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Biomedical subjects

A Marty

Publications and source records attributed to A Marty.

At least 19 recordsLinked to original sources
PubMed2006

Thermally activated depinning of a narrow domain wall from a single defect.

We describe the field induced depinning process of a magnetic domain wall (DW) from a single bidimensional nanometric defect. The DW propagates in a wire lithographed on a film with strong perpendicular anisotropy. We observe a statistical distribution of the relaxation time consistent with a Néel-Brown picture of magnetization reversal. This indicates that the nanometric DW can be considered as an ideal monodomain particle switching over a single energy barrier. Such a stochastic character of DW depinning has to be taken into account for spintronic applications.

Journal Article↗
PubMed2005

Hydrophobic substrate utilisation by the yeast Yarrowia lipolytica, and its potential applications.

The alkane-assimilating yeast Yarrowia lipolytica degrades very efficiently hydrophobic substrates such as n-alkanes, fatty acids, fats and oils for which it has specific metabolic pathways. An overview of the oxidative degradation pathways for alkanes and triglycerides in Y. lipolytica is given, with new insights arising from the recent genome sequencing of this yeast. This includes the interaction of hydrophobic substrates with yeast cells, their uptake and transport, the primary alkane oxidation to the corresponding fatty alcohols and then by different enzymes to fatty acids, and the subsequent degradation in peroxisomal beta-oxidation or storage into lipid bodies. Several enzymes involved in hydrophobic substrate utilisation belong to multigene families, such as lipases/esterases (LIP genes), cytochromes P450 (ALK genes) and peroxisomal acyl-CoA oxidases (POX genes). Examples are presented demonstrating that wild-type and genetically engineered strains of Y. lipolytica can be used for alkane and fatty-acid bioconversion, such as aroma production, for production of SCP and SCO, for citric acid production, in bioremediation, in fine chemistry, for steroid biotransformation, and in food industry. These examples demonstrate distinct advantages of Y. lipolytica for their use in bioconversion reactions of biotechnologically interesting hydrophobic substrates.

Alkanes↗
PubMed2004

Magnetic domain wall propagation unto the percolation threshold across a pseudorectangular disordered lattice.

The reversal process of thin FePt/Pt(001) layers with perpendicular magnetization was observed by magnetic imaging techniques. Reversal occurs through domain wall propagation across a strongly disordered rectangular lattice of linear anisotropy defects. Micromagnetic simulations of domain wall pinning allowed deriving an analytical model of the reversal process unto percolation threshold. Quantitative agreement is found between the calculated and experimental fractal dimension of the reversed domain.

Journal Article↗
PubMed2004

Hysteresis effect in FePd magnetic stripes studied by coherent soft X-ray resonant magnetic scattering.

An FePd thin film sample, showing magnetic stripe domains as imaged by magnetic force microscopy, has been measured by soft X-ray resonant magnetic scattering in reflection geometry. Illumination with coherent radiation, produced by inserting a 20 microm pinhole in front of the sample, leads to a magnetic speckle pattern in the scattered intensity that gives access to the domain morphology. Application of an in-plane magnetic field for a few seconds gives a strong change in the observed intensity fluctuations, which indicates a large degree of variation between the two patterns taken before and after field exposure. From the speckle pattern we calculate a degree of coherence of beta = 0.5 for the incident beam.

Anisotropy↗
PubMed2003

Association of matrix protein of respiratory syncytial virus with the host cell membrane of infected cells.

The matrix protein of paramyxoviruses plays an important role in virus assembly through its interactions with cell membrane, virus envelope and virus nucleocapsid. In the present study, we investigated the possible association of respiratory syncytial virus (RSV) matrix (M) protein with the plasma membrane of infected cells. Using confocal microscopy we found that M was present at the cytoplasmic side of the plasma membrane. We used flotation gradients to purify membranes from RSV infected cells and treated them with cold Triton X-100 to obtain lipid rafts in the insoluble fraction. Western blot of the lipid raft fraction with specific antibodies showed that it contained M, as well as G (attachment) and N (nucleocapsid) proteins. We also found that RSV purified on sucrose gradients contained lipid raft markers. Together, our data suggest that RSV uses lipid rafts for assembly and budding.

Cell Line↗
PubMed2003

Ordering intermetallic alloys by ion irradiation: a way to tailor magnetic media.

We show how, combining He ion irradiation and thermal mobility below 600 K, the transformation from chemical disorder to order in thin films of an intermetallic ferromagnet (FePd) may be triggered and controlled. Kinetic Monte Carlo simulations show that the initial directional short range order determines the transformation. Magnetic ordering perpendicular to the film plane was achieved, promoting the initially weak magnetic anisotropy to the highest values known for FePd films. Applications to ultrahigh density magnetic recording are suggested.

