Showing papers by "University of Ljubljana published in 1990"

Theoretical model of the frequency and temperature dependence of the complex dielectric constant of ferroelectric liquid crystals near the smectic-C * -smectic-A phase transition

Abstract: Using the generalized Landau model [Phys. Rev. A 36, 1484 (1987)], the temperature and frequency dependence of the complex dielectric constant of the ferroelectric smectic-${\mathit{C}}^{\mathrm{*}}$ (Sm-${\mathit{C}}^{\mathrm{*}}$) phase and the corresponding smectic-A (Sm-A) phase is calculated. It is demonstrated how the dielectric response of the Sm-${\mathit{C}}^{\mathrm{*}}$ phase generally consists of four modes---two high-frequency polarization modes and two modes of lower frequency that are connected to the reorientation of the director, commonly denoted the soft mode and the Goldstone mode. In the Sm-A phase only two modes are present---one doubly degenerate soft mode and one doubly degenerate polarization mode. The temperature dependences of the dielectric strengths and relaxation frequencies of the modes in question are calculated, and simplified expressions of these quantities are given. The most important feature of the generalized Landau model is the presence of a biquadratic coupling between tilt and polarization in the free-energy density of the system and we show how the general thermodynamic and dielectric properties of the system depend on the strength of this coupling. Comparing the results of the calculations with existing data, we finally conclude that the model provides a description of the Sm-${\mathit{C}}^{\mathrm{*}}$--Sm-A transition that takes all experimentally known features of the dielectric properties of the system into account in a qualitatively correct way.

Mucus Aggregates in the Adriatic Sea: An Overview of Stages and Occurrences

Abstract: . The wide range of macroscopic mucus aggregates in the Adriatic Sea is documented. These structures arc considered to be initially produced by diatoms. A differentiation into five states or stages: macroflocs, stringers, clouds, creamy surface layers, and gelatinous surface layers is proposed. This classification is based not only on size and shape, but also takes relative position in the water, stability, behavior, and effect on benthos into consideration. An interrelationship between these aggregates - as different manifestations or phases of the same phenomenon - is postulated. Macroflocs and stringers arc more common. Larger aggregates (clouds, creamy layers, gelatinous layers) arc less frequent. The general term “marine snow” is inadequate to describe the entire range of aggregate sizes and shapes, while the Italian designation “mare sporco” (dirty sea) is misleading. Two massive occurrences of dense clouds coupled with surface accumulations were recorded in the Adriatic in 1988 and 1989. The effect of the various stages on the shallow coastal ecosystem of the Northern Adriatic Sea is discussed.

Interface pinning and dynamics in random systems.

Abstract: A detailed low-temperature treatment of the domain wall or interface pinning by imperfections in disordered systems with discrete symmetry of the order parameter is presented. Crossover behavior as well as analogies between pinning mechanisms in different systems is analyzed. Pinning may arise from random bonds, when the disordering agents do not break the local symmetry of the order parameter, or from random fields, when the disordering agents do break this symmetry. The interface roughness and response to an external driving force are discussed. The model is explained for dilute magnetic systems in a uniform field where the magnetic domain walls are pinned by random fields and/or random bonds. The results are, however, more general and apply also to interfaces in other systems, e.g., in fluid-fluid interfaces, (anti)ferroelectrics, solitons in incommensurate systems, etc. The interface roughness and pinning pressure (force per unit area) are estimated for weak and strong pinning and their scaling relations to length scale, temperature, frequency, and disorder strength (concentration) are given. The interface contribution to the static and dynamic susceptibility at low temperatures is evaluated. Because of pinning, the low-temperature dynamical susceptibility of disordered ferromagnets in or out of equilibrium carries a [ln(1/\ensuremath{\omega})${]}^{2/\mathrm{\ensuremath{\theta}}}$ frequency dependence in addition to the Debye relaxation behavior. In particular, \ensuremath{\theta}=(d+1)/3 for random-field systems, and \ensuremath{\theta}(d=2)=1/3 and \ensuremath{\theta}(d=3)\ensuremath{\approxeq}0.83 for random-bond systems.

