18,776 research outputs found
Heuristic Refinement Method for the Derivation of Protein Solution Structures: Validation on Cytochrome B562
A method is described for determining the family of protein structures compatible with solution data obtained primarily from nuclear magnetic resonance (NMR) spectroscopy. Starting with all possible conformations, the method systematically excludes conformations until the remaining structures are only those compatible with the data. The apparent computational intractability of this approach is reduced by assembling the protein in pieces, by considering the protein at several levels of abstraction, by utilizing constraint satisfaction methods to consider only a few atoms at a time, and by utilizing artificial intelligence methods of heuristic control to decide which actions will exclude the most conformations. Example results are presented for simulated NMR data from the known crystal structure of cytochrome b562 (103 residues). For 10 sample backbones an average root-mean-square deviation from the crystal of 4.1 A was found for all alpha-carbon atoms and 2.8 A for helix alpha-carbons alone. The 10 backbones define the family of all structures compatible with the data and provide nearly correct starting structures for adjustment by any of the current structure determination methods
Quantum Dynamics of the Slow Rollover Transition in the Linear Delta Expansion
We apply the linear delta expansion to the quantum mechanical version of the
slow rollover transition which is an important feature of inflationary models
of the early universe. The method, which goes beyond the Gaussian
approximation, gives results which stay close to the exact solution for longer
than previous methods. It provides a promising basis for extension to a full
field theoretic treatment.Comment: 12 pages, including 4 figure
The Calibration of the HST Kuiper Belt Object Search: Setting the Record Straight
The limiting magnitude of the HST data set used by Cochran et al. (1995) to
detect small objects in the Kuiper belt is reevaluated, and the methods used
are described in detail. It is shown, by implanting artificial objects in the
original HST images, and re-reducing the images using our original algorithm,
that the limiting magnitude of our images (as defined by the 50% detectability
limit) is . This value is statistically the same as the value found in
the original analysis. We find that of the moving Kuiper belt objects
with are detected when trailing losses are included. In the same data
in which these faint objects are detected, we find that the number of false
detections brighter than is less than one per WFPC2 image. We show
that, primarily due to a zero-point calibration error, but partly due to
inadequacies in modeling the HST'S data noise characteristics and Cochran et
al.'s reduction techniques, Brown et al. 1997 underestimate the SNR of objects
in the HST dataset by over a factor of 2, and their conclusions are therefore
invalid.Comment: Accepted to ApJ Letters; 10 pages plus 3 figures, LaTe
A Damping of the de Haas-van Alphen Oscillations in the superconducting state
Deploying a recently developed semiclassical theory of quasiparticles in the
superconducting state we study the de Haas-van Alphen effect. We find that the
oscillations have the same frequency as in the normal state but their amplitude
is reduced. We find an analytic formulae for this damping which is due to
tunnelling between semiclassical quasiparticle orbits comprising both
particle-like and hole-like segments. The quantitative predictions of the
theory are consistent with the available data.Comment: 7 pages, 5 figure
Unquenched QCD with Light Quarks
We present recent results in unquenched lattice QCD with two degenerate light
sea quarks using the truncated determinant approximation (TDA). In the TDA the
infrared modes contributing to the quark determinant are computed exactly up to
some cutoff in quark off-shellness (typically 2). This approach
allows simulations to be performed at much lighter quark masses than possible
with conventional hybrid MonteCarlo techniques. Results for the static energy
and topological charge distributions are presented using a large ensemble
generated on very coarse (6) but physically large lattices. Preliminary
results are also reported for the static energy and meson spectrum on 10x20
lattices (lattice scale =1.15 GeV) at quark masses corresponding to
pions of mass 200 MeV. Using multiboson simulation to compute the
ultraviolet part of the quark determinant the TDA approach becomes an exact
with essentially no increase in computational effort. Some preliminary results
using this fully unquenched algorithm are presented.Comment: LateX, 39 pages, 16 eps figures, 1 ps figur
Reservation status and priorities for Tasmanian plants I. Angiospermae (Dicotyledonae)
Almost one-fifth of Tasmanian native dicotyledonous angiosperms are not known from any national park or equivalent reserve. Extinct, endangered, vulnerable and unreserved species are "most common among annuals and least common among woody plants. The unreserved species have their distributions concentrated between Launceston and Hobart in the dry, naturally grassy Midlands. A minimum reservation strategy is suggested for those species for which this option still exists
The Fading Radio Emission from SN 1961V: Evidence for a Type II Peculiar Supernova?
