77 research outputs found
Concerted loop motion triggers induced fit of FepA to ferric enterobactin
Spectroscopic analyses of fluorophore-labeled Escherichia coli FepA described dynamic actions of its surface loops during binding and transport of ferric enterobactin (FeEnt). When FeEnt bound to fluoresceinated FepA, in living cells or outer membrane fragments, quenching of fluorophore emissions reflected conformational motion of the external vestibular loops. We reacted Cys sulfhydryls in seven surface loops (L2, L3, L4, L5, L7 L8, and L11) with fluorophore maleimides. The target residues had different accessibilities, and the labeled loops themselves showed variable extents of quenching and rates of motion during ligand binding. The vestibular loops closed around FeEnt in about a second, in the order L3 > L11 > L7 > L2 > L5 > L8 > L4. This sequence suggested that the loops bind the metal complex like the fingers of two hands closing on an object, by individually adsorbing to the iron chelate. Fluorescence from L3 followed a biphasic exponential decay as FeEnt bound, but fluorescence from all the other loops followed single exponential decay processes. After binding, the restoration of fluorescence intensity (from any of the labeled loops) mirrored cellular uptake that depleted FeEnt from solution. Fluorescence microscopic images also showed FeEnt transport, and demonstrated that ferric siderophore uptake uniformly occurs throughout outer membrane, including at the poles of the cells, despite the fact that TonB, its inner membrane transport partner, was not detectable at the poles
Neutronic Design and Measured Performance of the Low Energy Neutron Source (LENS) Target Moderator Reflector Assembly
The Low Energy Neutron Source (LENS) is an accelerator-based pulsed cold
neutron facility under construction at the Indiana University Cyclotron
Facility (IUCF). The idea behind LENS is to produce pulsed cold neutron beams
starting with ~MeV neutrons from (p,n) reactions in Be which are moderated to
meV energies and extracted from a small solid angle for use in neutron
instruments which can operate efficiently with relatively broad (~1 msec)
neutron pulse widths. Although the combination of the features and operating
parameters of this source is unique at present, the neutronic design possesses
several features similar to those envisioned for future neutron facilities such
as long-pulsed spallation sources (LPSS) and very cold neutron (VCN) sources.
We describe the underlying ideas and design details of the
target/moderator/reflector system (TMR) and compare measurements of its
brightness, energy spectrum, and emission time distribution under different
moderator configurations with MCNP simulations. Brightness measurements using
an ambient temperature water moderator agree with MCNP simulations within the
20% accuracy of the measurement. The measured neutron emission time
distribution from a solid methane moderator is in agreement with simulation and
the cold neutron flux is sufficient for neutron scattering studies of
materials. We describe some possible modifications to the existing design which
would increase the cold neutron brightness with negligible effect on the
emission time distribution.Comment: This is a preprint version of an article which has been published in
Nuclear Instruments and Methods in Physics Research A 587 (2008) 324-341.
http://dx.doi.org/10.1016/j.nima.2007.12.04
Uncertainties in the Anti-neutrino Production at Nuclear Reactors
Anti-neutrino emission rates from nuclear reactors are determined from
thermal power measurements and fission rate calculations. The uncertainties in
these quantities for commercial power plants and their impact on the calculated
interaction rates in electron anti-neutrino detectors is examined. We discuss
reactor-to-reactor correlations between the leading uncertainties and their
relevance to reactor anti-neutrino experiments.Comment: Submitted to Phys Rev
An Ultra-Low Background PMT for Liquid Xenon Detectors
Results are presented from radioactivity screening of two models of
photomultiplier tubes designed for use in current and future liquid xenon
experiments. The Hamamatsu 5.6 cm diameter R8778 PMT, used in the LUX dark
matter experiment, has yielded a positive detection of four common radioactive
isotopes: 238U, 232Th, 40K, and 60Co. Screening of LUX materials has rendered
backgrounds from other detector materials subdominant to the R8778
contribution. A prototype Hamamatsu 7.6 cm diameter R11410 MOD PMT has also
been screened, with benchmark isotope counts measured at <0.4 238U / <0.3 232Th
/ <8.3 40K / 2.0+-0.2 60Co mBq/PMT. This represents a large reduction, equal to
a change of \times 1/24 238U / \times 1/9 232Th / \times 1/8 40K per PMT,
between R8778 and R11410 MOD, concurrent with a doubling of the photocathode
surface area (4.5 cm to 6.4 cm diameter). 60Co measurements are comparable
between the PMTs, but can be significantly reduced in future R11410 MOD units
