Showing papers by "École Polytechnique Fédérale de Lausanne published in 1983"

Enhanced fields on large metal particles: dynamic depolarization

Abstract: Influences of particle size on surface-enhancement processes are discussed in terms of a simple physical model. When the size of a silver sphere is increased, the magnitude of the enhancement exhibits a slight increase followed by a strong decrease. Simultaneously the plasmon resonance is shifted and severely broadened. To interpret these effects, a self-consistent calculation of the particle polarization is performed. Initial increase in magnitude and shift of the resonance are due to dynamic depolarization, whereas the decrease in magnitude and broadening are caused by radiation damping. The importance of higher-order multipoles is assessed by analyzing their contributions separately.

Determination of the absolute handedness of knots and catenanes of DNA

Abstract: DNA winds about itself in a right-handed or left-handed fashion at several structural levels. The double helix is generally right-handed and is given a (+) sign by convention, whereas supercoiling of the helix axis is always (-) in the cell. The winding in higher -order forms such as knots and catenanes is unknown, and this has impeded elucidation of the mechanisms of their formation and resolution by replication, recombination and topoisomerase action. We introduce here a procedure for determining the handedness of DNA winding by inspection of electron micrographs of DNA molecules coated with Escherichia coli RecA protein. We demonstrate the validity of the method and show that DNA topoisomerase I of E. coli generates an equal mixture of (+) and (-) duplex DNA knots, and that one product of recombination by resolvase of transposon Tn3 (refs 8, 9) is a catenane of uniquely (+) sign.

The sex-linked recessive lethal test for mutagenesis in Drosophila melanogaster: A report of the U.S. environmental protection agency Gene-Tox program

Abstract: The test for sex-linked recessive lethals (SLRL) in Drosophila melanogaster has been used to detect induced mutations since 1927. The advantage of the test for both screening and hazard evaluation is its objectivity in testing for transmissible mutations in the germ cells of a eukaryote. Statistical criteria for both positive and negative mutagenicity at the highest concentration tested under a particular exposure condition were developed by the Work Group, and a recommended protocol for future testing was agreed upon. For 421 compounds there were sufficient data available in the literature for analysis; 198 compounds were found to be positive and 46 negative at the highest concentration tested. Most experiments had been done for objectives of pure research rather than for deliberately screening for mutagenicity, although many of the 421 chemicals were selected for testing because of suspected mutagenicity. Therefore, the statement of 198 positive and 46 negative should not be taken as an example of the proportion of mutagens in the environment. In three sets of experiments with D. melanogaster that were done specifically for screening, one involving 40 compounds for the Environmental Protection Agency (EPA), the others involving 13 for the Food and Drug Administration (FDA), only 6 mutagens were discovered. After completion of the classification of compounds according to their response in the SLRL test, the compounds were classified as to their carcinogenic response according to the list of Griesemer and Cueto (1980). There were 62 compounds that could be classified as positive or negative for both carcinogenesis and mutagenesis. Of the 62 compounds, there was agreement between the carcinogenesis and mutagenesis classification in 56 (50 positive and 6 negative), or 90% would have been correctly classified as to carcinogenesis from only the SLRL test. Because of inadequate sample size, 177 compounds could not be classified as positive or negative according to the statistical criteria established by the Work Group. This large number of inadequately tested compounds reflects the fact that many of the experiments were not done for screening. Further work is needed on the compounds with inadequate sample size.(ABSTRACT TRUNCATED AT 400 WORDS)

Analytical and numerical techniques in the Green's function treatment of microstrip antennas and scatterers

Abstract: In the paper, the mode theory of wave propagation in stratified media is used to establish the spatial Green's functions associated with a microstrip structure. They correspond to the fields and potentials created by a horizontal electric dipole placed on the air/dielectric interface and are expressed as Sommerfeld integrals. Near- and far-field approximate analytical evaluations of these integrals are rigorously derived. They allow a quantitative discussion of the scope and limitations of the static microstrip theory, and point out the relevance of surface wave effects in microstrip. The most interesting situation when both source and observer are in the air/dielectric interface is studied extensively. Numerical methods to evaluate Sommerfeld integrals in such situations are reviewed, and several new accurate techniques are introduced and discussed in detail.

Embossing technique for fabricating integrated optical components in hard inorganic waveguiding materials.

