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  • 2000-2004  (102)
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  • 1
    Electronic Resource
    Electronic Resource
    Formation of unilamellar vesicles by repetitive freeze-thaw cycles: characterization by electron microscopy and 31P-nuclear magnetic resonance (2000)
    Springer
    European biophysics journal 29 (2000), S. 184-195 
    Details
    ISSN: 1432-1017
    Keywords: Key words Liposomes ; Phosphorus-31 NMR ; Large unilamellar vesicles ; Freeze-fracture electron microscopy ; Dioleoylphosphatidylcholine
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract  It has been reported that repetitive freeze-thaw cycles of aqueous suspensions of dioleoylphosphatidylcholine form vesicles with a diameter smaller than 200 nm. We have applied the same treatment to a series of phospholipid suspensions with particular emphasis on dioleoylphosphatidylcholine/dioleoylphosphatidic acid (DOPC/DOPA) mixtures. Freeze-fracture electron microscopy revealed that these unsaturated lipids form unilamellar vesicles after 10 cycles of freeze-thawing. Both electron microscopy and broad-band 31P NMR spectra indicated a disparity of the vesicle sizes with a highest frequency for small unilamellar vesicles (diameters ≤30 nm) and a population of larger vesicles with a frequency decreasing exponentially as the diameter increases. From 31P NMR investigations we inferred that the average diameter of DOPC/DOPA vesicles calculated on the basis of an exponential size distribution was of the order of 100 nm after 10 freeze-thaw cycles and only 60 nm after 50 cycles. Fragmentation by repeated freeze-thawing does not have the same efficiency for all lipid mixtures. As found already by others, fragmentation into small vesicles requires the presence of salt and does not take place in pure water. Repetitive freeze-thawing is also efficient to fragment large unilamellar vesicles obtained by filtration. If applied to sonicated DOPC vesicles, freeze-thawing treatment causes fusion of sonicated unilamellar vesicles into larger vesicles only in pure water. These experiments show the usefulness of NMR as a complementary technique to electron microscopy for size determination of lipid vesicles. The applicability of the freeze-thaw technique to different lipid mixtures confirms that this procedure is a simple way to obtain unilamellar vesicles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002490000077
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  • 2
    Electronic Resource
    Electronic Resource
    From High-Energy Ball-Milling to Surface Properties of TiO〈sub〉2〈/sub〉 Powders (2002)
    s.l. ; Stafa-Zurich, Switzerland
    Journal of metastable and nanocrystalline materials Vol. 12 (Aug. 2002), p. 27-36 
    Details
    ISSN: 1422-6375
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Type of Medium: Electronic Resource
    URL: http://www.tib-hannover.de/fulltexts/2011/0528/01/43/transtech_doi~10.4028%252Fwww.scientific.net%252FJMNM.12.27.pdf
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  • 3
    Electronic Resource
    Electronic Resource
    Domestication of wild Saccharomyces cerevisiae is accompanied by changes in gene expression and colony morphology (2003)
    Oxford,UK : Blackwell Science Ltd
    Molecular microbiology 47 (2003), S. 0 
    Details
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: Although colonies from Saccharomyces cerevisiae laboratory strains are smooth, those isolated from nature exhibit a structured fluffy pattern. Environmental scanning electron microscopy shows that the cells within wild fluffy colonies are connected by extracellular matrix (ECM) material. This material contains a protein of about 200 kDa unrelated to the flocculins, proteins involved in cell–cell adhesion in liquid media. The matrix material binds to concanavalin A. Within a few passages on rich agar medium, the wild strains switch from the fluffy to the smooth colony morphology. This domestication is accompanied by loss of the ECM and by extensive changes in gene expression as detected by DNA microarrays. The expression of about 320 genes was changed in smooth colonies. The major changes comprise carbohydrate metabolism, cell wall, water channels, Ty-transposons and subtelomeric genes, iron homeostasis, vitamin metabolism and cell cycle and polarity. The growth in fluffy colonies may represent a metabolic strategy for survival of yeast under unfavourable conditions that is switched off under felicitous laboratory conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2958.2003.03332.x
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  • 4
    Electronic Resource
    Electronic Resource
    Velocity model estimation for depth imaging: Comparison of three tomography methods on a 2D real data set (2004)
    PO Box 1354, 9600 Garsington Road , Oxford OX4 2XG , UK . : Blackwell Science Ltd
    Geophysical prospecting 52 (2004), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The estimation of velocity models is still crucial in seismic reflection imaging as it controls the quality of the depth-migrated image, which is the basis of geological interpretation. Among the numerous existing methods for velocity determination, tomographic methods are very attractive for their efficiency and ability to retrieve heterogeneities of the medium. We present three tomographic methods in order to estimate heterogeneous velocity models from 2D prestack PP reflection data: a traveltime tomography in the time-migrated domain, a traveltime and slope tomography in the non-migrated time domain, and a slope tomography in the depth-migrated domain.The first method (traveltime tomography in the time domain) is based on continuous picked events, whereas the two slope tomographic methods, one in the time domain and the other in the depth domain, are based on locally coherent events, with no assumptions about reflector geometry or the unknown velocity field. The purpose of this paper is not to describe in detail the theoretical basis and implementation of the methods, but to apply and compare their output using the same marine real data set. Based on the estimated velocity models, the migrated images and the common-image gathers from the three processing routes, the relative strengths and weaknesses of the methods are discussed. Finally, similarities are indicated and potential alternative approaches are proposed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.2004.00427.x