Journal Article↗
PubMed2003

[Bowel obstruction in pregnancy: value of Single Shot Fast Spin Echo MR sequence (SS-FSE)].

Bowel obstruction is a rare but serious complication of pregnancy for both the mother and the developing fetus. The authors report a case of small bowel obstruction in an early pregnant patient. Increasing delay between the onset of symptoms and the diagnosis remains a significant contributing factor to the high morbidity and mortality in the pregnant patient population. So, prompt diagnosis and therapy are essential. Single Shot Fast Spin Echo MR sequence confirmed the presence of bowel obstruction and identified the zone of transition.

Adult↗
PubMed2002

Esterification by immobilized lipase in solvent-free media: kinetic and thermodynamic arguments.

The aim of this study is to characterize, in solvent-free systems (SFS), the kinetic and thermodynamic performance of batch lipase-catalyzed esterification. SFS are compared to a conventional organic solvent, n-hexane. The esterification of oleic acid with ethanol was chosen as a model reaction. The TABEK (thermodynamic activity-based enzyme kinetics) approach was used to rationally analyze kinetics. Influence of the reaction medium on final conversions was also studied. Several factors, such as initial molar ratio of substrates, reactant availability, initial water content, and quantity of immobilized enzyme, were examined. Special attention was also turned to enzyme stability and reuse after reaction, this last item being a prerequisite in the development of industrial processes. SFS proved to be almost as efficient as n-hexane from a kinetic and thermodynamic point of view and offered a better volumetric production.

Enzymes, Immobilized↗
PubMed2002

Individual domain wall resistance in submicron ferromagnetic structures.

The resistance generated by individual domain walls is measured in a FePd nanostructure. Combining transport and magnetic imaging measurements, the intrinsic domain wall resistance is quantified. It is found positive and of a magnitude consistent with that predicted by models based on spin scattering effects within the walls. This magnetoresistance at a nanometer scale allows a direct counting of the number of walls inside the nanostructure. The effect is then used to measure changes in the magnetic configuration of submicron stripes under application of a magnetic field.

Journal Article↗
PubMed2001

Combining solvent engineering and thermodynamic modeling to enhance selectivity during monoglyceride synthesis by lipase-catalyzed esterification.

Monoglyceride synthesis by Rhyzomucor miehei lipase was investigated via direct esterification between glycerol (adsorbed onto silica gel) and oleic acid in organic solvents. The main difficulty is to avoid the unwanted production of di- and tri-glycerides. It was demonstrated that an increase in solvent polarity, using mixtures of n-hexane and 2-methyl-2-butanol (2M2B), improves drastically the selectivity toward monoglyceride formation. In pure n-hexane, the monoglyceride represents only 6 molar % of the total products at the thermodynamic equilibrium (34 and 60% for di- and tri-glyceride respectively). Use of an equivolume mixture of n-hexane/2M2B enables a product mixture to be obtained containing 94% of monoglyceride at equilibrium (2.4 and 0% for di- and tri-glyceride respectively). This positive effect is counterbalanced by a decrease both in initial velocities and in substrate conversion at thermodynamic equilibrium.A modeling, able to predict the three thermodynamic equilibria governing the 3 consecutive reactions, based on activity coefficient calculations using the UNIFAC model, is proposed. It takes into account both the partition of water between solvent and immobilized catalyst, and the partition of glycerol between solvent and silica gel. A good correlation with experimental data obtained in n-hexane/2M2B mixtures was observed.

Journal Article↗
PubMed2000

Role of the calcium-binding protein parvalbumin in short-term synaptic plasticity.

GABAergic (GABA = gamma-aminobutyric acid) neurons from different brain regions contain high levels of parvalbumin, both in their soma and in their neurites. Parvalbumin is a slow Ca(2+) buffer that may affect the amplitude and time course of intracellular Ca(2+) transients in terminals after an action potential, and hence may regulate short-term synaptic plasticity. To test this possibility, we have applied paired-pulse stimulations (with 30- to 300-ms intervals) at GABAergic synapses between interneurons and Purkinje cells, both in wild-type (PV+/+) mice and in parvalbumin knockout (PV-/-) mice. We observed paired-pulse depression in PV+/+ mice, but paired-pulse facilitation in PV-/- mice. In paired recordings of connected interneuron-Purkinje cells, dialysis of the presynaptic interneuron with the slow Ca(2+) buffer EGTA (1 mM) rescues paired-pulse depression in PV-/- mice. These data show that parvalbumin potently modulates short-term synaptic plasticity.

