2,526 research outputs found
Incorporation of excluded volume correlations into Poisson-Boltzmann theory
We investigate the effect of excluded volume interactions on the electrolyte
distribution around a charged macroion. First, we introduce a criterion for
determining when hard-core effects should be taken into account beyond standard
mean field Poisson-Boltzmann (PB) theory. Next, we demonstrate that several
commonly proposed local density functional approaches for excluded volume
interactions cannot be used for this purpose. Instead, we employ a non-local
excess free energy by using a simple constant weight approach. We compare the
ion distribution and osmotic pressure predicted by this theory with Monte Carlo
simulations. They agree very well for weakly developed correlations and give
the correct layering effect for stronger ones. In all investigated cases our
simple weighted density theory yields more realistic results than the standard
PB approach, whereas all local density theories do not improve on the PB
density profiles but on the contrary, deviate even more from the simulation
results.Comment: 23 pages, 7 figures, 1 tabl
Self-consistent Ornstein-Zernike approximation for three-dimensional spins
An Ornstein-Zernike approximation for the two-body correlation function
embodying thermodynamic consistency is applied to a system of classical
Heisenberg spins on a three-dimensional lattice. The consistency condition
determined in a previous work is supplemented by introducing a simplified
expression for the mean-square fluctuations of the spin on each lattice site.
The thermodynamics and the correlations obtained by this closure are then
compared with approximants based on extrapolation of series expansions and with
Monte Carlo simulations. The comparison reveals that many properties of the
model, including the critical temperature, are very well reproduced by this
simple version of the theory, but that it shows substantial quantitative error
in the critical region, both above the critical temperature and with respect to
its rendering of the spontaneous magnetization curve. A less simple but
conceptually more satisfactory version of the SCOZA is then developed, but not
solved, in which the effects of transverse correlations on the longitudinal
susceptibility is included, yielding a more complete and accurate description
of the spin-wave properties of the model.Comment: 32 pages, 12 figure
Adenosine Triphosphate (ATP) as a Metric of Microbial Biomass in Aquatic Systems: New Simplified Protocols, Laboratory Validation, and a Reflection on Data From the Literature
The use of adenosine triphosphate (ATP) as a universal biomass indicator is built on the premise that ATP concentration tracks biomass rather than the physiological condition of cells. However, reportedly high variability in ATP in response to environmental conditions is the main reason the method has not found widespread application. To test possible sources of this variability, we used the diatom Thalassiosira weissflogii as a model and manipulated its growth rate through nutrient limitation and through exposure to three different temperatures (15°C, 20°C, and 25°C). We simplified the ATP protocol with hotâwater or chemical extraction methods, modified a commercially available luciferinâluciferase assay, and employed singleâphoton counting in a scintillation counter, all of which increased sensitivity and throughput. Perâcell ATP levels remained relatively constant despite changes in growth rates by approximately 10âfold in the batch culture (i.e., nutrient limitation) experiments, and approximately 2âfold in response to temperature. The reâexamination of related literature values revealed that average cellular ATP levels differed little among taxonomic groups of aquatic microbes, even at the domain level, and correlated well with bulk properties such as elemental carbon or nitrogen. Fulfilling multiple cellular functions in addition to being the universal energy currency requires ATP to be maintained in a millimolar concentration range. Consequently, ATP relates directly to live cytoplasm volume, while elemental carbon and nitrogen are constrained by an indeterminate pool of detrital material and intracellular storage compounds. The ATPâbiomass indicator is sensitive, economical, and can be readily standardized among laboratories and across environments
H-1-MRS Measured Ectopic Fat in Liver and Muscle in Danish Lean and Obese Children and Adolescents