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  • 5
    Electronic Resource
    Electronic Resource
    Preparation and transport properties of polycrystalline Bi and Bi–SiO2 nanocomposites (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 3269-3275 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Bismuth–silica nanocomposites and polycrystalline bismuth were prepared via powder metallurgy in order to study the influence of silica inclusions on the thermoelectric properties of bismuth. Bi–SiO2 powders containing from 0.5 to 15 vol. % of silica and pure Bi powders were produced by an arc-plasma processing. Transmission electron microscopy investigations revealed the presence of a nanometric silica shell around the Bi grains. The powders were cold pressed and sintered close to the melting temperature of bismuth. The bulk microstructures are very different for the bismuth and the Bi–SiO2 nanocomposites because silica, which is primarily dispersed at grain boundaries, inhibits the grain growth during sintering. The electrical resistivity was measured from 5 to 300 K, while the thermoelectric power and the thermal conductivity were measured from 65 to 300 K on both polycrystalline bismuth and Bi–SiO2 samples containing 0.5, 4, and 15 vol. % of silica, respectively. The transport properties are mainly discussed with regard to the microstructures. In spite of a strong reduction of the thermal conductivity for the nanocomposites, the thermoelectric figure of merit is not improved compared to bismuth due to a dominating concurrent increase of the electrical resistivity resulting from a finite-size effect. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1289079
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  • 6
    Electronic Resource
    Electronic Resource
    Contrasted responses to carbohydrate limitation in tomato fruit at two stages of development (2002)
    Oxford, UK : Blackwell Science Ltd
    Plant, cell & environment 25 (2002), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: The metabolic consequences of long-term carbohydrate depletion have been well documented in many sink organs but not extensively in fruit. Therefore, in the present study the response to sugar limitation in tomato fruit (Lycopersicon esculentum Mill.) was investigated at two developmental stages; during the cell division and cell expansion phases. First, the response in excised fruit cultured in vitro was characterized. Sugar depletion caused an arrest of growth and an exhaustion of carbon reserves. The proteins that were degraded and the nitrogen released was transiently stored as asparagine and glutamine in both developmental stages and also as γ -aminobutyric acid (GABA) in expanding fruit. Fruit at the cell division stage appeared to be more sensitive to sugar limitation. The response to sugar depletion was then characterized in fruit from plants submitted to extended darkness. In planta, the effects of sugar-limitation were similar to those described in vitro but much more attenuated, especially in expanding fruit, which still accumulated dry matter. The expression of cell cycle genes, sugar- and nitrogen-related genes was reduced by darkness. Only asparagine synthetase gene expression was induced in both dark-treated fruit. Together the present data revealed that the effects of the carbon limitation are more pronounced in the youngest fruits as it is probably controlled by the relative sink strength of the fruit.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-3040.2002.00941.x
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  • 7
    Electronic Resource
    Electronic Resource
    Initial results of remediation activities to restore hypereutrophic Villerest Reservoir (Roanne, France) (2004)
    Oxford, UK : Blackwell Science Pty
    Lakes & reservoirs 9 (2004), S. 0 
    Details
    ISSN: 1440-1770
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geography
    Notes: We examined the impacts of remediation activities aimed at improving the water quality of hypereutrophic Villerest Reservoir, in which Microcystis aeruginosa dominated during the summer. We also compared nutrients and chlorophyll a data from this study with the results of a previous study on the reservoir. Between the two studies, the nitrogen and phosphorus loads into the reservoir from the main tributary decreased by 70% and 80%, respectively. Within the reservoir, the quantities of ammonia-nitrogen were similar in the two studies, and the total nitrogen was significantly higher in this study compared to the initial study. Both the phosphate-phosphorus and total phosphorus concentrations decreased significantly between the two studies. However, the statistically significant decrease in phosphate-phosphorus and total phosphorus did not always lead to a significant decrease in chlorophyll a concentrations. The nitrogen/phosphorus mass ratio during the present study remained well above five, the critical value below which summer blooms of Microcystis aeruginosa were observed in Villerest Reservoir. These study results indicated that the remediation activities being used to improve the water quality of Villerest Reservoir were off to a good start.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1440-1770.2004.00241.x
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  • 8
    Electronic Resource
    Electronic Resource
    Modeling of the Phase Transformation Induced by Ball Milling in Anatase TiO2 (2000)
    Springer
    Journal of materials synthesis and processing 8 (2000), S. 139-144 
    Details
    ISSN: 1573-4870
