312 research outputs found
Protein-DNA charge transport: Redox activation of a DNA repair protein by guanine radical
DNA charge transport (CT) chemistry provides a route to carry out oxidative DNA damage from a distance in a reaction that is sensitive to DNA mismatches and lesions. Here, DNA-mediated CT also leads to oxidation of a DNA-bound base excision repair enzyme, MutY. DNA-bound Ru(III), generated through a flash/quench technique, is found to promote oxidation of the [4Fe-4S](2+) cluster of MutY to [4Fe-4S](3+) and its decomposition product [3Fe-4S](1+). Flash/quench experiments monitored by EPR spectroscopy reveal spectra with g = 2.08, 2.06, and 2.02, characteristic of the oxidized clusters. Transient absorption spectra of poly(dGC) and [Ru(phen)(2)dppz](3+) (dppz = dipyridophenazine), generated in situ, show an absorption characteristic of the guanine radical that is depleted in the presence of MutY with formation instead of a long-lived species with an absorption at 405 nm; we attribute this absorption also to formation of the oxidized [4Fe-4S](3+) and [3Fe4S](1+) clusters. In ruthenium-tethered DNA assemblies, oxidative damage to the 5'-G of a 5'-GG-3' doublet is generated from a distance but this irreversible damage is inhibited by MutY and instead EPR experiments reveal cluster oxidation. With ruthenium-tethered assemblies containing duplex versus single-stranded regions, MutY oxidation is found to be mediated by the DNA duplex, with guanine radical as an intermediate oxidant; guanine radical formation facilitates MutY oxidation. A model is proposed for the redox activation of DNA repair proteins through DNA CT, with guanine radicals, the first product under oxidative stress, in oxidizing the DNA-bound repair proteins, providing the signal to stimulate DNA repair
RK-TBA studies at the RTA test facility
Construction of a prototype RF power source based on the RK-TBA concept, called the RTA, has commenced at the Lawrence Berkeley National Laboratory. This prototype will be used to study physics, engineering, and costing issues involved in the application of the RK-TBA concept to linear colliders. The status of the prototype is presented, specifically the 1-MV, 1.2-kA induction electron gun and the pulsed power system that are in assembly. The RTA program theoretical effort, in addition to supporting the development of the prototype, has been studying optimization parameters for the application of the RK-TBA concept to higher-energy linear colliders. An overview of this work is presented. 1 fig
Evidence-Based Professional Development of Science Teachers in Two Countries
The focus of this collaborative research project of King?s College London, and the Weizmann Institute, Israel. project is on investigating the ways in which teachers can demonstrate accomplished teaching in a specific domain of science and on the teacher learning that is generated through continuing professional development programs (CPD) that lead towards such practice. The interest lies in what processes and inputs are required to help secondary school science teachers develop expertise in a specific aspect of science teaching. `It focuses on the design of the CPD programmes and examines the importance of an evidence-based approach through portfolioconstruction in which professional dialogue pathes the way for teacher learning. The set of papers highlight the need to set professional challenge while tailoring CPD to teachers? needs to create the environment in which teachers can advance and transform their practice. The cross-culture perspective added to the richness of the development and enabled the researchers to examine which aspects were fundamental to the design by considering similarities and differences between the domains
The Electrical-Thermal Switching in Carbon Black-Polymer Composites as a Local Effect
Following the lack of microscopic information about the intriguing well-known
electrical-thermal switching mechanism in Carbon Black-Polymer composites, we
applied atomic force microscopy in order to reveal the local nature of the
process and correlated it with the characteristics of the widely used
commercial switches. We conclude that the switching events take place in
critical interparticle tunneling junctions that carry most of the current. The
macroscopic switched state is then a result of a dynamic-stationary state of
fast switching and slow reconnection of the corresponding junctions.Comment: 14 pages, 5 figures,Typographic correctio
Anti-Hyperon Enhancement through Baryon Junction Loops
The baryon junction exchange mechanism recently proposed to explain valence
baryon number transport in nuclear collisions is extended to study midrapidity
anti-hyperon production. Baryon junction-anti-junction (J anti-J) loops are
shown to enhance anti-Lambda, anti-Xi, anti-Omega yields as well as lead to
long range rapidity correlations. Results are compared to recent WA97 Pb + Pb
-> Y + anti-Y + X data.Comment: 10 pages, 4 figure
Non-universal current flow near the metal-insulator transition in an oxide interface
In systems near phase transitions, macroscopic properties often follow
algebraic scaling laws, determined by the dimensionality and the underlying
symmetries of the system. The emergence of such universal scaling implies that
microscopic details are irrelevant. Here, we locally investigate the scaling
properties of the metal-insulator transition at the LaAlO3/SrTiO3 interface. We
show that, by changing the dimensionality and the symmetries of the electronic
system, coupling between structural and electronic properties prevents the
universal behavior near the transition. By imaging the current flow in the
system, we reveal that structural domain boundaries modify the filamentary flow
close to the transition point, preventing a fractal with the expected universal
dimension from forming. Our results offer a generic platform to engineer
electronic transitions on the nanoscale.Comment: 19 pages, 6 figure
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Stacked insulator induction accelerator gaps
Stacked insulators, with alternating layers of insulating material and conducting film, have been shown to support high surface electrical field stresses. We have investigated the application of the stacked insulator technology to the design of induction accelerator modules for the Relativistic-Klystron Two-Beam Accelerator program. The rf properties of the accelerating gaps using stacked insulators, particularly the impedance at frequencies above the beam pipe cutoff frequency, are investigated. Low impedance is critical for Relativistic-Klystron Two-Beam Accelerator applications where a high current, bunched beam is trsnsported through many accelerating gaps. An induction accelerator module designs using a stacked insulator is presented
Tensile and fatigue strength of hydrogen-treated Ti-6Al-4V alloy
Tensile, fatigue and fractographic data on Ti-6Al-4V microstructures attained through a series of post-ÎČ-annealing treatments which used hydrogen as a temporary alloying element are presented. Hydrogen-alloying treatments break up the continuous grain boundary α and colony structure, and produce a homogeneous microstructure consisting of refined α-grains in a matrix of discontinuous ÎČ. These changes in microstructural morphology result in significant increases of the yield strength (974 to 1119 MPa), ultimate strength (1025 to 1152 MPa) and high cycle fatigue strength (643 to 669 MPa) compared to respective values for lamellar microstructures (902, 994, 497 MPa). The strengths are also significantly greater than the strengths of equiaxed microstructures (914, 1000, 590 MPa). The strengths of hydrogen-alloy treated samples are therefore superior to strengths attainable via other thermal cycling techniques.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44703/1/10853_2004_Article_BF00576523.pd
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