7,331 research outputs found
Local molecular field theory for the treatment of electrostatics
We examine in detail the theoretical underpinnings of previous successful
applications of local molecular field (LMF) theory to charged systems. LMF
theory generally accounts for the averaged effects of long-ranged components of
the intermolecular interactions by using an effective or restructured external
field. The derivation starts from the exact Yvon-Born-Green hierarchy and shows
that the approximation can be very accurate when the interactions averaged over
are slowly varying at characteristic nearest-neighbor distances. Application of
LMF theory to Coulomb interactions alone allows for great simplifications of
the governing equations. LMF theory then reduces to a single equation for a
restructured electrostatic potential that satisfies Poisson's equation defined
with a smoothed charge density. Because of this charge smoothing by a Gaussian
of width sigma, this equation may be solved more simply than the detailed
simulation geometry might suggest. Proper choice of the smoothing length sigma
plays a major role in ensuring the accuracy of this approximation. We examine
the results of a basic confinement of water between corrugated wall and justify
the simple LMF equation used in a previous publication. We further generalize
these results to confinements that include fixed charges in order to
demonstrate the broader impact of charge smoothing by sigma. The slowly-varying
part of the restructured electrostatic potential will be more symmetric than
the local details of confinements.Comment: To be published in J Phys-Cond Matt; small misprint corrected in Eq.
(12) in V
Drug-like analogues of the parasitic worm-derived immunomodulator ES-62 are therapeutic in the MRL/Lpr model of systemic lupus erythematosus
Introduction ES-62, a phosphorylcholine (PC)-containing immunomodulator secreted by the parasitic worm Acanthocheilonema viteae, protects against nephritis in the MRL/Lpr mouse model of systemic lupus erythematosus (SLE). However, ES-62 is not suitable for development as a therapy and thus we have designed drug-like small molecule analogues (SMAs) based around its active PC-moiety. To provide proof of concept that ES-62-based SMAs exhibit therapeutic potential in SLE, we have investigated the capacity of two SMAs to protect against nephritis when administered to MRL/Lpr mice after onset of kidney damage.
Methods SMAs 11a and 12b were evaluated for their ability to suppress antinuclear antibody (ANA) generation and consequent kidney pathology in MRL/Lpr mice when administered after the onset of proteinuria.
Results SMAs 11a and 12b suppressed development of ANA and proteinuria. Protection reflected downregulation of MyD88 expression by kidney cells and this was associated with reduced production of IL-6, a cytokine that exhibits promise as a therapeutic target for this condition.
Conclusions SMAs 11a and 12b provide proof of principle that synthetic compounds based on the safe immunomodulatory mechanisms of parasitic worms can exhibit therapeutic potential as a novel class of drugs for SLE, a disease for which current therapies remain inadequate
Stretched exponentials and power laws in granular avalanching
We introduce a model for granular avalanching which exhibits both stretched exponential and power law avalanching over its parameter range. Two modes of transport are incorporated, a rolling layer consisting of individual particles and the overdamped, sliding motion of particle clusters. The crossover in behaviour observed in experiments on piles of rice is attributed to a change in the dominant mode of transport. We predict that power law avalanching will be observed whenever surface flow is dominated by clustered motion.
Comment: 8 pages, more concise and some points clarified
Time Variability in the X-ray Nebula Powered by Pulsar B1509-58
We use new and archival Chandra and ROSAT data to study the time variability
of the X-ray emission from the pulsar wind nebula (PWN) powered by PSR B1509-58
on timescales of one week to twelve years. There is variability in the size,
number, and brightness of compact knots appearing within 20" of the pulsar,
with at least one knot showing a possible outflow velocity of ~0.6c (assuming a
distance to the source of 5.2 kpc). The transient nature of these knots may
indicate that they are produced by turbulence in the flows surrounding the
pulsar. A previously identified prominent jet extending 12 pc to the southeast
of the pulsar increased in brightness by 30% over 9 years; apparent outflow of
material along this jet is observed with a velocity of ~0.5c. However, outflow
alone cannot account for the changes in the jet on such short timescales.