This cross sectional study aims to investigate the associations between ectopic lipid accumulation in liver and skeletal muscle and biochemical measures, estimates of insulin resistance, anthropometry, and blood pressure in lean and overweight/obese children.Fasting plasma glucose, serum lipids, serum insulin, and expressions of insulin resistance, anthropometry, blood pressure, and magnetic resonance spectroscopy of liver and muscle fat were obtained in 327 Danish children and adolescents aged 8-18 years.In 287 overweight/obese children, the prevalences of hepatic and muscular steatosis were 31% and 68%, respectively, whereas the prevalences in 40 lean children were 3% and 10%, respectively. A multiple regression analysis adjusted for age, sex, body mass index z-score (BMI SDS), and pubertal development showed that the OR of exhibiting dyslipidemia was 4.2 (95%CI: [1.8; 10.2], p = 0.0009) when hepatic steatosis was present. Comparing the simultaneous presence of hepatic and muscular steatosis with no presence of steatosis, the OR of exhibiting dyslipidemia was 5.8 (95%CI: [2.0; 18.6], p = 0.002). No significant associations between muscle fat and dyslipidemia, impaired fasting glucose, or blood pressure were observed. Liver and muscle fat, adjusted for age, sex, BMI SDS, and pubertal development, associated to BMI SDS and glycosylated hemoglobin, while only liver fat associated to visceral and subcutaneous adipose tissue and intramyocellular lipid associated inversely to high density lipoprotein cholesterol.Hepatic steatosis is associated with dyslipidemia and liver and muscle fat depositions are linked to obesity-related metabolic dysfunctions, especially glycosylated hemoglobin, in children and adolescents, which suggest an increased cardiovascular disease risk
Natural iron enrichment around the Antarctic Peninsula in the Southern Ocean
As part of the US-AMLR program in January-February of 2006, 99 stations in
the South Shetland Islands-Antarctic Peninsula region were sampled to
understand the variability in hydrographic and biological properties related
to the abundance and distribution of krill in this area. Concentrations of
dissolved iron (DFe) and total acid-leachable iron (TaLFe) were measured in
the upper 150 m at 16 of these stations (both coastal and pelagic waters) to
better resolve the factors limiting primary production in this area and in
downstream waters of the Scotia Sea. The concentrations of DFe and TaLFe in
the upper mixed layer (UML) were relatively high in Weddell Sea Shelf Waters
(~0.6 nM and 15 nM, respectively) and low in Drake Passage waters
(~0.2 nM and 0.9 nM, respectively). In the Bransfield
Strait, representing a mixture of waters from the Weddell Sea and the
Antarctic Circumpolar Current (ACC), concentrations of DFe were ~0.4 nM
and of TaLFe ~1.7 nM. The highest concentrations of DFe and TaLFe
in the UML were found at shallow coastal stations close to Livingston Island
(~1.6 nM and 100 nM, respectively). The ratio of TaLFe:DFe varied
with the distance to land: ~45 at the shallow coastal stations, ~15
in the high-salinity waters of Bransfield Strait, and ~4 in ACC
waters. Concentrations of DFe increased slightly with depth in the water
column, while that of TaLFe did not show any consistent trend with depth.
Our Fe data are discussed in regard to the hydrography and water circulation
patterns in the study area, and with the hypothesis that the relatively high
rates of primary production in the central regions of the Scotia Sea are
partially sustained by natural iron enrichment resulting
from a northeasterly flow of iron-rich coastal waters originating in
the South Shetland Islands-Antarctic Peninsula region
Magnetic ground state and magnon-phonon interaction in multiferroic h-YMnO
Inelastic neutron scattering has been used to study the magneto-elastic
excitations in the multiferroic manganite hexagonal YMnO. An avoided
crossing is found between magnon and phonon modes close to the Brillouin zone
boundary in the -plane. Neutron polarization analysis reveals that this
mode has mixed magnon-phonon character. An external magnetic field along the
-axis is observed to cause a linear field-induced splitting of one of the
spin wave branches. A theoretical description is performed, using a Heisenberg
model of localized spins, acoustic phonon modes and a magneto-elastic coupling
via the single-ion magnetostriction. The model quantitatively reproduces the
dispersion and intensities of all modes in the full Brillouin zone, describes
the observed magnon-phonon hybridized modes, and quantifies the magneto-elastic
coupling. The combined information, including the field-induced magnon
splitting, allows us to exclude several of the earlier proposed models and
point to the correct magnetic ground state symmetry, and provides an effective
dynamic model relevant for the multiferroic hexagonal manganites.Comment: 12 pages, 10 figure
Dose-response of myofibrillar protein synthesis to ingested whey protein during energy restriction in overweight postmenopausal women: a randomized, controlled trial
BackgroundDiet-induced weight loss is associated with a decline in lean body mass, as mediated by an impaired response of muscle protein synthesis (MPS). The dose-response of MPS to ingested protein, with or without resistance exercise, is well characterized during energy balance but limited data exist under conditions of energy restriction in clinical populations.ObjectiveTo determine the dose-response of MPS to ingested whey protein following short-term diet-induced energy restriction in overweight, postmenopausal, women at rest and postexercise.DesignForty middle-aged (58.6Âą0.4 y), overweight (BMI: 28.6Âą0.4), postmenopausal women were randomly assigned to 1 of 4 groups: Three groups underwent 5 d of energy restriction (âź800 kcal/d). On day 6, participants performed a unilateral leg resistance exercise bout before