Using the Very Large Array (VLA), we have detected radio emission from the
site of SN 1961V in the Sc galaxy NGC 1058. With a peak flux density of 0.063
+/- 0.008 mJy/beam at 6 cm and 0.147 +/- 0.026 mJy/beam at 18 cm, the source is
non-thermal, with a spectral index of -0.79 +/- 0.23. Within errors, this
spectral index is the same value reported for previous VLA observations taken
in 1984 and 1986. The radio emission at both wavelengths has decayed since the
mid 1980's observations with power-law indices of beta(20cm) = -0.69 +/- 0.23
and beta(6cm) = -1.75 +/- 0.16. We discuss the radio properties of this source
and compare them with those of Type II radio supernovae and luminous blue
variables.Comment: 19 pages, 3 figures; To appear in the Astronomical Journa
Heavy-light baryonic mass splittings from the lattice
We present lattice estimates of the mass of the heavy-light baryons
and obtained using propagating heavy quarks. For
our result is GeV, after
extrapolation to the continuum limit and in the quenched approximation.Comment: 3 pages postscript, Contribution to Lattice'9
Sensitivity Improvement of a 1-ÎŒm Ladar System Incorporating an Active Optical Fiber Preamplifier
In an effort to increase the SNR of a continuous wave, 1-ÎŒm all solid state ladar system, a rare-earth-doped optical fiber amplifier is investigated as a preamplifier for ladar return signals. The experimental system is detailed and a theoretical analysis of the fiber amplifier\u27s effect on both heterodyne and direct detection schemes is provided. Beginning with the optical powers incident on the detector, the signal and noises are analyzed, through the detector electronics, to predict the SNR. The SNR is then plotted as a function of the return signal power, and a SNR threshold is defined to determine a minimum detectable signal power. The return signals required to attain the SNR threshold are then compared for four cases: direct detection with and without the fiber amplifier and heterodyne detection with and without the fiber amplifier. For the direct detection scheme considered, our results predict a sensitivity increase of 20.6 dB with the addition of the fiber amplifier, yet for heterodyne detection the predicted sensitivity increase is only 3.1 dB
Comparative study of selected indoor concentration from selective laser sintering process using virgin and recycled polyamide nylon (pa12)
Additive manufacturing (AM) stands out as one of the promising technologies that
have huge potential towards manufacturing industry. The study on additive manufacturing
impact on the environment and occupational exposure are attracting growing attention recently.
However, most of the researcher focus on desktop and fused deposition modelling type and less
attention given to the industrial type of AM. Usually, during the selective laser sintering process,
recycle powder will be used again to reduce cost and waste. This article compares the PM 2.5,
carbon dioxide (CO2) and total volatile organic compound (TVOC) concentration between virgin
and recycles powder using polyamide-nylon (PA12) towards indoor concentration. Four phases
of sampling involve during air sampling accordingly to the Industry Code of Practice on Indoor
Air Quality 2010 by DOSH Malaysia. It was found that PM 2.5 and CO2
concentration are mainly
generated during the pre-printing process. The recycle powder tended to appear higher compared
to virgin powder in terms of PM 2.5, and CO2. The peak value of PM 2.5 is 1452 ÎŒg/m3 and CO2
is 1218 ppm are obtained during the pre-printing process during 8 hours of sampling. TVOC
concentration from recycling powder is slightly higher during the post- printing phase where
confirm the influence of the powder cake and PA12 temperature from the printing process. In
summary, this work proves that elective laser sintering (SLS) machine operators are exposed to
a significant amount of exposure during the SLS printing process. Mitigation strategies and
personal protective equipment are suggested to reduce occupational exposure
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