Showing papers by "Pierre-and-Marie-Curie University published in 1994"

Classification of chemical bonds based on topological analysis of electron localization functions

Abstract: THE definitions currently used to classify chemical bonds (in terms of bond order, covalency versus ionicity and so forth) are derived from approximate theories1–3 and are often imprecise. Here we outline a first step towards a more rigorous means of classification based on topological analysis of local quantum-mechanical functions related to the Pauli exclusion principle. The local maxima of these functions define 'localization attractors', of which there are only three basic types: bonding, non-bonding and core. Bonding attractors lie between the core attractors (which themselves surround the atomic nuclei) and characterize the shared-electron interactions. The number of bond attractors is related to the bond multiplicity. The spatial organization of localization attractors provides a basis for a well-defined classification of bonds, allowing an absolute characterization of covalency versus ionicity to be obtained from observable properties such as electron densities.

Atherosclerotic disease of the aortic arch and the risk of ischemic stroke.

Abstract: Background Atherosclerotic disease of the aortic arch has been suspected to be a potential source of cerebral emboli. We conducted a study to quantify the risk of ischemic stroke associated with atherosclerotic disease of the aortic arch. Methods Using transesophageal echocardiography, we performed a prospective case-control study of the frequency and thickness of atherosclerotic plaques in the ascending aorta and proximal arch in 250 consecutive patients admitted to the hospital with ischemic stroke and 250 consecutive controls, all over the age of 60 years. Results Atherosclerotic plaques ≥ 4 mm in thickness were found in 14.4 percent of the patients but in only 2 percent of the controls. After adjustment for atherosclerotic risk factors, the odds ratio for ischemic stroke among patients with such plaques was 9.1 (95 percent confidence interval, 3.3 to 25.2; P<0.001). Among the 78 patients who had brain infarcts with no obvious cause, 28.2 percent had plaques ≥ 4 mm in thickness, as compared with 8.1 pe...

The importance of Prochlorococcus to community structure in the central North Pacific Ocean

Abstract: Assessments of plankton community structure in the oligotrophic oceans based solely on microscopy may overstate the importance of heterotrophic bacterial biomass. Using flow cytometry to distinguish heterotrophic bacteria from the photosynthetic procaryotes Prochlorococcus spp., we found that Prochlorococcus contributed 31% of total bacterial counts in the upper 100 m at station ALOHA (22°45′N, 158°W). In terms of carbon, procaryotic biomass was the largest component (≥ 80%) of the microbial community, but almost half of this was photosynthetic biomass contributed by Prochlorococcus. Overall, the total 200-m integrated photosynthetic biomass exceeded heterotrophic bacterial biomass (55 vs. 45%). We suggest that the relative proportion of photosynthetic to heterotrophic bacterial biomass varies among oligotrophic regions of the ocean and that dominance by heterotrophic bacteria is not typical.

Diffuse reflectance of oceanic shallow waters: influence of water depth and bottom albedo

Abstract: We used simplifying assumptions to derive analytical formulae expressing the reflectance of shallow waters as a function of observation depth and of bottom depth and albedo. These formulae also involve two apparent optical properties of the water body: a mean diffise attenuation coefficient and a hypothetical reflectance which would be observed if the bottom was infinitely deep. The validity of these approximate formulae was tested by comparing their outputs with accurate solutions of the radiative transfer obtained under the same boundary conditions by Monte Carlo simulations. These approximations were also checked by comparing the reflectance spectra for varying bottom depths and compositions determined in coastal lagoons with those predicted by the formulae. These predictions were based on separate determinations of the spectral albedos of typical materials covering the floor, such as coral sand and various green or brown algae. The simple analytical expressions are accurate enough for most practical applications and also allow quantitative discussion of the limitations of remote-sensing techniques for bottom recognition and bathymetry. As early as 1944, Duntley used a spectrograph mounted in a glass-bottomed boat or flown in an airplane to analyze radiances emerging from the ocean and shallow waters. He evidenced the influence of the water depth on the spectral composition of the upward flux (Duntley 1963). Using a Monte Carlo technique, Plass and Kattawar (1972) calculated the radiative field in the atmosphere+cean system and in particular examined the dependence of the upward flux on the albedo of the ocean floor. Gordon and Brown (1974) studied the diffuse reflectance of a shallow ocean using Monte Carlo simulations and a probabilistic approach. Gordon and Brown provided an analysis based on photon history of the light field as modified by the presence of a reflecting bottom. In, addition, Ackleson and Klemas (1986) developed a two-flow model that simulated the light field within a canopy of bottom-adhering plants. A single scattering approximation for irradiance reflectance was also