Exclusive hadronic decays of B mesons

Abstract: Using the ARGUS detector at thee + e − storage ring DORIS II, we have measuredB decays into exclusive final states containing aD orD * meson plus several pions, or containing aJ/ψ or ψ′ meson plus a strange particle. Some of these channels have not been seen before, while others represent updated measurements of previous results. The branching ratios are compared with the predictions of the model of Bauer, Stech and Wirbel. Using the cleanest decay channels, the mass of the charged and neutralB meson are found to bem B −=(5280.5±1.0±2.0) MeV/c2 andm B 0=(5279.6±0.7±2.0) MeV/c2 respectively, yielding a mass differencem B 0−m B − =(−0.9±1.2±0.5) MeV/c2.

Solid‐liquid mass transfer in packed beds with cocurrent downward two‐phase flow

Abstract: This study determine how liquid-solid mass transfer rate is affected by the gas flow in a trickle-bed operated in both the low and high interaction regimes. The volumetric liquid-solid mass transfer coefficients were measured by means of the dissolution of naphtalene cylinders into water. Dynamic liquid holdup was measured simultaneously to evaluate the intrinsic velocity of a liquid. The liquid-solid mass transfer rates were also studied in liquid-full operation. Results from both modes were used to determine the fraction of external surface area of packing that was wetted in a three-phase system. A single correlation for the liquid-solid mass transfer coefficient is proposed.

Cytotoxicity of equinatoxin II from the sea anemone Actinia equina involves ion channel formation and an increase in intracellular calcium activity.

Abstract: Equinatoxin Il is a 20-kDa basic protein isolated from the sea anemoneActinia equina. The aim of our work was to investigate the primary molecular basis for the cytotoxic effects of equinatoxin II in two model systems: single bovine lactotrophs and planar lipid bilayers. Previous work has shown that equinatoxin II produces rapid changes in cell morphology, which are dependent on external calcium. It has also been reported that addition of equinatoxin II increases membrane electrical conductance, which suggests that the cytotoxic action of equinatoxin II involves an increase in the permeability of membranes to Ca2+. Extensive changes in cytosolic Ca2+ activity are thought to invoke irreversible changes in cell physiology and morphology. In this paper, we show that morphological changes brought about by equinatoxin II in bovine lactotrophs are associated with a rapid rise in cytosolic Ca2+ activity, monitored with a fura-2 video imaging apparatus. Moreover, incorporation of equinatoxin II into planar lipid bilayers produces Ca2+ permeable ion channels. This suggests that the mode of equinatoxin II cytotoxicity involves the formation of cation (Ca2+) permeable channels in cell membranes.

Cross-sectional approach for clustering time varying data

Abstract: Cluster analysis is to be performed on a three-mode data matrix of type: units, variables, time. A general model for calculating the distance between two units varying in time is proposed. One particular model is developed and used in an example concerned with clustering of 23 European countries according to the similarity of energy consumption in the years 1976–1982.

Self-organization of neurons described by the maximum-entropy principle

Abstract: In the article the maximum-entropy principle and Parzen windows are applied to derive an optimal mapping of a continuous into a descrete random variable. The mapping can be performed by a network of self-organizing information processing units similar to biological neurons. Each neuron is selectively sensitized to one prototype from the sample space of the discrete random variable. The continuous random variable is applied as the input signal exciting the neurons. The response of the network is described by the excitation vector which represents the encoded input signal. Due to the interaction between neurons adaptive changes of prototypes are caused by the excitations. The derived mathematical model explains this interaction in detail; a simplified self-organization rule derived from it corresponds to that of Kohonen. One and two-dimensional examples of self-organization simulated on a computer are shown in the article.