2-Amino-5-phosphonovalerate↗
PubMed2000

Anisotropy of domain wall resistance

The resistive effect of domain walls in FePd films with perpendicular anisotropy was studied experimentally as a function of field and temperature. The films were grown directly on MgO substrates, which induces an unusual virgin magnetic configuration composed of 60 nm wide parallel stripe domains. This allowed us to carry out the first measurements of the anisotropy of domain wall resistivity in the two configurations of current perpendicular and parallel to the walls. At 18 K, we find 8.2% and 1.3% for the domain wall magnetoresistance normalized to the wall width (8 nm) in these two respective configurations. These values are consistent with the predictions of Levy and Zhang.

Journal Article↗
PubMed2000

Quantal currents at single-site central synapses.

The mode of operation of synaptic transmission has been primarily worked out at the vertebrate neuromuscular junction, thus providing a framework for the interpretation of studies at central synapses. However, differences have been found between the two systems, and a coherent model is still lacking for central synapses. Research in this area revolves around several questions. (1) Is the variability of quantal amplitudes determined pre- or postsynaptically? (2) What is the occupancy of postsynaptic receptors following the release of a synaptic vesicle? And (3) does multivesicular release occur at single release sites following one presynaptic action potential? To answer these questions, it is essential to investigate synaptic processes at the level of single release sites. This is technically difficult because of the complex morphology and small dimensions of central synapses. Nevertheless significant advances have been made in the past few years.

Action Potentials↗
PubMed2000

Magnetization profile of ultrathin FePd films.

The method of circular dichroism in X-ray resonant magnetic scattering is presented which allows a straightforward determination of the magnetization profile of magnetic patterns in ultrathin films. Application to single crystalline FePd layers shows unambiguously the presence of magnetic flux closure domains whose thickness can constitute a significant fraction ( approximately 25%) of the total film.

Journal Article↗
PubMed2000

Presynaptic calcium stores underlie large-amplitude miniature IPSCs and spontaneous calcium transients.

The cellular mechanisms responsible for large miniature currents in some brain synapses remain undefined. In Purkinje cells, we found that large-amplitude miniature inhibitory postsynaptic currents (mIPSCs) were inhibited by ryanodine or by long-term removal of extracellular Ca2+. Two-photon Ca2+ imaging revealed random, ryanodine-sensitive intracellular Ca2+ transients, spatially constrained at putative presynaptic terminals. At high concentration, ryanodine decreased action-potential-evoked rises in intracellular Ca2+. Immuno-localization showed ryanodine receptors in these terminals. Our data suggest that large mIPSCs are multivesicular events regulated by Ca2+ release from ryanodine-sensitive presynaptic Ca2+ stores.

Animals↗
PubMed1999

Extractive fermentation of aroma with supercritical CO2

This work deals with the feasibility of achieving an extractive fermentation of 2-phenylethyl alcohol, the rose aroma, coupling fermentation with Kluyveromyces marxianus and supercritical carbon dioxide (SCCO2) extraction. The extractive process is, in this case, of special interest due to the strong yeast inhibition by 2-phenylethyl alcohol. First results confirmed that direct SCCO2 extraction is not possible, due to a drastic CO2 effect on cell viability. It is therefore necessary to perform cell separation prior to the extraction. Aroma extraction conditions from a synthetic mixture were then optimized, a pressure of 200 bar and a temperature in the range 35-45 degrees C being chosen. Under these conditions, the distribution coefficient Kd is 2 times higher than during the extraction using a conventional organic solvent, n-hexane. Using a simple model of aroma partition between aqueous and SCCO2 phases, the parameters of a continuous extraction from a synthetic broth were defined. The two substrates, glucose and phenylalanine, are not extracted whatever the conditions. As predicted by the model, more than 90% of 2-phenylethyl alcohol can be extracted, while the extraction of ethanol, the second main product, can be easily tuned with respect to operating conditions, as a function of its influence on the fermentation. Finally, the feasibility of the aroma recovery using two depressurization steps at the outflow of the extraction vessel was demonstrated; 97% of the extracted aroma was recovered, and a mass purity of 91% was achieved. Copyright 1999 John Wiley & Sons, Inc.

Journal Article↗
PubMed1999

Chiral magnetic domain structures in ultrathin FePd films

The magnetization profile of magnetically ordered patterns in ultrathin films was determined by circular dichroism in x-ray resonant magnetic scattering (CDXRMS). When this technique was applied to single crystalline iron palladium alloy layers, magnetic flux closure domains were found whose thickness can constitute a large fraction ( approximately 25 percent) of the total film.

Journal Article↗