957 research outputs found
Selective labeling of peptides, metalloproteins, and proteolytic enzymes with Pt(II) complexes and electron-transfer reactions of native and cross-linked metalloproteins
The transition-metal complex (Pt(trpy)Cl) [superscript]+ selectively labels the amino acids Cys and His, but is unreactive toward Met. The (Pt(trpy)Cl) [superscript]+ complex also forms complexes with nonbiological ligands to form novel compounds with metal-metal or [pi]-[pi] interactions;The homologous gold reagent, (Au(trpy)Cl) [superscript]2+ reacts with Trp, Cys, His, and Met. The oxidative properties of the gold complex makes it an invasive reagent toward proteins, however;In reactions with proteins, the complex (Pt(trpy)Cl) [superscript]+ exhibits unexpected selectivity toward amino acid side chains in cytochromes c. His residues are labeled in greater yield than the Cys residues. The Pr(trpy)[superscript]2+ tags are stable and are easily detected and quantitated. The new reagent does not alter the structural and redox properties of the cytochromes c;Subsequent research involves modification of the active sites of serine and sulfhydryl proteases with (Pt(trpy)Cl) [superscript]+. Although the tagging modifies the catalytic triad and disrupts the charge relay, the platinated enzymes retain significant activity for substrates. The (Pt(trpy)Cl) [superscript]+ complex labels both Cys and His residues in papain. Selective removal of these tags with judiciously chosen nucleophiles provides a new approach for site-selective modification of proteins;Cyt c was also modified with the negatively charged complex (Pt(sbpaphy)Cl) [superscript]-. The yield of protein derivatives is low compared with (Pt(trpy)Cl) [superscript]+ derivatives;Electron-transfer between covalently tethered cyt c and pc is investigated. The tethers prohibit surface diffusion of the proteins and therefore prohibit a productive electron-transfer complex from forming. Rates of electron transfer between the heterodimeric proteins and a third, free, monomeric protein provide an empirical approach for understanding how the 2 proteins are cross-linked and why they cannot participate in intracomplex electron transfer;Methylene blue has long been known to be a photoreductant, but its reactions with biomolecules have not been quantitated. Rates of reaction of methylene blue and derivatives of methylene blue with cyt c are measured
Ultrafast excitonic dynamics in DNA: Bridging correlated quantum dynamics and sequence dependence
After photo-excitation of DNA, the excited electron (in the LUMO) and the
remaining hole (in the HOMO) localized on the same DNA base form a bound pair,
called the Frenkel exciton, due to their mutual Coulomb interaction. In this
study, we demonstrate that a tight-binding (TB) approach, parametrized by ab
initio data, allows to correlate relaxation properties, average charge
separation, and dipole moments to a large ensemble of double-stranded DNA
sequences (all 16,384 possible sequences with 14 nucleobases). This way, we are
able to identify a relatively small sub-ensemble of sequences responsible for
long-lived excited states, high average charge separation, and high dipole
moment. Further analysis shows that these sequences are particularly T-rich. By
systematically screening the impact of electron-hole interaction (Coulomb
forces), we verify that these correlations are relatively robust against
finite-size variations of the interaction parameter, not directly accessible
experimentally. This methodology combines simulation methods from quantum
physics and physical chemistry with statistical analysis known from genetics
and epigenetics, thus representing a powerful bridge to combine information
from both fields.Comment: 15 pages, 13 figure
Contingency and Its Intersections in Writing Centers: An Introduction
Introduction to WCJ 41.1, which is a special issue on contingency in writing centers
Contingent Writing Center Work: Benefits, Risks, and the Need for Equity and Institutional Change
This study investigates and reports on the personal, professional, and programmatic benefits and risks associated with contingent writing center work. Interviews were conducted with 48 contingent writing centers workers, including directors, assistant directors, associate directors, graduate student workers, and tutors. Survey data of the interview participants showed contingent writing center workers are usually White women with advanced degrees. Most of this article focuses on interview data, analyzed using grounded theory. Interviews revealed participants’ understanding of what contingency means and revealed their struggles with instability, insecurity, and uncertainty even while they lauded the flexibility, freedom, and autonomy their contingency afforded them. The interview data also further revealed the ways in which these working conditions were created and maintained by the institution. These findings suggest the need for collective action across the composition and writing center fields—from professional organizations, tenure-line writing center workers, and contingent workers themselves. Through collective action, we can create equitable working conditions for all writing center workers
Noise-based core monitoring and diagnostics: overview of the cortex project
This paper gives an overview of the CORTEX project, which is a Research and Innovation Action