Mitochondrial Hsp70/MIM44 complex facilitates protein import

Abstract: Protein translocation into mitochondria requires the mitochondria! protein Hsp70. This molecular chaperone of the mitochondrial matrix is recruited to the protein import machinery by MIM44, a component associated with the inner membrane of the mitochondria. Formation of the mt-Hsp70/MIM44 complex is regulated by ATP. MIM44 and mt-Hsp70 interact in a sequential manner with incoming segments of unfolded preproteins and thereby facilitate stepwise vectorial translocation of proteins across the mitochondrial membranes. The complex appears to act as a molecular ratchet which is energetically driven by the hydrolysis of ATP.

Carotid and vertebral artery dissections: three-dimensional time-of-flight MR angiography and MR imaging versus conventional angiography.

Abstract: PURPOSE: To compare three-dimensional time-of-flight magnetic resonance (MR) angiography, MR imaging, and transfemoral four-vessel angiography in diagnosis and follow-up of cervical artery dissection. MATERIALS AND METHODS: Eighteen patients with 19 extracranial internal carotid artery and five vertebral artery dissections were included in this prospective study. Blind interpretation of MR and angiographic images was made by two independent radiologists. RESULTS: MR angiography was more accurate than MR imaging in diagnosis of carotid dissection, although the difference was not statistically significant. In carotid artery dissection, respective sensitivity and specificity were 95% and 99% for MR angiography and 84% and 99% for MR imaging and in vertebral artery dissection were 20% and 100% for MR angiography and 60% and 98% for MR imaging. CONCLUSION: MR angiography is a reliable, noninvasive method for use in diagnosis and follow-up of extracranial internal carotid artery dissection. In vertebral artery ...

Heating rate within the upper ocean in relation to its bio-optical state

Abstract: Solar radiation absorption and local heating within the upper layers of the open ocean are strongly influenced by the abundance of phytoplankton as depicted by the chlorophyll concentration According to whether this concentration is high or low, the heat deposition occurs within a layer that may vary in thickness from low than 10 m to more than 100 m A simple parameterization, accounting for this dependence, is developed It allows the vertical profiles of heating rate to be predicted from the phytoplanktonic pigment concentration, as it can (and will) be remotely detected from space, by using ocean color sensors This computationally efficient parameterization has been validated in reference to the results of a full spectral model In the simplified computation, the solar spectrum is partitioned into two domains, below and above the wavelength 075 µm For the infrared waveband, not influenced by biological materials the irradiance profile is described by a single exponential function For the

Quantum-noise reduction using a cavity with a movable mirror

Abstract: Because of radiation pressure, an optical cavity with harmonically bound mirrors has an intensity-dependent length and behaves as an effective Kerr medium. We determine the quantum fluctuations of the field reflected by such a cavity, taking into account the input field fluctuations and the mirror Brownian motion. In the regions of parameter space close to bistability turning points, we obtain a significant quantum-noise reduction effect.

Exact spectra, spin susceptibilities, and order parameter of the quantum Heisenberg antiferromagnet on the triangular lattice

Abstract: Exact spectra of periodic samples are computed up to N=36. Evidence of an extensive set of low-lying levels, lower than the softest magnons, is exhibited. These low-lying quantum states are degenerated in the thermodynamic limit; their symmetries and dynamics as well as their finite-size scaling are strong arguments in favor of N\'eel order. It is shown that the N\'eel order parameter agrees with first-order spin-wave calculations. A simple explanation of the low-energy dynamics is given as well as the numerical determinations of the energies, order parameter, and spin susceptibilities of the studied samples. It is shown how suitable boundary conditions, which do not frustrate N\'eel order, allow the study of samples with N=3p+1 spins. A thorough study of these situations is done in parallel with the more conventional case N=3p.

Modelling phytoplankton development in whole drainage networks: the RIVERSTRAHLER Model applied to the Seine river system

Abstract: Phytoplankton development in river systems is under the control of various meteorological, hydrological, chemical and biological factors. Because of the continuity of the aquatic systems which progress from headwaters to the largest rivers, the interplay of these control factors can only be understood at the scale of the entire drainage network. The RIVERSTRAHLER Model, based on the concept of stream-order, has been established for that purpose. It has been applied here on two rivers from the Seine basin: rivers Marne and Oise. It is shown that hydrological factors determine the time of onset, and the position within the drainage network, of the spring algal bloom. Phosphorus availability, when limiting, controls the intensity of the bloom. During summer, top-down control, linked to grazing and other causes of mortality, has a marked impact on algal dynamics.

