1,760 research outputs found
Processes on the emergent landscapes of biochemical reaction networks and heterogeneous cell population dynamics: differentiation in living matters.
The notion of an attractor has been widely employed in thinking about the nonlinear dynamics of organisms and biological phenomena as systems and as processes. The notion of a landscape with valleys and mountains encoding multiple attractors, however, has a rigorous foundation only for closed, thermodynamically non-driven, chemical systems, such as a protein. Recent advances in the theory of nonlinear stochastic dynamical systems and its applications to mesoscopic reaction networks, one reaction at a time, have provided a new basis for a landscape of open, driven biochemical reaction systems under sustained chemostat. The theory is equally applicable not only to intracellular dynamics of biochemical regulatory networks within an individual cell but also to tissue dynamics of heterogeneous interacting cell populations. The landscape for an individual cell, applicable to a population of isogenic non-interacting cells under the same environmental conditions, is defined on the counting space of intracellular chemical composition
Transport critical current density in Fe-sheathed nano-SiC doped MgB2 wires
The nano-SiC doped MgB2/Fe wires were fabricated using a powder-in-tube
method and an in-situ reaction process. The depression of Tc with increasing
SiC doping level remained rather small due to the counterbalanced effect of Si
and C co-doping. The high level SiC co-doping allowed creation of the
intra-grain defects and nano-inclusions, which act as effective pinning
centers, resulting in a substantial enhancement in the Jc(H) performance. The
transport Jc for all the wires is comparable to the magnetic Jc at higher
fields despite the low density of the samples and percolative nature of
current. The transport Ic for the 10wt% SiC doped MgB2/Fe reached 660A at 5K
and 4.5T (Jc = 133,000A/cm2) and 540A at 20K and 2T (Jc = 108,000A/cm2). The
transport Jc for the 10wt% SiC doped MgB2 wire is more than an order of
magnitude higher than for the state-the-art Fe-sheathed MgB2 wire reported to
date at 5K and 10T and 20K and 5T respectively. There is a plenty of room for
further improvement in Jc as the density of the current samples is only 50%.Comment: 4 pages, 7 figures, presented at ASC 2002, Housto
Determining Relative Dynamic Stability of Cell States Using Boolean Network Model.
Cell state transition is at the core of biological processes in metazoan, which includes cell differentiation, epithelial-to-mesenchymal transition (EMT) and cell reprogramming. In these cases, it is important to understand the molecular mechanism of cellular stability and how the transitions happen between different cell states, which is controlled by a gene regulatory network (GRN) hard-wired in the genome. Here we use Boolean modeling of GRN to study the cell state transition of EMT and systematically compare four available methods to calculate the cellular stability of three cell states in EMT in both normal and genetically mutated cases. The results produced from four methods generally agree but do not totally agree with each other. We show that distribution of one-degree neighborhood of cell states, which are the nearest states by Hamming distance, causes the difference among the methods. From that, we propose a new method based on one-degree neighborhood, which is the simplest one and agrees with other methods to estimate the cellular stability in all scenarios of our EMT model. This new method will help the researchers in the field of cell differentiation and cell reprogramming to calculate cellular stability using Boolean model, and then rationally design their experimental protocols to manipulate the cell state transition
Crystal and Electronic Structures of LiNHâ‚‚
The crystal structure of LiNH2 was reinvestigated using powder neutron diffraction with high sensitivity. The compound crystallizes in the tetragonal space group I4 with lattice parameters α = b= 5.034 42 (24) Å, c = 10.255 58 (52) Å. It is found that H atoms occupy 8g1(0.2429, 0.1285, 0.1910) and 8g2 (0.3840, 0.3512, 0.1278) sites. The bond lengths between the nearest nitrogen and hydrogen atoms are 0.986 and 0.942 Å, respectively. The bond angle between H-N-H is about 99.97°. These results are significantly different from those of previous experiments. The electronic structure was calculated according to the revised structural data. The calculated density of states and charge density distribution show strong ionic characteristics between the ionic Li+ cation and the covalent bonded [NH2]- anion
Growth and Magnetic Properties of MnO₂-δ Nanowire Microspheres
We report the synthesis of MnO2-delta microspheres using hydrothermal and conventional chemical reaction methods. The microspheres of MnO2-delta consist of nanowires having a diameter of 20-50 nm and a length of 2-8 µm. The value of oxygen vacancy delta estimated from x-ray photoelectron spectrum is 0.3. The magnetization versus temperature curve indicates a magnetic transition at about 13 K. It is found that a parasitic ferromagnetic component is imposed on the antiferromagnetic structure of MnO2-delta, which might result from distortion of the lattice structure due to oxygen vacancies. The magnetic transition temperature TN is about 10 K lower than that of the bulk MnO2 single crystal
Cell Population Growth Kinetics in the Presence of Stochastic Heterogeneity of Cell Phenotype
Recent studies at individual cell resolution have revealed phenotypic
heterogeneity in nominally clonal tumor cell populations. The heterogeneity
affects cell growth behaviors, which can result in departure from the idealized
uniform exponential growth of the cell population. Here we measured the
stochastic time courses of growth of an ensemble of populations of HL60
leukemia cells in cultures, starting with distinct initial cell numbers to
capture a departure from the {uniform exponential growth model for the initial
growth (``take-off'')}. Despite being derived from the same cell clone, we
observed significant variations in the early growth patterns of individual
cultures with statistically significant differences in growth dynamics, which
could be explained by the presence of inter-converting subpopulations with
different growth rates, and which could last for many generations. Based on the
hypothesis of existence of multiple subpopulations, we developed a branching
process model that was consistent with the experimental observations
Large Scale Growth and Magnetic Properties of Fe and Fe₃O₄ Nanowires
Fe and Fe3O4 nanowires have been synthesized by thermal decomposition of Fe(CO)5, followed by heat treatments. The Fe wires are formed through the aggregation of nanoparticles generated by decomposition of Fe(CO)5. A core-shell structure with an iron oxide shell and Fe core is observed for the as-prepared Fe wires. Annealing in air leads to the formation of Fe2O3/Fe3O4 wires, which after heat treatment in a N2/alcohol atmosphere form Fe3O4 wires with a sharp Verwey [Nature (London) 144, 327 (1939)] transition at 125 K. The Fe3O4 wires have coercivities of 261 and 735 Oe along the wire axis at RT and 5 K, respectively. The large increase of coercivity at 5 K as compared to RT is due to the increase of anisotropy resulting from the Verwey transition
The Effect of Cu-Doping on the Magnetic and Transport Properties of La₀.₇Sr₀.₃MnO₃
The effects of Cu-doping on the structural, magnetic, and transport properties of La0.7Sr0.3Mn1xCuxO3 (0\u3c=x\u3c=0.20) have been studied using neutron diffraction, magnetization, and magnetoresistance (MR) measurements. All samples show the rhombohedral structure with the R[overline 3]c space-group from 10 K to room temperature (RT). Neutron diffraction data suggest that some of the Cu ions have a Cu3+ state in these compounds. The substitution of Mn by Cu affects the MnO bond length and Mn-O-Mn bond angle resulting from the minimization of the distortion of the MnO6 octahedron. Resistivity measurements show that a metal to insulator transition occurs for the x\u3e=0.15 samples. The x=0.15 sample shows the highest MR([approximate]80%), which might result from the co-existence of Cu3-Cu2+ and the dilution effect of Cu-doping on the double exchange interactio
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