Phase space dynamics of unmagnetized plasmas: collisionless and collisional regimes

Eulerian electrostatic kinetic simulations of unmagnetized plasmas (kinetic electrons and motionless protons) with high-frequency equilibrium perturbations have been employed to investigate the phase space energy transfer across spatial and velocity scales, associated with the resonant interaction of electrons with the self-induced electric field. Numerical runs cover a wide range of collisionless and weakly collisional plasma regimes. An analysis technique based on the Fourier-Hermite transform of the particle distribution function allows to point out how kinetic processes trigger the phase space energy cascade, which is instead inhibited at finer scales when collisions are turned on. Numerical results are presented and discussed for the cases of linear wave Landau damping, nonlinear electron trapping, bump-on-tail and two-stream instabilities. A more realistic situation of turbulent Langmuir fluctuations is also discussed in detail. Fourier-Hermite transform shows an energy spread, highly conditioned by collisions, which involves velocity scales more quickly than the spatial scales, even when nonlinear effects are dominant. This results in anisotropic spectra whose slopes are compatible with theoretical expectations. Finally, an exact conservation law has been derived, which describes the time evolution of the free energy of the system, taking into account the collisional dissipation.Comment: 15 pages, 7 figure

BMP signaling mediates glioma stem cell quiescence and confers treatment resistance in glioblastoma.

Despite advances in therapy, glioblastoma remains an incurable disease with a dismal prognosis. Recent studies have implicated cancer stem cells within glioblastoma (glioma stem cells, GSCs) as mediators of therapeutic resistance and tumor progression. In this study, we investigated the role of the transforming growth factor-β (TGF-β) superfamily, which has been found to play an integral role in the maintenance of stem cell homeostasis within multiple stem cell systems, as a mediator of stem-like cells in glioblastoma. We find that BMP and TGF-β signaling define divergent molecular and functional identities in glioblastoma, and mark relatively quiescent and proliferative GSCs, respectively. Treatment of GSCs with BMP inhibits cell proliferation, but does not abrogate their stem-ness, as measured by self-renewal and tumorigencity. Further, BMP pathway activation confers relative resistance to radiation and temozolomide chemotherapy. Our findings define a quiescent cancer stem cell population in glioblastoma that may be a cellular reservoir for tumor recurrence following cytotoxic therapy

Obtaining the structure and bond rotational potential of a substituted ethane by NMR spectroscopy of solutions in nematic liquid-crystalline solvents

Partially averaged dipolar couplings (also referred to as residual dipolar couplings) Dij can be obtained from the analysis of the NMR spectra of molecules dissolved in liquid-crystalline solvents. Their values for a nonrigid molecule depend upon the bond lengths and angles, the rotational potentials, and the orientational order of the molecules. The molecule studied, 1-chloro-2-bromoethane, is one of the simplest example of a substituted alkane in which the rotational potential has three minimum-energy positions, trans and gauche±conformations, and the present investigation explores the problems inherent in deriving the form of the potential and the molecular geometry from the set of partially averaged couplings between the protons, and between protons and 13C nuclei. The geometrical parameters and the rotational potential obtained are compared with the results from a density-functional theory method

Short- and long-range contributions to the ordering of rigid planar solutes dissolved in a 55wt% ZLI1132+EBBA nematic mixture

Nine well studied (polar and non-polar) planar solutes have been used as probes to investigate the mechanism for orientational ordering in the nematic phase. Second-rank orientational order parameters have been derived from proton dipolar coupling constants, obtained from the NMR spectra of these molecules dissolved in a 55 wt% ZLI1132 + EBBA nematic mixture. The experimental data are interpreted in terms of an orienting potential experienced by molecules in a liquid crystal environment. A model for the orienting potential is presented which includes, in addition to short-range repulsion interactions described by the surface tensor model, long-range contributions treated according to a generalized reaction field approach. Least-squares fits of solvent dependent parameters to the experimental data have been performed to estimate the weight of the various contributions to the solute orientational order, and the results obtained in this way are discussed. Finally, the results obtained with the surface tensor model are compared with those from the size-shape potential suggested by other workers

The conformational distribution in diphenylmethane determined by nuclear magnetic resonance spectroscopy of a sample dissolved in a nematic liquid crystalline solvent

The deuterium decoupled, proton nuclear magnetic resonance spectrum of a sample of diphenylmethane-d(3) dissolved in a nematic liquid crystalline solvent has been analyzed to yield a set of dipolar couplings, D-ij. These have been used to test models for the conformational distribution generated by rotation about the two ring-CH2 bonds through angles tau(1) and tau(2). Conformational distributions, particularly when obtained from a quantum chemistry calculation, are usually described in terms of the potential energy surface, V(tau(1),tau(2)), which is then used to define a probability density distribution, P(tau(1),tau(2)). It is shown here that when attempting to obtain P(tau(1),tau(2)) from experimental data it can be an advantage to do this directly without going through the intermediate step of trying to characterize V(tau(1,)tau(2)). When applied to diphenylmethane this method shows that the dipolar couplings are consistent with a conformational distribution centered on tau(1)=tau(2)=56.5+/-0.5degrees, which is close to the values calculated for an isolated molecule of 57.0degrees, and significantly different from the asymmetric structure found in the crystalline state

Toward a generalized algorithm for the automated analysis of complex anisotropic NMR spectra

An existing algorithm, founded on the works of Stephenson and Binsch, for the automatic analysis of isotropic or simple anisotropic NMR spectra has been improved to treat very complex NMR spectra of molecules dissolved in nematic solvents. The main options added to the original algorithm are a wider choice of smoothing functions; the use of the principal component regression method; and the possibility of selecting molecular coordinates, order parameters, and spectral parameters as variables of the problem. By means of these new options, it has been possible to analyze automatically NMR spectra (even depending on 27 spectral parameters) of 16 molecules in an anisotropic environment. Details of each case are discussed

Toward a Generalized Algorithm for the Automated Analysis of Complex Anisotropic NMR Spectra

An existing algorithm, founded on the works of Stephenson and Binsch, for the automatic analysis of isotropic or simple anisotropic NMR spectra has been improved to treat very complex NMR spectra of molecules dissolved in nematic solvents. The main options added to the original algorithm are a wider choice of smoothing functions; the use of the principal component regression method; and the possibility of selecting molecular coordinates, order parameters, and spectral parameters as variables of the problem. By means of these new options, it has been possible to analyze automatically NMR spectra (even depending on 27 spectral parameters) of 16 molecules in an anisotropic environment. Details of each case are discussed. © 1998 Academic Press

The NMR Spectra of Samples Dissolved in Liquid-Crystalline Phases: Automatic Analysis with the Aid of Multiple Quantum Spectra - The Case of Flexible Molecules

Homonuclear NS = 0 and heteronuclear NS ≠ 0 multiple quantum spectra, involving changes in the magnetic number mI by (NI-1), (NI-2), and (NI-3), with NI and NS the number of interacting nuclei of magnetogyric ratio γI and γS, are used for the automatic analysis of 1H NMR spectra of flexible molecules dissolved in liquid-crystalline phases. The automatic procedure has been applied to study molecules of general formula Ph-CH2-X starting from a parameter set having all the spectral parameters set to zero. The results of such an analysis are then used as starting parameters for analysis of the single quantum spectrum. The method was first tested when X = Br and X = H in order to compare strategies differing for the types of parameters used and was then applied to the analysis of 3-phenylprop-1-yne. © 2000 Academic Press