. Previous studies have demonstrated that at
$\beta_{\rm p} \lesssim 1$, particles are energized primarily by time
variations in the electrostatic potential and thermal-proton gyro-orbits are
stochasticized primarily by gyroscale fluctuations in the electrostatic
potential. In contrast, at $\beta_{\rm p} \gtrsim 1$, particles are energized
primarily by the solenoidal component of the electric field and thermal-proton
gyro-orbits are stochasticized primarily by gyroscale fluctuations in the
magnetic field.
The efficiency of organic bulk heterojunction solar cells strongly depends on the multiscale morphology of the interpenetrating polymer–fullerene network. Understanding the molecular assembly and the ...identification of influencing parameters is essential for a systematic optimization of such devices. Here, we investigate the molecular ordering during the drying of doctor-bladed polymer–fullerene blends on PEDOT:PSS-coated substrates simultaneously using in situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry. In the process of blend crystallization, we observe the nucleation of well-aligned P3HT crystallites in edge-on orientation at the interface at the instant when P3HT solubility is crossed. A comparison of the real-time GIXD study at ternary blends with the binary phase diagrams of the drying blend film gives evidence of strong polymer–fullerene interactions that impede the crystal growth of PCBM, resulting in the aggregation of PCBM in the final drying stage. A systematic dependence of the film roughness on the drying time after crossing P3HT solubility has been shown. The highest efficiencies have been observed for slow drying at low temperatures which showed the strongest P3HT interchain π–π-ordering along the substrate surface. By adding the “unfriendly” solvent cyclohexanone to a chlorobenzene solution of P3HT:PCBM, the solubility can be crossed prior to the drying process. Such solutions exhibit randomly orientated crystalline structures in the freshly cast film which results in a large crystalline orientation distribution in the dry film that has been shown to be beneficial for solar cell performance.
Acetylation of histone H3K23 has emerged as an essential posttranslational modification associated with cancer and learning and memory impairment, yet our understanding of this epigenetic mark ...remains insufficient. Here, we identify the native MORF complex as a histone H3K23-specific acetyltransferase and elucidate its mechanism of action. The acetyltransferase function of the catalytic MORF subunit is positively regulated by the DPF domain of MORF (MORF
). The crystal structure of MORF
in complex with crotonylated H3K14 peptide provides mechanistic insight into selectivity of this epigenetic reader and its ability to recognize both histone and DNA. ChIP data reveal the role of MORF
in MORF-dependent H3K23 acetylation of target genes. Mass spectrometry, biochemical and genomic analyses show co-existence of the H3K23ac and H3K14ac modifications in vitro and co-occupancy of the MORF complex, H3K23ac, and H3K14ac at specific loci in vivo. Our findings suggest a model in which interaction of MORF
with acylated H3K14 promotes acetylation of H3K23 by the native MORF complex to activate transcription.
Intestinal stem cells, characterized by high Lgr5 expression, reside between Paneth cells at the small intestinal crypt base and divide every day. We have carried out fate mapping of individual stem ...cells by generating a multicolor Cre-reporter. As a population, Lgr5hi stem cells persist life-long, yet crypts drift toward clonality within a period of 1–6 months. We have collected short- and long-term clonal tracing data of individual Lgr5hi cells. These reveal that most Lgr5hi cell divisions occur symmetrically and do not support a model in which two daughter cells resulting from an Lgr5hi cell division adopt divergent fates (i.e., one Lgr5hi cell and one transit-amplifying TA cell per division). The cellular dynamics are consistent with a model in which the resident stem cells double their numbers each day and stochastically adopt stem or TA fates. Quantitative analysis shows that stem cell turnover follows a pattern of neutral drift dynamics.
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► An Intestinal crypt contains 14 equal Lgr5hi stem cells that all divide each day ► A multicolor Cre-reporter mouse was developed for lineage tracing of stem cells ► Most Lgr5hi stem cell divisions yield a symmetric cell fate outcome ► Homeostasis results from competition between symmetrically dividing Lgr5hi stem cells
Over the last decade, numerous histone acyl post-translational modifications (acyl-PTMs) have been discovered, of which the functional significance is still under intense study. Here, we use ...high-resolution mass spectrometry to accurately quantify eight acyl-PTMs in vivo and after in vitro enzymatic assays. We assess the ability of seven histone acetyltransferases (HATs) to catalyze acylations on histones in vitro using short-chain acyl-CoA donors, proving that they are less efficient towards larger acyl-CoAs. We also observe that acyl-CoAs can acylate histones through non-enzymatic mechanisms. Using integrated metabolomic and proteomic approaches, we achieve high correlation (R
> 0.99) between the abundance of acyl-CoAs and their corresponding acyl-PTMs. Moreover, we observe a dose-dependent increase in histone acyl-PTM abundances in response to acyl-CoA supplementation in in nucleo reactions. This study represents a comprehensive profiling of scarcely investigated low-abundance histone marks, revealing that concentrations of acyl-CoAs affect histone acyl-PTM abundances by both enzymatic and non-enzymatic mechanisms.
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