Abstract: We describe a novel embossing technique for fabricating integrated optical components in hard and resistant inorganic waveguiding materials of good optical quality. The desired structure is embossed into a dip-coated deformable gel film prepared from organometallic solutions. Subsequent heat treatment transforms it into inorganic hard oxide material. We have successfully fabricated surface-relief gratings with 1200 lines/mm on SiO2–TiO2 waveguides (with refractive index nF ≃ 1.8, thickness dF ≃ 120 nm, and loss <1 dB/cm). The embossed gratings served as input and output grating couplers and as Bragg reflectors. We propose to fabricate other integrated optical components, for example, channel waveguides, with this method.

Electronic properties of alkali trimers

Abstract: The electronic properties of the alkali trimers Li3, Na3, and K3 are studied using the pseudopotential and the local‐spin‐density approximations. More than 100 configurations were calculated for each trimer in order to obtain a complete picture of the adiabatic Born–Oppenheimer surfaces. The equilibrium geometry of the trimers are Jahn–Teller distortions of an equilateral triangle. Although the three surfaces are quite similar, Li3 is more affected than Na3 or K3 by the dynamical character of the Jahn–Teller distortion. The calculated ionization potentials agree very well with the experimental values and the qualitative features of the Born–Oppenheimer surface are confirmed by recent ESR experiments.

Collective effects in a random-site electric dipole system: KTaO3: Li

Abstract: Li, substituting for K in KTa${\mathrm{O}}_{3}$, creates a local electric dipole, due to its off-center position with respect to the cubic site. We have studied such crystals with different amounts of Li (and in a few cases also doped with Nb, substituting for Ta) by nuclear magnetic resonance, dielectric relaxation, pyroelectricity, ultrasound, and birefringence methods. Birefringence and dielectric susceptibility results show that collective effects between the Li dipoles occur below a fairly well-defined concentration-dependent temperature of the order of 50 K, but nuclear magnetic and dielectric relaxation indicate the absence of criticality at the onset of these effects. These collective effects are related to those arising in spin-glasses. We discuss the data in the light of theoretical models and computer simulations of systems of randomly interacting moments, which predict an apparent condensation into a system of metastable clusters without long-range order.

Thyroid hormones, blood plasma metabolites and haematological parameters in relationship to milk yield in dairy cows

Abstract: To study their relationship to milk yield, the concentrations, in jugular venous blood, of thyroxine iodine (T4I), thyroxine (T4), 3,5,3'-tri-iodothyronine (T3), glucose, non-esterified fatty acids (NEFA), triglycerides, phospholipids, cholesterol, total protein, albumin, urea, haemoglobin and packed cell volume (PCV) have been measured in 36 cows (Simmental, Swiss Brown, Holstein and Simmental × Holstein) of different ages during a full lactation, pregnancy, dry period, parturition and 150 days of the ensuing lactation. Thyroid hormones and triglycerides were negatively, and total protein, globulin, cholesterol and phospholipids were positively, correlated with uncorrected or corrected milk yield during several periods of lactation, whereas glucose, NEFA, albumin, urea, haemoglobin and packed cell volume were not correlated with milk yield. The 10 animals with the highest milk yield (18·9 to 23·5 kg/day) exhibited significantly lower values of T4I, T4, T3 and glucose, significantly higher levels of total protein and globulin and tended to have higher levels of NEFA than the 10 cows with the lowest milk yield (10·9 to 14·3 kg/day) throughout or during certain periods of lactation, whereas concentrations of triglycerides, phospholipids, cholesterol, albumin, haemoglobin and PCV did not differ. Changes in T4I, T4, T3, glucose and total protein during lactation were also influenced by age, presumably associated with an increase in milk production with age. T3 was consistently lowest and cholesterol and phospholipids, during later stages of lactation, were highest in Holsteins, which had the highest milk yields of all breeds. Changes of blood parameters were mainly caused by shifts in energy and protein metabolism in association with level of milk production

Noise analysis of switched capacitor networks

Abstract: Noise generated in switched capacitor (SC) networks has its origin in the thermal fluctuations of charged particles in the channels of the MOS switch transistors on one hand, in the operational amplifiers on the other hand. Using a SC integrator as vehicle, it is shown that the output noise spectrum consists in general of a broad-band component due to a continuous-time noise signal and of a narrow-band contribution predominating in the baseband of the SC network resulting from a sampled-data noise signal. The ratio of undersampling is introduced and it is shown that the latter noise contribution can be evaluated by sampled-data techniques using the z -transform transfer function. These results are applied to the SC integrator and excellent concordance to the measurements made on a laboratory model is established.

