Stereoselective synthesis of alpha-C-glucosyl serine and alanine via a cross-metathesis/cyclization strategy

C-Glycosyl amino acids represent stable mimics of monomeric units within natural O-linked glycoproteins. Olefin cross-metathesis has been used to provide alkene precursors for a mercury(II)-mediated cyclization, yielding alpha-C-glucosyl serine and alanine.     

A mild and general one-pot preparation of cyanoethyl-protected tetrazoles

Described herein is a mild and general one-pot procedure for the conversion of cyanoethyl amides to cyanoethyl-protected tetrazoles with azidotrimethylsilane via the intermediacy of imidoyl chlorides generated in situ with phosphorus pentachloride. This synthetic sequence works well with sterically hindered amides and is compatible with acid sensitive functionality.     

Oriented matroids

In this paper, the basic properties of oriented matroids are examined. A topological representation theorem for oriented matroids is proven, utilizing the notion of an “arrangement of pseudo-hemispheres”. The duality theorem of linear programming is extended to oriented matroids.     

Overwinter changes to near-surface bulk density, penetration resistance and infiltration rates in compacted soil

Previous studies at Yakima Training Center (YTC), in Washington State, suggest freeze-thaw (FT) cycles can ameliorate soil compacted by tracked military vehicles [J. Terramechanics 38 (2001) 133]. However, we know little about the short-term effects of soil freezing over a single winter. We measured bulk density (BD), soil penetration resistance (SPR), and steady-state runoff rates in soil newly tracked by an Abrams tank and in uncompacted soil, before and after a single winter at YTC. We similarly measured BD, SPR and saturated hydraulic conductivity (k_fs) in simulated tank tracks at another site near Lind Washington. Average BD was significantly greater in tank ruts at YTC and in simulated tracks at the Lind site than in uncompacted soil soon after tracking and did not change significantly during the winter of 1997–1998. Measurements of SPR were strongly influenced by soil moisture. When soil was moist or tracks were newly formed, SPR was significantly higher in tank ruts than in uncompacted soil from the surface to a depth of about 10–15 cm. The greatest average SPR in compacted soil was observed between 4 and 6 cm depth. We observed less difference in SPR between tank ruts and uncompacted soil near-surface at YTC as the time after trafficking increased. We observed highest SPR ratios (compacted rut:undisturbed) in fresh tracks near the surface, with lower ratios associated with increasing track age or soil depth, indicating that some recovery had occurred at YTC near-surface. However, we did not observe a similar over-winter change in SPR profiles at the Lind site. Rainfall simulator data from YTC showed higher steady-state runoff rates in tank ruts than over uncompacted soil both before and after winter. However, more time was required to reach steady-state flow in tank ruts and the proportion of runoff was slightly lower in May 1998 than in August 1997. At the Lind site, k_fs was lower in newly compacted soil than in one-year old compacted soil or uncompacted soil. Our data suggest that indices of water infiltration such as steady-state runoff rates or k_fs, are more sensitive indicators of soil recovery after compaction than are BD or SPR.     

High performance set associative translation lookaside buffers for low power microprocessors

A fast low power four-way set-associative translation lookaside buffer (TLB) is proposed. The proposed TLB allows single clock phase accesses at clock frequencies above 1 GHz. Comparisons to a conventional fully associative CAM tagged TLB, which is the type most commonly used in embedded processors, with the same number of entries on a 65 nm low standby power process show that the proposed design has 28% lower delay and up to 50% lower energy delay product. Unlike previous set-associative TLBs, which replicate the TLB to support multiple page sizes, multiple page sizes are supported on a way-by-way basis. Alternative conventional CAM tagged and set-associative TLBs are investigated with regard to access latency, power dissipation and hit rates.     

On Modeling the Single Motor Unit Action Potential

A model has been proposed for the generation of single motor unit potentials routinely observed in the clinical EMG examination of the normal biceps brachii muscle. A dipole representation was chosen for the single fiber activity. The motor unit was constructed from a uniform random array of single fibers. Motor unit potentials generated by this array have been observed at various distances both inside and outside the array. The effects of single fiber dipole axial dispersions on the potentials observed at increasing distances from the array have also been investigated. Motor unit potentials generated by the model have been compared with existing data from multielectrode studies in the biceps brachii.     

Electrocutaneous Nerve Stimulation?I: Model and Experiment

The behavior of nerve fibers in the skin during electrocutaneous stimulation has been studied in this investigation for the purpose of understanding the influence of various current pulse parameters. One part of the investigation assessed the amount of stimulus charge, for various pulse parameters, required to depolarize the nodal membrane of myelinated nerve, using the Frankenhaeuser-Huxley model for Xenopus Laevis (toad). The second part of the investigation involved the experimental application of biphasic current pulses via a concentric electrode to the skin of a number of human subjects. The subjects were asked to match the intensity of sensation as stimulus parameters were varied. The experimental results were compared to the predictions of the model by the process of normalization for temperature differences and the loss of electrode current in adjacent passive tissues.     

Electric current controlled liquid phase epitaxy of GaAs on N+ and semi-insulating substrates

Electric current controlled liquid phase epitaxy (LPE) of GaAs has been performed on both n⁺ and semi-insulating substrates. Growth is induced by current flow across the substrate-melt interface. The furnace temperature is held constant during growth so that direct electrical control of the growth process is achieved. The dependence of the growth rate on both the electric current density across the substrate-melt interface and the ambient furnace temperature was determined. Current densities from 5 to 20 A/cm² were employed and furnace temperatures ranging from 680 to 800°C were used. Sustained steady state growth rates as small as 0.022μm/min and as large as 1.4μm/min were obtained. For a given furnace temperature and current density, the measured growth rates on semi-insulating substrates range from 48% to 77% of the rates obtained on n⁺ n substrates. The surface morphology of the epitaxial layers is observed to depend on the electric current density employed during growth. Electric current controlled doping modulation was studied in epitaxial layers grown from unintentionally doped melts. The degree of doping modulation achieved is approximately proportional to the change in applied current density. Approximately a 40% increase in the net electron concentration is obtained by changing the current density from 10 to 30 A/cm² during growth. Preliminary experiments with tin doped epitaxial layers indicate that similar changes in the amount of tin incorporation can be achieved.     

Neurophysiological Model of the Normal and Abnormal Human Pupil

A model of the pathways controlling the size of the human pupil is presented. Computer simulation of this model demonstrates the role played by each of the elements in the pupil pathways. Simulations of the effects of drugs and a few common abnormalities in the system also help to illustrate the workings of the internal processes. Computer models of this type can be used as teaching aids or as tools for testing of hypotheses regarding the system.     

Parametric studies and optimization of the beta-voltaic cell—II open-circuit voltage and power efficiencies

Theoretical analyses on the silicon beta-violtaic cell have shown that contrary to the short-circuit current the open-circuit voltage is more sensitive to junction parameters than to bulk parameters. Optimum maximum power is obtained by a compromise between high current and high voltage. The resistivity of the silicon substrate under the optimum power condition is found to be about 0·3 Ω . cm. Optimization of overall efficiency involves maximizing the effectivness of the junction diode to collect generated carriers and minimizing the self-absorption in the active source layer; a layer of radioactive Pm₂O₃ with 1·0–1·5 Ci/cm² appears to be optimum. The corresponding overall power efficiency is 2·5 per cent for a cell irradiated from the front side. Higher efficiency is possible when beta particles are injected from both the front and the back of the silicon slice; under these conditions there is an optimum substrate thickness whcih is about 125 μm for maximum efficiency.