    Keywords: TiO2 ; phase transformations ; mechanical alloying ; kinetics ; modeling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A high-pressure and high-temperature phase of TiO2 : TiO2 II is formed transiently during room-temperature high-energy ball milling of anatase TiO2 : TiO2 anatase → TiO2 II → TiO2 rutile. Rutile is the only phase present after prolonged ball milling. The present paper focuses on the influences of physical and chemical processing conditions on the transformation kinetics. The effects of two milling parameters on the kinetics of phase transformation of anatase TiO2 were investigated: the nature of milling tools and the powder-to-ball weight ratio R. Granulometric characterizations and TEM observations have demonstrated that the transformation of TiO2 anatase into TiO2 II occurs without fracturing of particles and that TiO2 II nanograins form at the surface of anatase particles. The parameter R affects only the transformation rate. For a given R, the transformation rate is the largest with alumina grinding tools, intermediate with zirconia tools, and the smallest with steel tools. The parameters involved in current models of the mechanical alloying process do not suffice to explain the differences in transformation rates observed here. A parameter, which takes into account the influence of the mechanical properties of grinding materials, is considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1011351807629
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  • 9
    Electronic Resource
    Electronic Resource
    Numerical modeling of thermal stratification in a lake reservoir. Methodology and case study (2000)
    Springer
    Aquatic sciences 62 (2000), S. 105-124 
    Details
    ISSN: 1420-9055
    Keywords: Key words: Numerical modeling, hydrodynamics, lake.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract: A numerical model of the hydrodynamic and thermal structure of an artificial lake was designed and developed as a basis for an ecological water-quality model. It allows the quantification of the vertical mixing processes that govern not only the thermal structure but also the nutrient exchanges, and more generally the distribution of dissolved and particulate matter between the different parts of the lake. The vertical temperature profiles were calculated by solving the one-dimensional heat transfer equation that takes into account the internal heat sources and sinks, advection due to inflow and outflow and the molecular and eddy diffusions. A finite-difference discretization of first-order in time and second-order in space was chosen. The numerical time-step was three hours and layers were one-meter in thickness. These time- and space-scales are well-suited to perform a precise simulation of the different processes occurring over a seasonal period. Moreover, this simulation requires only a reasonable amount of computer time.¶This model was used to study an artificial lake, (i.e. a reservoir), located in the high Loire valley (Roanne, France). To precisely identify the physical processes followed with an accurate numerical modeling, on-site data were acquired intensively over three years. Temperatures were monitored hourly at 11 different levels in the three main reaches of the reservoir to study the lake hydrodynamics and thermal behaviour. Meterological measurements were made every 20 minutes. One-year data were used for calibration, whereas the model was validated using the data collected over the other two years.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s000270050001
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  • 10
    Electronic Resource
    Electronic Resource
    Plastid differentiation during androgenesis in albino and non-albino producing cultivars of barley (Hordeum vulgare L.) (2000)
    Springer
    Sexual plant reproduction 13 (2000), S. 95-104 
    Details
    ISSN: 1432-2145
    Keywords: Key words Androgenesis ; Albinism ; Hordeum vulgare L. ; Plastid differentiation ; Plastid DNA ; Pollen development
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract  In order to better understand androgenic albinism in barley, we compared plastid differentiation during anther culture in two cultivars, an albino (spring cultivar Cork) and a non-albino (winter cultivar Igri) producing cultivar. The ultrastructure of plastids and the relative amount of DNA containing plastids were followed in both cultivars during the androgenic process and correlated with the proportion of regenerated chlorophyllous plantlets. For androgenesis, anthers were collected at the uninucleate stage, during mid- or late-microspore vacuolation. At this stage DNA was detected in 15.3 ± 2. 7% of microspore plastid sections in the winter cultivar Igri, compared to 1.7 ± 0.5% in the spring cultivar Cork. In the winter cultivar Igri, starch was broken down after anther pretreatment but plastids divided rapidly during anther culture and thylakoids developed in the stroma. Prior to regeneration, plastids contained 2.0 ± 0.2 thylakoids per plastid and starch represented 26.1 ± 3.3% of the plastid volume. In the spring cultivar Cork, plastids followed a different developmental pathway. After anther pretreatment, microspore plastids differentiated exclusively into amyloplasts, accumulating starch and losing their thylakoids as well as their capacity to divide. This developmental pattern became progressively more marked, so that by the end of anther culture plastids contained 0.5 ± 0.4 thylakoids per plastid and starch represented up to 90.3 ± 4.3% of plastid volume. Following androgenesis, the response was similar in both cultivars except that the winter cultivar Igri provided 87.8% of chlorophyllous plantlets compared to 99.7% albino plantlets in the cultivar Cork. The results presented here suggest that the exclusive regeneration of albino plantlets in the spring cultivar Cork may be due to degradation of microspore plastid DNA during early pollen development, preventing the plastids from differentiating into chloroplasts under culture conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004970000043
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