Lignin: Biosynthesis, application, and biodegradation

Abstract: During the last few decades, research on lignin has focused on its biosynthesis, its possible applications, and its biodegradation. In order to better evaluate and understand the composition and possible uses of this material, this article reviews many of the most recent discoveries in lignin research and discusses several other aspects of lignin: its distribution and function in plants, the occurrence and sources of lignin, processes used to recover lignin, and its chemical structure. Since significant research is concentrated on the biological degradation, this aspect is also emphasized.

The structure of membrane crystals of the light-harvesting chlorophyll a/b protein complex.

Abstract: Membrane crystals of the light-harvesting chlorophyll a/b protein complex from pea chloroplasts were investigated using electron microscopy and image analysis. The membrane crystals formed upon precipitation of the detergent-solubilized complex with mono- and divalent cations in the presence of small amounts of Triton X-100. The crystalline fraction contained two polypeptides of 25,000 and 27,000 mol wt. Freeze-dried and freeze-etched specimens showed a periodic honeycomb structure on the surface of membrane crystals. Double replicas of freeze-fractured sheets showed a hexagonal lattice of particles on both fracture faces. Image analysis of negatively stained membrane crystals suggested that they had threefold rather than sixfold symmetry in projection. A projection map at 20-A resolution revealed two triangular structural units of opposite handedness per crystallographic unit cell. The structural units appeared to be inserted bidirectionally into the membrane, alternating in orientation perpendicular to the membrane plane.

Influence of oxygen on the growth of Saccharomyces cerevisiae in continuous culture.

Abstract: Baker's yeast, Saccharomyces cerevisiae, was investigated for the combined influence of dissolved oxygen and glucose concentration in continuous culture. A reactor was operated at a range of dilution rates (0.1, 0.2, 0.25, 0.27, and 3.0 h-1), above and below the critical value that separates the oxidative and fermentation regions. For each diltuion rate (D), steady states were established at each of five to ten different dissolved oxygen concentrations (DO) in the range of 0.01-5 mg/l. The use of on-line mass spectrometry facilitated the measurement of gaseous and dissolved O/sub 2/, CO/sub 2/, and ethanol. Intracellular carbohydrate, protein, RNA, DNA, lipid, and cytochrome concentrations were measured. Cell size measurements were reduced to specific surface areas. Cytochrome content showed up to 100% variation during a 20-day period of adaptation at D = 0.2 h/sup -1/ to low DO. Eventually, the culture behaved the same at DO = 0.05 mg/l as it did initially at 3 mg/l. At D = 0.2, 0.25, and 0.27 h/sup -1/, the transition between oxidation and fermentation was characterized by a critical DO which decreased with decreasing D. The X-D curves were shifted such that the critical D value was reduced with decreasing DO. Specific oxygen update ratesmore » varied with DO according to the saturation kinetics. Specific cell surface areas increased with decreasing DO. Cytochrome content generally decreased with decreasing DO, and Q O/sub 2/ could be linearly related to the total cytochrome content, which exhibited a maximum at D = 0.27 h/sup -1/.« less

Origin of calcite in syntectonic veins as determined from carbon-isotope ratios

Abstract: A particular feature of many Alpine tectonic thrust sheets composed of carbonates is the development of distinct generations of calcite-filled veins. In the Doldenhorn nappe, one of the Helvetic nappes of Switzerland, we were able to distinguish five different generations of calcite-filled veins. Determination of carbon-isotope ratios in these calcite veins and their adjacent limestone matrix indicates that the earlier vein systems contain carbonate material not in isotopic equilibrium with the adjacent matrix. In the case of the later vein systems, the carbon-13 content approaches that of the nearby matrix. The isotope data indicate a transition from a chemically open system to a closed system; this trend is related to an overall modification of the deformation mechanism of the nappe, resulting in the increasing importance of pressure solution with time.

Problem of oxygen transfer during degradation of lignin by Phanerochaete chrysosporium

Abstract: Partial oxygen limitation was shown to be the main reason for slow and incomplete degradation of lignin by Phanerochaete chrysosporium in non-agitated cultures. No oxygen could be measured in the mycelial mat deeper than 1 mm from the surface although the cultures were incubated under a 100% oxygen atmosphere. When the depth of the mycelial mat was reduced below the limiting thickness, the organism was able to degrade lignin in air at a rate comparable to that measured under 100% oxygen atmosphere.

Singlet Oxygen and Superoxide: Experimental Differentiation and Analysis

Abstract: Reference LPI-ARTICLE-1983-007doi:10.1002/hlca.19830660236View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12