Dielectric method for determining the rotational viscosity in thick samples of ferroelectric chiral smectic- C * liquid crystals

Abstract: An alternative method, based on dielectric measurements on thick samples, for determining the Goldstone-mode rotational viscosity and the twist elastic constant of ferroelectric, chiral smectic-${\mathit{C}}^{\mathrm{*}}$ liquid crystals is presented. By the use of this method the temperature dependence of these quantities for p-decyloxibenzilidine-p'-amino-1-methylpropyl-cinamate (DOBA-1-MPC) and p-n-decyloxybenzylidine-p-amino-2-methyl-butyl-cinamate (DOBAMBC) has been determined. An Arrhenius-type behavior of the Goldstone-mode rotational viscosity was obtained with activation energies \ensuremath{\mu}=0.33 eV for DOBA-1-MPC and \ensuremath{\mu}=0.51 eV for DOBAMBC.

Advances in Pyridazine Chemistry

Abstract: Publisher Summary This chapter discusses the synthetic methods for the synthesis of pyridazine. Numerous syntheses of pyridazines follow the well-known approaches, but there are some significant developments and new methods, such as syntheses of pyridazines by cycloaddition, which have become powerful synthetic tools in the hands of organic chemists. Pyridazines have been investigated intensely, from a theoretical standpoint, by spectroscopic methods. Several aspects of pyridazine chemistry and activity are included in this chapter. The chapter deals with aromaticity and antiaromaticity of azines and substituent effects in azines. The chapter covers unusual organic compounds including azoalkanes, cycloadditions of azadienes, diazoquinones, thermal and photochemical decomposition of azoalkanes, and one-electron oxidations. The pyridazine nucleosides, some biologically active pyridazines, antihypertensive 3-hydrazinopyridazines, and pyridazines with antimicrobial activity are also reviewed in the chapter

Reducing uncertainty of objects by robot pushing

Abstract: A sensorless manipulation task eliminating geometrical uncertainty is proposed. It involves a two-fingered tool attached to a robot end effector. The object is assumed to be a convex polyhedron, although this assumption is later relaxed. Assuming that position and orientation are approximately known prior to operation, the initial position of the tool and the direction of movement are chosen. As the tool advances it strikes the object. If the trajectory is chosen properly, the fingers will contact the object with its corner between them. It is shown that such a configuration of contact points uniquely constrains the position of an object in all degrees of freedom. >

Proton NMR study of the state of water in fibrin gels, plasma, and blood clots.

Abstract: A proton NMR relaxation and pulsed field gradient self-diffusion study of water in fibrin gels, plasma, and blood clots has been performed with special emphasis on the effect of the sol-gel and shrinkage transitions. Deuteron NMR in fibrin gels was also studied to supplement the proton data. It is shown that a measurement of the water proton or deuteron T1/T2 ratio allows for a determination of the bound water fraction in all these systems. The change in the T1/T2 ratio at the shrinkage transition further allows for a determination of the surface fractal dimension of the gel if the change in the volume of the gel is known. The self-diffusion coefficient of water in these systems, which determines the transport properties of the gel, is found to be proportional to the free water fraction in both the nonshrunken and shrunken state.

Regulation of potato meristem development by jasmonic acid in vitro.

Abstract: The influence of jasmonic acid (JA) on differentiation of meristems of the potato,Solanum tuberosum cv. Vesna, was investigated in vitro. Meristems were grown on Murashige and Skoog (MS) medium supplemented with indole-3-acetic acid (IAA) (10 μM), kinetin (10 μM), gibberellic acid (3 μM), as modified by Bang. Addition of JA in concentrations of 0.5–10 μM increased the number of meristems that developed into buds, particularly in meristems isolated from shoots grown from tubers in the dark. JA had no noticeable effect on meristems from germs grown in light. All added concentrations of JA retarded callus and root formation. The inhibitory effect on rhizogenesis disappeared immediately after transfer of the developed buds to medium without JA.

