Gibbs ensemble Monte Carlo simulations are reported for the vapor-liquid phase coexistence of argon, krypton, and xenon. The calculations employ accurate two-body potentials in addition to contributions from three-body dispersion interactions resulting from third-order triple-dipole, dipole-dipole-quadrupole, dipole-quadrupole-quadrupole, quadrupole-quadrupole-quadrupole, and fourth-order triple-dipole terms. It is shown that vapor-liquid equilibria are affected substantially by three-body interactions. The addition of three-body interactions results in good overall agreement of theory with experimental data. In particular, the subcritical liquid-phase densities are predicted accurately.
Female mammals are born with a lifetime's supply of oocytes individually enveloped in flattened epithelial cells to form primordial follicles. It is not clear how sufficient primordial follicles are maintained to sustain the reproductive lifespan, while providing an adequate supply of mature oocytes for ovulation. Locally produced growth factors are thought to be critical regulators of early follicle growth, but knowledge of their identity and source remains incomplete. Here, we have used a simple approach of spatial analysis of structures in histological tissue sections to identify likely sources of such regulatory molecules, narrowing the field for future screening for candidate growth factors or antagonists. We have quantified the relative spatial positions of primordial (resting) follicles and growing follicles in mice on days 4, 8, and 12 after birth, and calculated interfollicular distances. Follicles were significantly less likely to have started growing if they had 1 or more primordial follicles close by (within 10 m), predicting that primordial follicles inhibit each other. This approach allows us to hypothesize that primordial follicles produce a diffusible inhibitor that prevents neighboring primordial follicles from growing. Such an approach has wide applicability within many branches of developmental and cell biology for studying spatial signaling within tissues and cells.diffusing inhibitor ͉ follicle growth ͉ signal gradient
We present a unified method to estimate current–voltage characteristics of insulators starting from ab initio electronic calculations of the properties of the dielectric material. The method consists of three stages: (1) computation of trap energy distributions for excess electrons by means of density functional theory, (2) computation of local electron mobilities from a multiple trapping electron transport model which includes trap filling effects and (3) macroscopic integration of the Poisson and current–field equations, using local electron mobility data from stage (2) to predict the current–voltage characteristics for a material of a given width. The only input to this procedure is the chemical composition of the insulating material. We compare our model results with experimental studies of the current–voltage curve of cross-linked polyethylene.
Nonequilibrium molecular dynamics simulations are reported at different strain rates (gamma;) for a shearing atomic fluid interacting via accurate two- and three-body potentials. We report that the hydrostatic pressure has a strain-rate dependence of gamma;(2), in contrast to the gamma;(3/2) dependence predicted by mode-coupling theory. Our results indicate that the pressure and energy of real fluids may display an analytic dependence on the strain rate. This is in contrast to previous work using either Lennard-Jones or Weeks-Chandler-Anderson potentials that had shown a gamma;(3/2) dependence of pressure and energy.
Serotonin (5-HT) is well known to be a key neurotransmitter within the gastrointestinal (GI) tract, where it is responsible for influencing motility. Obtaining dynamic information about the neurotransmission process (specifically the release and reuptake of 5-HT) requires the development of new approaches to measure the extracellular 5-HT concentration profile. In this work constant-potential amperometry has been utilised at +650 mV vs. Ag|AgCl to measure in vitro the overflow of 5-HT. Steady-state levels of 5-HT have been observed, due to continuous mechanical stimulation of the tissue from the experimental protocol. Measurements are conducted at varying tissue-electrode distances in the range of 5 to 1100 microm. The difference in the current from the bulk media and that from each tissue-electrode distance is obtained, and the natural log of this current is plotted versus the tissue-electrode distance. The linear fit to the log of the current is derived, and its intercept, I(0), with the vertical axis and its slope are calculated. The reciprocal of the slope, indicated as slope(-1), is used as a marker of reuptake. The ratio between intercept, I(0), and the reciprocal of the slope, I(0)/slope(-1), is a measure of the flux at the tissue surface and it can be used as a marker for the 5-HT release rate. Current measurements for ileum and colon tissue indicated a significantly higher reuptake rate in the colon, showed by a lower slope(-1). In addition, the ratio, I(0)/slope(-1), indicated that the colon has a higher 5-HT flux compared to the ileum. Following the application of the serotonin selective reuptake inhibitor (SSRI), fluoxetine, both tissues showed a higher value of slope(-1), as the reuptake process is blocked preventing clearance of 5-HT. No differences were observed in the ratio, I(0)/slope(-1), in the ileum, but a decrease was observed in the colon. These results indicate that ileum and colon are characterised by different reuptake and release processes. The new approach we propose provides pivotal information on the variations in the signalling mechanism, where steady state levels are observed and can be a vital tool to study differences between normal and diseased tissue and also the efficacy of pharmacological agents.
Molecular simulation data are reported that indicate that there is a simple empirical relationship between two-body and three-body interaction energies. The significance of this relationship is that three-body interactions can be estimated accurately from two-body interactions without incurring the computational penalty of three-body calculations. The relationship is tested by performing Gibbs ensemble simulations for the vapor-liquid equilibria of argon. The results are in good agreement with calculations that explicitly evaluate all three-body interactions.
We describe a model of the mechanical properties of the cell plasma membrane using a finite-temperature particle-dynamics simulation of the whole cell, in which a two-dimensional network of virtual particles embedded in a three-dimensional closed surface represents the membrane. The particles interact via harmonic potential and dihedral angle potential and are subject to a constant area constraint. The evolution of the positions of the particles yields the equilibrium state of the membrane and allows determination of the membrane thermal fluctuations and the elastic moduli. We show that time-averaging of the cell-model configurations allows quantitative comparison with experimental data on membrane fluctuations and elastic moduli of the red blood cell.
Gibbs ensemble simulations using ab initio intermolecular potentials are reported for the vapourÈliquid phase coexistence of neon and argon. For neon two di †erent quantum chemical ab initio potentials of well-known quality are used to investigate the e †ect of the quality of pair interactions. In addition calculations are also reported for neon using a potential that includes three-body interactions. For argon, simulations are compared with results obtained from NPH-ensemble molecular dynamics simulations. It is found that the results of a perfect pair potential must occur outside the experimental temperatureÈdensity phase envelope. Therefore, if a perfect pair potential is used, many-body interactions and quantum e †ects must be considered to obtain good agreement with experiment.
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