Magnetohydrodynamic sausage or kink instabilities are feasible explanations for
the jet variability with timescale of ~1.3-2 years. An arc structure, located
30"-45" north of the pulsar, shows transverse structural variations and appears
to have moved inward with a velocity of ~0.03c over three years. The overall
structure and brightness of the diffuse PWN exterior to this arc and excluding
the jet has remained the same over the twelve year span. The photon indices of
the diffuse PWN and possibly the jet steepen with increasing radius, likely
indicating synchrotron cooling at X-ray energies.Comment: accepted to ApJ, 14 pages, 8 figure
Spectral Density of Sparse Sample Covariance Matrices
Applying the replica method of statistical mechanics, we evaluate the
eigenvalue density of the large random matrix (sample covariance matrix) of the
form , where is an real sparse random matrix.
The difference from a dense random matrix is the most significant in the tail
region of the spectrum. We compare the results of several approximation
schemes, focusing on the behavior in the tail region.Comment: 22 pages, 4 figures, minor corrections mad
The crystal and molecular structure of the bis(4-N, N1-dimethylaminopyridine) solvate of disalicylicacidatobis(nitrotodioxouranium)(VI)
The structure of the title compound [(C7H4NO8U)(C7H11N2)]2 has been determined by Patterson and Fourier methods from single crystal X-ray diffraction data collected on a four-circle diffractometer. Full-matrix least-squares refinement yielded a final conventional R of 0.041 for 2189 reflections. The complex crystallizes in the space group P with a = 11.004(5), b = 9.981(5), c = 9.928(5) Å, α = 119.6(3), β = 107.7(3), γ = 81.9(3)°, Dm = 2.17, Dc = 2.173g cm−3. The structure is dimeric. The uranium atoms are eight-coordinate and are bridged via centrosymmetrically related carboxylic oxygen atoms. The nitrate group is bidentate and the average U---O (ligand) distance is 2.463 Å. Hydrogen bonding of the type N---HO links two dimethyl-aminopyridine molecules to the dimer
The structure of aliphatic amine adducts of uranyl acetylacetonate. III. Dioxobis(2,4-pentanedionato)mono (2-N-isopropylaminopentan-4-one)uranium(VI)
Introduction: In two earlier structural determinations of compounds of this type we have shown that the conformation of the adduct moiety is dependent on the formation of intramolecular N-H...0 hydrogen bonds (part I: Haigh, Nassimbeni, Pauptit, Rodgers & Sheldrick, 1976; part II: Nassimbeni, Orpen, Pauptit, Rodgers & Haigh, 1977). We have carried out the present analysis to study the conformational effects on the ligand brought about by the steric influence of an isopropyl substituent at N
The Anisotropic Bak-Sneppen model
The Bak-Sneppen model is shown to fall into a different universality class with the introduction of a preferred direction, mirroring the situation in spin systems. This is first demonstrated by numerical simulations and subsequently confirmed by analysis of the multitrait version of the model, which admits exact solutions in the extremes of zero and maximal anisotropy. For intermediate anisotropies, we show that the spatiotemporal evolution of the avalanche has a power law `tail' which passes through the system for any non-zero anisotropy but remains fixed for the isotropic case, thus explaining the crossover in behaviour. Finally, we identify the maximally anisotropic model which is more tractable and yet more generally applicable than the isotropic system
Nonlinear models of the bump cepheid HV 905 and the distance modulus to the large magellanic cloud
Nonlinear pulsation models have been used to simulate the light curve of the LMC bump Cepheid HV 905. In order to reproduce the light curve accurately, tight constraints on the input parameters M, L, and T-eff are required. The results, combined with accurate existing V and I photometry, yield an LMC distance modulus of 18.51 +/- 0.05, and they show that the luminosity of HV 905 is much higher than expected from the mass-luminosity relation of stellar evolution theory. If we assume that the pulsation models are accurate, this suggests that there is a larger amount of convective core overshoot during the main-sequence evolution of stars with M similar to 5 M. than is usually assumed
Competitive random sequential adsorption of point and fixed-sized particles: analytical results
We study the kinetics of competitive random sequential adsorption (RSA) of
particles of binary mixture of points and fixed-sized particles within the
mean-field approach. The present work is a generalization of the random car
parking problem in the sense that it considers the case when either a car of
fixed size is parked with probability q or the parking space is partitioned
into two smaller spaces with probability (1-q) at each time event. This allows
an interesting interplay between the classical RSA problem at one extreme
(q=1), and the kinetics of fragmentation processes at the other extreme (q=0).
We present exact analytical results for coverage for a whole range of q values,
and physical explanations are given for different aspects of the problem. In
addition, a comprehensive account of the scaling theory, emphasizing on
dimensional analysis, is presented, and the exact expression for the scaling
function and exponents are obtained.Comment: 7 pages, latex, 3 figure
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