Dielectric Behavior of Nonspherical Cell Suspensions

Recent experiments revealed that the dielectric dispersion spectrum of fission yeast cells in a suspension was mainly composed of two sub-dispersions. The low-frequency sub-dispersion depended on the cell length, whereas the high-frequency one was independent of it. The cell shape effect was qualitatively simulated by an ellipsoidal cell model. However, the comparison between theory and experiment was far from being satisfactory. In an attempt to close up the gap between theory and experiment, we considered the more realistic cells of spherocylinders, i.e., circular cylinders with two hemispherical caps at both ends. We have formulated a Green function formalism for calculating the spectral representation of cells of finite length. The Green function can be reduced because of the azimuthal symmetry of the cell. This simplification enables us to calculate the dispersion spectrum and hence access the effect of cell structure on the dielectric behavior of cell suspensions.Comment: Preliminary results have been reported in the 2001 March Meeting of the American Physical Society. Accepted for publications in J. Phys.: Condens. Matte

On the feasibility of cooling and trapping metastable alkaline-earth atoms

Metastability and long-range interactions of Mg, Ca, and Sr in the lowest-energy metastable $^3P_2$ state are investigated. The calculated lifetimes are 38 minutes for Mg*, 118 minutes for Ca*, and 17 minutes for Sr*, supporting feasibility of cooling and trapping experiments. The quadrupole-quadrupole long-range interactions of two metastable atoms are evaluated for various molecular symmetries. Hund's case (c) 4_g potential possesses a large 100-1000 K potential barrier. Therefore magnetic trap losses can possibly be reduced using cold metastable atoms in a stretched M=2 state. Calculations were performed in the framework of ab initio relativistic configuration interaction method coupled with the random-phase approximation.Comment: 8 pages, 2 figures; to appear in PR

Classical and quantum chaos in a circular billiard with a straight cut

We study classical and quantum dynamics of a particle in a circular billiard with a straight cut. This system can be integrable, nonintegrable with soft chaos, or nonintegrable with hard chaos, as we vary the size of the cut. We use a quantum web to show differences in the quantum manifestations of classical chaos for these three different regimes.Comment: LaTeX2e, 8 pages including 3 Postscript figures and 4 GIF figures, submitted to Phys. Rev.

North-south asymmetry of the high-latitude thermospheric density:IMF BY effect

Previous studies have established that the y component of the interplanetary magnetic field (IMF By) plays a role in the north-south asymmetry of the high-latitude plasma convection and wind. The effect of the positive/negative IMF By in the Northern Hemisphere resembles the effect that the negative/positive IMF By would have in the Southern Hemisphere. In this study, we demonstrate that the IMF By effect can also contribute to the hemispheric asymmetry of the thermospheric density. We use high-accuracy air drag measurements from the CHAllenging Minisatellite Payload (CHAMP) satellite and SuperMAG AE index during the period 2001–2006 to examine the response of the high-latitude thermospheric density to geomagnetic activity. Our statistical analysis reveals that the density response at 400 km is greater in the Southern Hemisphere under positive IMF By conditions, and greater in the Northern Hemisphere under negative IMF By conditions. The results suggest that the IMF By effect needs to be taken into account in upper atmospheric modeling for an accurate description of high-latitude densities during periods of enhanced geomagnetic activity

Tetrathiotetracene thin film morphology and electrical properties

The electrical properties of organic thin films are determined by their chemical constituents and the morphology of the films deposited. In this paper the morphology of vacuum sublimed (7∙10-6 mbar) tetrathiotetracene (TTT) thin films is shown to be strongly affected by the thermal deposition temperature (222-350 K) and rate of deposition. Mostly needle-like morphologies are identified by scanning electron microscopy. Optimal TTT purity (a pre-requisite for device preparation via subsequent oxidation) is evidenced by their initially low electrical conductivity. Altering the TTT morphology, by variation of the evaporation parameters, strongly affects this base electrical conductivity. Four probe conductivity measurements and charge extraction by linear increasing voltage methods are used to characterize film electrical properties. In-plane conductivity of up to 7.03∙10-5 S/cm is achieved for pure TTT thin films. Subsequent aerial oxidation resulted in a 3.4-fold increase in electrical conductivity

