Acoustic methods for high‐throughput protein crystal mounting at next‐generation macromolecular crystallographic beamlines

To take full advantage of advanced data collection techniques and high beam flux at next‐generation macromolecular crystallography beamlines, rapid and reliable methods will be needed to mount and align many samples per second. One approach is to use an acoustic ejector to eject crystal‐containing droplets onto a solid X‐ray transparent surface, which can then be positioned and rotated for data collection. Proof‐of‐concept experiments were conducted at the National Synchrotron Light Source on thermolysin crystals acoustically ejected onto a polyimide `conveyor belt'. Small wedges of data were collected on each crystal, and a complete dataset was assembled from a well diffracting subset of these crystals. Future developments and implementation will focus on achieving ejection and translation of single droplets at a rate of over one hundred per second.     

Lithium monosilicide (LiSi), a low-dimensional silicon-based material prepared by high pressure synthesis: NMR and vibrational spectroscopy and electrical properties characterization

Lithium monosilicide (LiSi) was formed at high pressures and high temperatures (1.0-2.5 GPa and 500-700°C) in a piston-cylinder apparatus. This compound was previously shown to have an unusual structure based on 3-fold coordinated silicon atoms arranged into interpenetrating sheets. In the present investigation, lowered synthesis pressures permitted recovery of large (150-200 mg) quantities of sample for structural studies via NMR spectroscopy (²⁹Si and ⁷Li), Raman spectroscopy and electrical conductivity measurements. The ²⁹Si chemical shift occurs at −106.5 ppm, intermediate between SiH₄ and Si(Si(CH₃)₃)₄, but lies off the trend established by the other alkali monosilicides (NaSi, KSi, RbSi, CsSi), that contain isolated Si₄⁴⁻ anions. Raman spectra show a strong peak at 508 cm⁻¹ due to symmetric Si-Si stretching vibrations, at lower frequency than for tetrahedrally coordinated Si frameworks, due to the longer Si-Si bonds in the 3-coordinated silicide. Higher frequency vibrations occur due to asymmetric stretching. Electrical conductivity measurements indicate LiSi is a narrow-gap semiconductor (E_b∼0.057 eV). There is a rapid increase in conductivity above T=450 K, that might be due to the onset of Li⁺ mobility.     

Pulsed discharge photoionization detector: Application to analysis of chloro alkanes/alkenes

A windowless pulsed discharge photoionization detector (PDPID) is described which uses the emission spectra from the discharge in helium and Ar/Kr doped helium. The emission from helium is a continuum ranging from 13.5–17.7 eV which ionizes all compounds except neon. The emission from 5.4% Ar/He ranges from 9.3–11.8 eV and ionizes most organic compounds and many inorganic compounds. The emission from 1.36% Kr/He consists principally of the resonance lines at 10.6 and 10.1 eV. These PDPIDs are used to analyze a 12 component mixture containing principally chloro alkane/alkene. The relative responses of the PDPID combined with the relative retention time can be used to qualitatively identify the chloro compounds.     

Applications using the chlorine-selective pulsed discharge emission detector

The use of the chlorine-selective pulsed discharge emission detector (Cl-PDED) for the GC analyses of EPA mixtures 502, 612, 624, organochlorine pesticides, and polychlorinated biphenyls has been demonstrated. The Cl-PDED is the most sensitive chlorine-selective detector with a minimum detectability of 50 fg Cl/s. A constant response/pg Cl was observed for these mixtures regardless of the number of Cl atoms/molecule and structure of the compound to which the Cl atoms are attached. The analysis of standard samples of polychlorinated biphenyls using the Cl-PDED have sensitivities comparable to those of the electron-capture detector; however, the predictable response/pg Cl from the Cl-PDED is preferred over the extremely variable response from the electron capture detector.     

Pulsed discharge helium ionization detector

Summary A pulsed discharge helium ionization detector (PDHID) (patent pending) for gas chromatography has been developed. This detector uses a non-radioactive pulsed high voltage discharge source for generation of electrons and pulsed collection of these electrons. We have evaluated this detector for the analysis of a wide range of chemical compounds. In this paper the analytes are passed through the discharge since the permanent gases are difficult to ionize. The initial results for the permanent gases indicate that the PDHID can be used as a universal detector of contaminant traces at detection levels on the order of 1–20 pg. The response in this mode of operation is linear over four orders of magnitude.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.     

Solitary wave solutions to nonlinear evolution equations in mathematical physics

This paper obtains solitons as well as other solutions to a few nonlinear evolution equations that appear in various areas of mathematical physics. The two analytical integrators that are applied to extract solutions are tan–cot method and functional variable approaches. The soliton solutions can be used in the further study of shallow water waves in (1+1) as well as (2+1) dimensions.     

NC-Approximation Schemes for NP- and PSPACE-Hard Problems for Geometric Graphs

We present NC-approximation schemes for a number of graph problems when restricted to geometric graphs including unit disk graphs and graphs drawn in a civilized manner. Our approximation schemes exhibit the same time versus performance trade-off as the best known approximation schemes for planar graphs. We also define the concept of λ-precision unit disk graphs and show that for such graphs the approximation schemes have a better time versus performance trade-off than the approximation schemes for arbitrary unit disk graphs. Moreover, compared to unit disk graphs, we show that for λ-precision unit disk graphs many more graph problems have efficient approximation schemes.Our NC-approximation schemes can also be extended to obtain efficient NC-approximation schemes for several PSPACE-hard problems on unit disk graphs specified using a restricted version of the hierarchical specification language of Bentley, Ottmann, and Widmayer. The approximation schemes for hierarchically specified unit disk graphs presented in this paper are among the first approximation schemes in the literature for natural PSPACE-hard optimization problems.