Influence of some oils and surfactants on ligninolytic activity, growth and lipid fatty acids of Phanerochaete chrysosporium.

Abstract: Ligninase production by Phanerochaete chrysosporium MZKIBK-B 186 was increased when the culture medium was supplemented with an emulsion of oleic acid. Addition of linseed oil enhanced fungal biomass synthesis. Under the growth conditions used in our tests, the fungus was capable of accumulating fatty acids from the culture medium into cell lipids. Addition of oleic acid, Tween 80, or 3-[(cholamidopropyl)-dimethylammonio]-1-propanesulphonate (CHAPS), which are known to increase ligninase production by fungi, resulted in oleic acid enrichment of whole cell and polar lipids.

Genesis of the (111) twin in barium titanate

Abstract: On the basis of the topotaxy between BaTiO{sub 3} and Ba{sub 6}Ti{sub 17}O{sub 40} found recently, a model of a nonconservative (111) twin in TiO{sub 2{minus}} rich BaTiO{sub 3} was constructed. The model consists of several (001) layers of Ba{sub 6}Ti{sub 17}O{sub 40} intergrown between (111) layers of BaTiO{sub 3}, the core of the twin being a slightly modified double layer of Ba{sub 6}Ti{sub 17}O{sub 40} containing face-sharing octahedra. Using this model, anomalous grain growth below the eutectic temperature and preferential growth of (111) twins in a reducing atmosphere are explained, as well as nucleation of butterfly twins.

New method for isolation of venom from the sea anemone Actinia cari. Purification and characterization of cytolytic toxins

Abstract: 1. Venom from the sea anemone Actinia cari was obtained by the "milking" method. Two lethal and hemolytic polypeptide toxins, caritoxins I (CTX I) and II (CTX II), were isolated with gel and ion exchange chromatography. 2. The mol. wt of the pure toxin was 19,800. The isoelectric points of CTX I and II were 9.45 and 10.0, respectively. The toxins had similar amino acid compositions lacking cysteine. 3. The intravenous CTX I and CTX II lethal dose (LD50) in mice was found to be 54 +/- 25 and 90 +/- 1 micrograms/kg, respectively. Their hemolytic activity was inhibited by sphingomyelin.

Dynamics of a fermion in the Kondo-lattice model for strongly correlated systems.

Abstract: The motion of a single fermion coupled to the planar antiferromagnetic spin background is examined within the Kondo-lattice model, including the local spin-fermion coupling and the spin-dependent hopping. The perturbation theory using lowest-order magnon processes and the self-consistent calculation of the quasiparticle spectra, yields several features obtained by an analogous treatment of the t-J model by Kane et al. Qualitative differences are, however, found in low excited states with the self-energy showing a nonanalytical behavior \ensuremath{\Sigma}''(\ensuremath{\omega}g0)\ensuremath{\propto}${\mathrm{\ensuremath{\omega}}}^{\mathrm{\ensuremath{\alpha}}}$, \ensuremath{\alpha}\ensuremath{\le}1, due to the local Kondo-like coupling. Within the perturbation theory we get \ensuremath{\alpha}=1, while the self-consistent treatment yields \ensuremath{\alpha}=1/2. The modified self-consistent equations lead also to better results for the quasiparticle mass.

Studies of the uptake and binding of trace metals in fungi. Part I: Accumulation and characterization of mercury and silver in the cultivated mushroom, Agaricus bisporus

Abstract: The champignon mushroom Agaricus bisporus was cultivated on compost labelled with the radioactive traers 110mAg and 203Hg and the uptake of these two metals studied. The radiotracer then served as an aid in elucidation and characterization of the fungal metal–protein species using gel chromatography. In the case of the silver–protein complex, but not of mercury, the majority of the metal was contained in the fractions of intermediate (ca 8000–10000 Da) molecular weight possibly corresponding to metallothioneins (MT) or MT-like species.