2,760 research outputs found
Stochastic theory of large-scale enzyme-reaction networks: Finite copy number corrections to rate equation models
Chemical reactions inside cells occur in compartment volumes in the range of
atto- to femtolitres. Physiological concentrations realized in such small
volumes imply low copy numbers of interacting molecules with the consequence of
considerable fluctuations in the concentrations. In contrast, rate equation
models are based on the implicit assumption of infinitely large numbers of
interacting molecules, or equivalently, that reactions occur in infinite
volumes at constant macroscopic concentrations. In this article we compute the
finite-volume corrections (or equivalently the finite copy number corrections)
to the solutions of the rate equations for chemical reaction networks composed
of arbitrarily large numbers of enzyme-catalyzed reactions which are confined
inside a small sub-cellular compartment. This is achieved by applying a
mesoscopic version of the quasi-steady state assumption to the exact
Fokker-Planck equation associated with the Poisson Representation of the
chemical master equation. The procedure yields impressively simple and compact
expressions for the finite-volume corrections. We prove that the predictions of
the rate equations will always underestimate the actual steady-state substrate
concentrations for an enzyme-reaction network confined in a small volume. In
particular we show that the finite-volume corrections increase with decreasing
sub-cellular volume, decreasing Michaelis-Menten constants and increasing
enzyme saturation. The magnitude of the corrections depends sensitively on the
topology of the network. The predictions of the theory are shown to be in
excellent agreement with stochastic simulations for two types of networks
typically associated with protein methylation and metabolism.Comment: 13 pages, 4 figures; published in The Journal of Chemical Physic
Search for Sterile Neutrinos with a Radioactive Source at Daya Bay
The far site detector complex of the Daya Bay reactor experiment is proposed
as a location to search for sterile neutrinos with > eV mass. Antineutrinos
from a 500 kCi 144Ce-144Pr beta-decay source (DeltaQ=2.996 MeV) would be
detected by four identical 20-ton antineutrino targets. The site layout allows
flexible source placement; several specific source locations are discussed. In
one year, the 3+1 sterile neutrino hypothesis can be tested at essentially the
full suggested range of the parameters Delta m^2_{new} and sin^22theta_{new}
(90% C.L.). The backgrounds from six nuclear reactors at >1.6 km distance are
shown to be manageable. Advantages of performing the experiment at the Daya Bay
far site are described
Post-Archean granitic rocks: contrasting petrogenetic processes and tectonic environments
Granitic rocks represent a ubiquitous component of upper continental crust but their origin remains highly controversial. This controversy stems from the fact that the granites may result from fractionation of mantle-derived basaltic magmas or partial melting of different crustal protoliths at contrasting pressure\u2013temperature conditions, either water-fluxed or fluid-absent. Consequently, many different mechanisms have been proposed to explain the compositional variability of granites ranging from whole igneous suites down to mineral scale. This Special Publication presents an overview of the state of the art and envisages future avenues towards a better understanding of granite petrogenesis
Can uptake length in strams be determined by nutrient addition experiments? Results from an interbiome comparison study
Nutrient uptake length is an important parnmeter tor quantifying nutrient cycling in streams. Although nutrient tracer additions are the preierred method for measuring uptake length under ambient nutrient concentrations, short-term nutrient addition experiments have more irequently been used to estimate uptake length in streams. Theoretical analysis of the relationship between uptake length determined by nutrient addition experiments (Sw\u27) and uptake length determined by tracer additions (Sw)predicted that Sw\u27 should be consistently longer than 5, , and that the overestimate of uptake length by Sw( should be related to the level of nutrient addition above ambient concentrations and the degree of nutrient limitation. To test these predictions, we used data irom an interbiorne study of NH,- uptake length in which 15NH,- tracer and short-term NH,-a ddition experiments were performed in 10 streams using a uniform experimental approach. The experimental results largely contirmed the theoretical predictions: sw\u27 was consistently longer than Sw and Sw\u27:Sw ratios were directly related to the level of NH,- addition and to indicatvrs of N limitation. The experimentally derived Sw\u27:Sw, ratios were used with the theoretical results to infer the N limitation status of each stream. Together, the theoretical and experimental results showed the tracer experiments should be used whenever possible to determine nutrient uptake length in streams. Nutrient addition experiments may be useful for comparing uptake lengths between different streams or cliiferent times in the same stream. however, provided that nutrient additions are kept as low as possible and of similar miagnitude
Coaxial Wire Measurements In NLC Accelerating Structures
The coaxial wire method provides an experimental way of measuring wake fields
without the need for a particle beam. A special setup has been designed and is
in the process of being fabricated at SLAC to measure the loss factors and
synchronous frequencies of dipole modes in both traveling and standing wave
structures for the Next Linear Collider (NLC). The method is described and
predictions based on electromagnetic field simulations are discussedComment: Paper presented at the 2002 8th European Particle Accelerator
Conference (EPAC 2002) Paris, France, June 3rd -June 7th, 200
Gamma-Ray Emissions from Pulsars: Spectra of the TEV Fluxes from Outer-Gap Accelerators
We study the gamma-ray emissions from an outer-magnetospheric potential gap
around a rotating neutron star. Migratory electrons and positrons are
accelerated by the electric field in the gap to radiate copious gamma-rays via
curvature process. Some of these gamma-rays materialize as pairs by colliding
with the X-rays in the gap, leading to a pair production cascade. Imposing the
closure condition that a single pair produces one pair in the gap on average,
we explicitly solve the strength of the acceleration field and demonstrate how
the peak energy and the luminosity of the curvature-radiated, GeV photons
depend on the strength of the surface blackbody and the power-law emissions.
Some predictions on the GeV emission from twelve rotation-powered pulsars are
presented. We further demonstrate that the expected pulsed TeV fluxes are
consistent with their observational upper limits. An implication of high-energy
pulse phase width versus pulsar age, spin, and magnetic moment is discussed.Comment: Revised to compute absolute TeV spectra (22 pages, 9 figures
Slow light propagation in trapped atomic quantum gases
We study semi-classical slow light propagation in trapped two level atomic
quantum gases. The temperature dependent behaviors of both group velocity and
transmissions are compared for low temperature Bose, Fermi, and Boltzman gases
within the local density approximation for their spatial density profile.Comment: 9 pages, 2 figure
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