ingesting either a bolus of 15g (ERW15, n = 10), 35g (ERW35, n = 10) or 60g (ERW60, n = 10) of whey protein. The fourth group (n = 10) ingested a 35g whey protein bolus after 5 d of an energy balanced diet (EBW35, n = 10). Myofibrillar fractional synthetic rate (FSR) was calculated under basal, fed (FED) and postexercise (FED-EX) conditions by combining an L-[ring-13C6] phenylalanine tracer infusion with the collection of bilateral muscle biopsies.ResultsMyofibrillar FSR was greater in ERW35 (0.043Âą0.003%/h, P = 0.013) and ERW60 (0.042Âą0.003%/h, P = 0.026) than ERW15 (0.032 Âą 0.003%/h), with no differences between ERW35 and ERW60 (P = 1.000). Myofibrillar FSR was greater in FED (0.044 Âą 0.003%/h, P < 0.001) and FED-EX (0.048 Âą 0.003%/h, P < 0.001) than BASAL (0.027 Âą 0.003%/h), but no differences were detected between FED and FED-EX (P = 0.732) conditions. No differences in myofibrillar FSR were observed between EBW35 (0.042 Âą 0.003%/h) and ERW35 (0.043 Âą 0.003%/h, P = 0.744).ConclusionA 35 g dose of whey protein, ingested with or without resistance exercise, is sufficient to stimulate a maximal acute response of MPS following short-term energy restriction in overweight, postmenopausal women, and thus may provide a per serving protein recommendation to mitigate muscle loss during a weight loss program.Trial registryclinicaltrials.gov (ID: NCT03326284)
Screening of Spherical Colloids beyond Mean Field -- A Local Density Functional Approach
We study the counterion distribution around a spherical macroion and its
osmotic pressure in the framework of the recently developed
Debye-H"uckel-Hole-Cavity (DHHC) theory. This is a local density functional
approach which incorporates correlations into Poisson-Boltzmann theory by
adding a free energy correction based on the One Component Plasma. We compare
the predictions for ion distribution and osmotic pressure obtained by the full
theory and by its zero temperature limit with Monte Carlo simulations. They
agree excellently for weakly developed correlations and give the correct trend
for stronger ones. In all investigated cases the DHHC theory and its
computationally simpler zero temperature limit yield better results than the
Poisson-Boltzmann theory.Comment: 10 pages, 4 figures, 2 tables, RevTeX4-styl
Pseudorapidity distributions of charged particles from Au+Au collisions at the maximum RHIC energy, Sqrt(s_NN) = 200 GeV
We present charged particle densities as a function of pseudorapidity and
collision centrality for the 197Au+197Au reaction at Sqrt{s_NN}=200 GeV. For
the 5% most central events we obtain dN_ch/deta(eta=0) = 625 +/- 55 and
N_ch(-4.7<= eta <= 4.7) = 4630+-370, i.e. 14% and 21% increases, respectively,
relative to Sqrt{s_NN}=130 GeV collisions. Charged-particle production per pair
of participant nucleons is found to increase from peripheral to central
collisions around mid-rapidity. These results constrain current models of
particle production at the highest RHIC energy.Comment: 4 pages, 5 figures; fixed fig. 5 caption; revised text and figures to
show corrected calculation of and ; final version accepted for
publicatio
A simple and fast heuristic for protein structure comparison
Background
Protein structure comparison is a key problem in bioinformatics. There exist several methods for doing protein comparison, being the solution of the Maximum Contact Map Overlap problem (MAX-CMO) one of the alternatives available. Although this problem may be solved using exact algorithms, researchers require approximate algorithms that obtain good quality solutions using less computational resources than the formers.
Results
We propose a variable neighborhood search metaheuristic for solving MAX-CMO. We analyze this strategy in two aspects: 1) from an optimization point of view the strategy is tested on two different datasets, obtaining an error of 3.5%(over 2702 pairs) and 1.7% (over 161 pairs) with respect to optimal values; thus leading to high accurate solutions in a simpler and less expensive way than exact algorithms; 2) in terms of protein structure classification, we conduct experiments on three datasets and show that is feasible to detect structural similarities at SCOP's family and CATH's architecture levels using normalized overlap values. Some limitations and the role of normalization are outlined for doing classification at SCOP's fold level.
Conclusion
We designed, implemented and tested.a new tool for solving MAX-CMO, based on a well-known metaheuristic technique. The good balance between solution's quality and computational effort makes it a valuable tool. Moreover, to the best of our knowledge, this is the first time the MAX-CMO measure is tested at SCOP's fold and CATH's architecture levels with encouraging results.
Software is available for download at http://modo.ugr.es/jrgonzalez/msvns4maxcmo webcite.This work is supported by Projects HeuriCosc TIN2005-08404-C04-01, HeuriCode TIN2005-08404-C04-03, both from the Spanish Ministry of Education and Science.
JRG acknowledges financial support from Project TIC2002-04242-C03-02.
Authors thank N. Krasnogor and ProCKSi project (BB/C511764/1) for their support
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