through further material selection. Assuming PMT activity equal to the measured
90% upper limits, Monte Carlo estimates indicate that replacement of R8778 PMTs
with R11410 MOD PMTs will change LUX PMT electron recoil background
contributions by a factor of \times1/25 after further material selection for
60Co reduction, and nuclear recoil backgrounds by a factor of \times 1/36. The
strong reduction in backgrounds below the measured R8778 levels makes the
R11410 MOD a very competitive technology for use in large-scale liquid xenon
detectors.Comment: v2 updated to include content after reviewer comments (Sep 2012
Insights into SusCD-mediated glycan import by a prominent gut symbiont
In Bacteroidetes, one of the dominant phyla of the mammalian gut, active uptake of large nutrients across the outer membrane is mediated by SusCD protein complexes via a “pedal bin” transport mechanism. However, many features of SusCD function in glycan uptake remain unclear, including ligand binding, the role of the SusD lid and the size limit for substrate transport. Here we characterise the β2,6 fructo-oligosaccharide (FOS) importing SusCD from Bacteroides thetaiotaomicron (Bt1762-Bt1763) to shed light on SusCD function. Co-crystal structures reveal residues involved in glycan recognition and suggest that the large binding cavity can accommodate several substrate molecules, each up to ~2.5 kDa in size, a finding supported by native mass spectrometry and isothermal titration calorimetry. Mutational studies in vivo provide functional insights into the key structural features of the SusCD apparatus and cryo-EM of the intact dimeric SusCD complex reveals several distinct states of the transporter, directly visualising the dynamics of the pedal bin transport mechanism
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McSKY: A hybrid Monte-Carlo lime-beam code for shielded gamma skyshine calculations
McSKY evaluates skyshine dose from an isotropic, monoenergetic, point photon source collimated into either a vertical cone or a vertical structure with an N-sided polygon cross section. The code assumes an overhead shield of two materials, through the user can specify zero shield thickness for an unshielded calculation. The code uses a Monte-Carlo algorithm to evaluate transport through source shields and the integral line source to describe photon transport through the atmosphere. The source energy must be between 0.02 and 100 MeV. For heavily shielded sources with energies above 20 MeV, McSKY results must be used cautiously, especially at detector locations near the source
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SKYDOSE: A code for gamma skyshine calculations using the integral line-beam method
SKYDOS evaluates skyshine dose from an isotropic, monoenergetic, point photon source collimated by three simple geometries: (1) a source in a silo; (2) a source behind an infinitely long, vertical, black wall; and (3) a source in a rectangular building. In all three geometries, an optical overhead shield may be specified. The source energy must be between 0.02 and 100 MeV (10 MeV for sources with an overhead shield). This is a user`s manual. Other references give more detail on the integral line-beam method used by SKYDOSE
Does birth weight predict childhood diet in the Avon longitudinal study of parents and children?
Study objective: Low birth weight predicts cardiovascular disease in adulthood, and one possible explanation is that children with lower birth weight consume more fat than those born heavier. Therefore, the objective of this study was to investigate associations between birth weight and childhood diet, and in particular, to test the hypothesis that birth weight is inversely related to total and saturated fat intake. Design: Prospective cohort study. Setting: South west England. Participants: A subgroup of children enrolled in the Avon longitudinal study of parents and children, with data on birth weight and also diet at ages 8, 18, 43 months, and 7 years (1152, 998, 848, and 771 children respectively). Main results: Associations between birth weight and diet increased in strength from age 8 to 43 months, but had diminished by age 7 years. Fat, saturated fat, and protein intakes were inversely, and carbohydrate intake was positively associated with birth weight at 43 months of age, after adjusting for age, sex, and energy intake. After adjustment for other confounders, all associations were weakened, although there was still a suggestion of a relation with saturated fat (–0.48 (95% CI –0.97, 0.02) g/day per 500 g increase in birth weight. Similar patterns were seen in boys and girls separately, and when the sample was restricted to those with complete data at all ages. Conclusions: A small inverse association was found between birth weight and saturated fat intake in children at 43 months of age but this was not present at 7 years of age. This study therefore provides little evidence that birth weight modifies subsequent childhood diet
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