Assessment of the direct effects of DDAH I on tumour angiogenesis in vivo

Nitric oxide (NO) has been strongly implicated in glioma progression and angiogenesis. The endogenous inhibitors of NO synthesis, asymmetric dimethylarginine (ADMA) and N-monomethyl-l-arginine (l-NMMA), are metabolized by dimethylarginine dimethylaminohydrolase (DDAH), and hence, DDAH is an intracellular factor that regulates NO. However, DDAH may also have an NO-independent action. We aimed to investigate whether DDAH I has any direct role in tumour vascular development and growth independent of its NO-mediated effects, in order to establish the future potential of DDAH inhibition as an anti-angiogenic treatment strategy. A clone of rat C6 glioma cells deficient in NO production expressing a pTet Off regulatable element was identified and engineered to overexpress DDAH I in the absence of doxycycline. Xenografts derived from these cells were propagated in the presence or absence of doxycycline and susceptibility magnetic resonance imaging used to assess functional vasculature in vivo. Pathological correlates of tumour vascular density, maturation and function were also sought. In the absence of doxycycline, tumours exhibited high DDAH I expression and activity, which was suppressed in its presence. However, overexpression of DDAH I had no measurable effect on tumour growth, vessel density, function or maturation. These data suggest that in C6 gliomas DDAH has no NO-independent effects on tumour growth and angiogenesis, and that the therapeutic potential of targeting DDAH in gliomas should only be considered in the context of NO regulation

Ultrafast relaxation of photoexcited carriers in semiconductor quantum wires: A Monte Carlo approach

A detailed analysis of the cooling and thermalization process for photogenerated carriers in semiconductor quantum wires is presented. The energy relaxation of the nonequilibrium carrier distribution is investigated for the ‘‘realistic'' case of a rectangular multisubband quantum-wire structure. By means of a direct ensemble Monte Carlo simulation of both the carrier and the phonon dynamics, all the nonlinear phenomena relevant for the relaxation process, such as carrier-carrier interaction, hot-phonon effects, and degeneracy, are investigated. The results of these simulated experiments show a significant reduction of the carrier-relaxation process compared to the bulk case, which is mainly due to the reduced efficiency of carrier-carrier scattering; on the contrary, the role of hot-phonon effects and degeneracy seems to be not so different from that played in bulk semiconductors

ZO-1 Stabilizes the Tight Junction Solute Barrier through Coupling to the Perijunctional Cytoskeleton

ZO-1 binds numerous transmembrane and cytoplasmic proteins and is required for assembly of both adherens and tight junctions, but its role in defining barrier properties of an established tight junction is unknown. We depleted ZO-1 in MDCK cells using siRNA methods and observed specific defects in the barrier for large solutes, even though flux through the small claudin pores was unaffected. This permeability increase was accompanied by morphological alterations and reorganization of apical actin and myosin. The permeability defect, and to a lesser extent morphological changes, could be rescued by reexpression of either full-length ZO-1 or an N-terminal construct containing the PDZ, SH3, and GUK domains. ZO-2 knockdown did not replicate either the permeability or morphological phenotypes seen in the ZO-1 knockdown, suggesting that ZO-1 and -2 are not functionally redundant for these functions. Wild-type and knockdown MDCK cells had differing physiological and morphological responses to pharmacologic interventions targeting myosin activity. Use of the ROCK inhibitor Y27632 or myosin inhibitor blebbistatin increased TER in wild-type cells, whereas ZO-1 knockdown monolayers were either unaffected or changed in the opposite direction; paracellular flux and myosin localization were also differentially affected. These studies are the first direct evidence that ZO-1 limits solute permeability in established tight junctions, perhaps by forming a stabilizing link between the barrier and perijunctional actomyosin