Structure of the sporulation histidine kinase inhibitor Sda from Bacillus subtilis and insights into its solution state

The crystal structure of the DNA-damage checkpoint inhibitor of sporulation, Sda, from Bacillus subtilis, has been solved by the MAD technique using selenomethionine-substituted protein. The structure closely resembles that previously solved by NMR, as well as the structure of a homologue from Geobacillus stearothermophilus solved in complex with the histidine kinase KinB. The structure contains three molecules in the asymmetric unit. The unusual trimeric arrangement, which lacks simple internal symmetry, appears to be preserved in solution based on an essentially ideal fit to previously acquired scattering data for Sda in solution. This interpretation contradicts previous findings that Sda was monomeric or dimeric in solution. This study demonstrates the difficulties that can be associated with the characterization of small proteins and the value of combining multiple biophysical techniques. It also emphasizes the importance of understanding the physical principles behind these techniques and therefore their limitations

Results of applying a non-evaporative mitigation technique to laser-initiated surface damage on fused-silica

We present results from a study to determine an acceptable CO{sub 2} laser-based non-evaporative mitigation protocol for use on surface damage sites in fused-silica optics. A promising protocol is identified and evaluated on a set of surface damage sites created under ICF-type laser conditions. Mitigation protocol acceptability criteria for damage re-initiation and growth, downstream intensification, and residual stress are discussed. In previous work, we found that a power ramp at the end of the protocol effectively minimizes the residual stress (<25 MPa) left in the substrate. However, the biggest difficulty in determining an acceptable protocol was balancing between low re-initiation and problematic downstream intensification. Typical growing surface damage sites mitigated with a candidate CO{sub 2} laser-based mitigation protocol all survived 351 nm, 5 ns damage testing to fluences >12.5 J/cm{sup 2}. The downstream intensification arising from the mitigated sites is evaluated, and all but one of the sites has 100% passing downstream damage expectation values. We demonstrate, for the first time, a successful non-evaporative 10.6 {micro}m CO{sub 2} laser mitigation protocol applicable to fused-silica optics used on fusion-class lasers like the National Ignition Facility (NIF)

A Compact Beam Stop for a Rare Kaon Decay Experiment

We describe the development and testing of a novel beam stop for use in a rare kaon decay experiment at the Brookhaven AGS. The beam stop is located inside a dipole spectrometer magnet in close proximity to straw drift chambers and intercepts a high-intensity neutral hadron beam. The design process, involving both Monte Carlo simulations and beam tests of alternative beam-stop shielding arrangements, had the goal of minimizing the leakage of particles from the beam stop and the resulting hit rates in detectors, while preserving maximum acceptance for events of interest. The beam tests consisted of measurements of rates in drift chambers, scintilation counter hodoscopes, a gas threshold Cherenkov counter, and a lead glass array. Measurements were also made with a set of specialized detectors which were sensitive to low-energy neutrons, photons, and charged particles. Comparisons are made between these measurements and a detailed Monte Carlo simulation.Comment: 39 pages, 14 figures, submitted to Nuclear Instruments and Method

Downstream Intensification Effects Associated with CO2 Laser Mitigation of Fused Silica

Mitigation of 351nm laser-induced damage sites on fused silica exit surfaces by selective CO{sub 2} treatment has been shown to effectively arrest the exponential growth responsible for limiting the lifetime of optics in high-fluence laser systems. However, the perturbation to the optical surface profile following the mitigation process introduces phase contrast to the beam, causing some amount of downstream intensification with the potential to damage downstream optics. Control of the laser treatment process and measurement of the associated phase modulation is essential to preventing downstream 'fratricide' in damage-mitigated optical systems. In this work we present measurements of the surface morphology, intensification patterns and damage associated with various CO{sub 2} mitigation treatments on fused silica surfaces. Specifically, two components of intensification pattern, one on-axis and another off-axis can lead to damage of downstream optics and are related to rims around the ablation pit left from the mitigation process. It is shown that control of the rim structure around the edge of typical mitigation sites is crucial in preventing damage to downstream optics

An instrument for in situ time-resolved X-ray imaging and diffraction of laser powder bed fusion additive manufacturing processes

In situ X-ray-based measurements of the laser powder bed fusion (LPBF) additive manufacturing process produce unique data for model validation and improved process understanding. Synchrotron X-ray imaging and diffraction provide high resolution, bulk sensitive information with sufficient sampling rates to probe melt pool dynamics as well as phase and microstructure evolution. Here, we describe a laboratory-scale LPBF test bed designed to accommodate diffraction and imaging experiments at a synchrotron X-ray source during LPBF operation. We also present experimental results using Ti-6Al-4V, a widely used aerospace alloy, as a model system. Both imaging and diffraction experiments were carried out at the Stanford Synchrotron Radiation Lightsource. Melt pool dynamics were imaged at frame rates up to 4 kHz with a ∼1.1 μm effective pixel size and revealed the formation of keyhole pores along the melt track due to vapor recoil forces. Diffraction experiments at sampling rates of 1 kHz captured phase evolution and lattice contraction during the rapid cooling present in LPBF within a ∼50 × 100 μm area. We also discuss the utility of these measurements for model validation and process improvement

Outer-Sphere Contributions to the Electronic Structure of Type Zero Copper Proteins

Bioinorganic canon states that active-site thiolate coordination promotes rapid electron transfer (ET) to and from type 1 copper proteins. In recent work, we have found that copper ET sites in proteins also can be constructed without thiolate ligation (called “type zero” sites). Here we report multifrequency electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), and nuclear magnetic resonance (NMR) spectroscopic data together with density functional theory (DFT) and spectroscopy-oriented configuration interaction (SORCI) calculations for type zero Pseudomonas aeruginosa azurin variants. Wild-type (type 1) and type zero copper centers experience virtually identical ligand fields. Moreover, O-donor covalency is enhanced in type zero centers relative that in the C112D (type 2) protein. At the same time, N-donor covalency is reduced in a similar fashion to type 1 centers. QM/MM and SORCI calculations show that the electronic structures of type zero and type 2 are intimately linked to the orientation and coordination mode of the carboxylate ligand, which in turn is influenced by outer-sphere hydrogen bonding

Start-up scenario studies in gyrotron oscillator using a novel linear and spectral code

A linear and spectral model has recently been developed [1], describing the self-consistent wave-particle interaction in a gyrotron oscillator. The spectral approach, compared to commonly used time-evolution approaches, has the possibility to describe all of the stable and unstable modes, respectively, with negative and positive growth rates. Moreover, this approach is numerically efficient and thus appropriate for parameter scans or start-up scenario studies. The model has been successfully benchmarked against real experiments for gyrotron cavity interaction, in particular concerning start-up scenario studies. In order to study backward-wave instabilities in smooth-wall beam ducts, the numerical model has been recently extended to include a higher order finite element discretization. The model, its numerical implementation and simulation results for high power gyrotrons as well as first results for smooth-wall beam ducts will be presented