funded by the European Union in the Euratom 2016-2017 work program, under the Horizon 2020
framework. CORTEX, which stands for CORe monitoring Techniques and EXperimental validation
and demonstration, aims at developing an innovative core monitoring technique that allows
detecting anomalies in nuclear reactors, such as excessive vibrations of core internals, flow
blockage, coolant inlet perturbations, etc. The technique is based on primarily using the inherent
fluctuations in neutron flux recorded by in-core and ex-core instrumentation (often referred to as
neutron noise), from which the anomalies will be differentiated depending on their type, location
and characteristics. In addition to be non-intrusive and not requiring any external perturbation of the
system, the method allows the detection of operational problems at a very early stage. Proper
actions could thus be taken by utilities before such problems have any adverse effect on plant safety
and reliability
Polymorphism and selection of rpoS in pathogenic Escherichia coli
<p>Abstract</p> <p>Background</p> <p>Though RpoS is important for survival of pathogenic <it>Escherichia coli </it>in natural environments, polymorphism in the <it>rpoS </it>gene is common. However, the causes of this polymorphism and consequential physiological effects on gene expression in pathogenic strains are not fully understood.</p> <p>Results</p> <p>In this study, we found that growth on non-preferred carbon sources can efficiently select for loss of RpoS in seven of ten representative verocytotoxin-producing <it>E. coli </it>(VTEC) strains. Mutants (Suc<sup>++</sup>) forming large colonies on succinate were isolated at a frequency of 10<sup>-8 </sup>mutants per cell plated. Strain O157:H7 EDL933 yielded mainly mutants (about 90%) that were impaired in catalase expression, suggesting the loss of RpoS function. As expected, inactivating mutations in <it>rpoS </it>sequence were identified in these mutants. Expression of two pathogenicity-related phenotypes, cell adherence and RDAR (red dry and rough) morphotype, were also attenuated, indicating positive control by RpoS. For the other Suc<sup>++ </sup>mutants (10%) that were catalase positive, no mutation in <it>rpoS </it>was detected.</p> <p>Conclusion</p> <p>The selection for loss of RpoS on poor carbon sources is also operant in most pathogenic strains, and thus is likely responsible for the occurrence of <it>rpoS </it>polymorphisms among <it>E. coli </it>isolates.</p
Towards Quantum Sensing of Chiral-Induced Spin Selectivity: Probing Donor-Bridge-Acceptor Molecules with NV Centers in Diamond
Photoexcitable donor-bridge-acceptor (D-B-A) molecules that support
intramolecular charge transfer are ideal platforms to probe the influence of
chiral-induced spin selectivity (CISS) in electron transfer and resulting
radical pairs. In particular, the extent to which CISS influences spin
polarization or spin coherence in the initial state of spin-correlated radical
pairs following charge transfer through a chiral bridge remains an open
question. Here, we introduce a quantum sensing scheme to measure directly the
hypothesized spin polarization in radical pairs using shallow nitrogen-vacancy
(NV) centers in diamond at the single- to few-molecule level. Importantly, we
highlight the perturbative nature of the electron spin-spin dipolar coupling
within the radical pair, and demonstrate how Lee-Goldburg decoupling can
preserve spin polarization in D-B-A molecules for enantioselective detection by
a single NV center. The proposed measurements will provide fresh insight into
spin selectivity in electron transfer reactions.Comment: 7 pages and 4 pages appendix including an extensive description of
the initial spin state of photo-generated radical pair
Tracking the precession of single nuclear spins by weak measurements
Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for
analyzing the structure and function of molecules, and for performing
three-dimensional imaging of the spin density. At the heart of NMR
spectrometers is the detection of electromagnetic radiation, in the form of a
free induction decay (FID) signal, generated by nuclei precessing around an
applied magnetic field. While conventional NMR requires signals from 1e12 or
more nuclei, recent advances in sensitive magnetometry have dramatically
lowered this number to a level where few or even individual nuclear spins can
be detected. It is natural to ask whether continuous FID detection can still be
applied at the single spin level, or whether quantum back-action modifies or
even suppresses the NMR response. Here we report on tracking of single nuclear
spin precession using periodic weak measurements. Our experimental system
consists of carbon-13 nuclear spins in diamond that are weakly interacting with
the electronic spin of a nearby nitrogen-vacancy center, acting as an optically
readable meter qubit. We observe and minimize two important effects of quantum
back-action: measurement-induced decoherence and frequency synchronization with
the sampling clock. We use periodic weak measurements to demonstrate sensitive,
high-resolution NMR spectroscopy of multiple nuclear spins with a priori
unknown frequencies. Our method may provide the optimum route for performing
single-molecule NMR at atomic resolution.Comment: 29 pages including methods and extended data figures; for
supplementary material, see v1 of this submissio
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