Guided waves by electromagnetic gratings and non‐uniqueness examples for the diffraction problem

Abstract: We consider the two-dimensional scalar problem of the diffraction of a plane wave by an infinite grating of conducting bodies immersed in a periodical dielectric medium. A Fredholm-type formulation is derived and studied. The existence of a solution is proved and some uniqueness results are established. A detailed description of the guided modes of the grating is carried out. Finally, various non-uniqueness examples for the diffraction problem are exhibited.

Survival benefit of repeat liver resections for recurrent colorectal metastases: 143 cases. Association Francaise de Chirurgie.

Abstract: PURPOSEResection is the only chance for cure in patients with colorectal liver metastases (LM). Five-year survival rates are close to 25%. Unfortunately, recurrences occur in most patients. Some recurrent LM are technically resectable. The aim of this study was to determine the risks and benefits of repeat resections for recurrent LM.PATIENTS AND METHODSData from 130 patients who received 143 repeat liver resections for recurrent LM were collected. In 116 patients, only the liver was involved, while 14 had both liver and extrahepatic recurrences.RESULTSIn the first group, the operative mortality and morbidity rates were 0.9% and 24.7%, respectively. Two- and 3-year survival rates were 57% and 33%, respectively. Recurrences were observed in 66% of patients. Twelve patients underwent a third hepatectomy for recurrence. The mortality rate was nil, and the mean survival time was 12.5 months. In the group with liver and extrahepatic metastases, the operative mortality and morbidity rates were 0% and 25%. The m...

Polyglutamylation of tubulin as a progressive regulator of in vitro interactions between the microtubule-associated protein Tau and tubulin.

Abstract: The multiple functions of microtubules are mediated by various structural and motor microtubule-associated proteins (MAPs). To harmonize these functions in different places of a single cell, the key problem is to regulate the interactions of these proteins with microtubules. The chemical diversity of tubulin isoforms, which constitute the microtubule wall, could represent a molecular basis for this control. Using an in vitro assay of ligand blotting, we found that the microtubule-associated protein Tau interacts differentially with the diverse posttranslationally-modified isotubulins: its binding is mainly restricted to moderately-modified alpha- and beta-tubulin isoforms. We obtained evidence that the recently-discovered polyglutamylation, which consists of the sequential, posttranslational addition of one to six glutamyl units to both alpha- and beta-tubulin subunits, regulates the binding of Tau as a function of its chain length. The relative affinity of Tau, very low for unmodified tubulin, increases progressively for isotubulins carrying from one to three glutamyl units, reaches an optimal value, and then decreases progressively when the polygutamyl chain lengthens up to six residues. Our results suggest that the unmodified C-terminus of tubulin exerts a constitutive inhibition on Tau binding, probably by locking the MAP-binding site, and that this inhibition could be first released and then restored as the polyglutamyl chain grows. As the posttranslational chain does not appear to interact directly with Tau, it is thought that the growth of this chain from one to six glutamyl units causes a progressive, conformational shift in the structure of the C-terminal domain of tubulin, thus leading to the observed modulation of affinity.

Xanthophyll biosynthesis in chromoplasts: isolation and molecular cloning of an enzyme catalyzing the conversion of 5,6-epoxycarotenoid into ketocarotenoid

Abstract: Summary The late steps of carotenoid biosynthesis in plants involve the formation of xanthophyUs. Little is known about the enzymology of these steps. This paper reports the purification to homogeneity of a xantho- phyll biosynthetic enzyme from Capsicum annuum chromoplasts, which catalyzes the conversion of the ubiquitous 5,6.epoxycarotenoids, antheraxanthin and violaxanthin, into capsanthin and capsorubin, respectively. Owing to its bifunctionallty, the name capsanthin-capsorubin synthase is proposed for this new enzyme. The purified enzyme is a monomer with a molecular mass of 50 kDa. Antibodies raised against this enzyme allowed the isolation of a full- length cDNA clone encoding a capsanthin capserubin synthase high molecular weight precursor. The primary deduced structure reveals the presence of a consensus nucleotide binding site. The capeanthin- capsorubin synthase gene is specifically expressed during chromoplast development in fruits accumu- lating ketocarotenoids, but not in mutants impaired in this biosynthetic step. phytofluene desaturase (Hugueney

The Eastern Boundary of the Subtropical North Atlantic

Abstract: A quasi-meridional hydrographic section carried out between 60° and 20°N offshore from the European and African continental slopes is analyzed in terms of water masses and zonal transports in and out of the eastern boundary. Outstanding features of the meridional distribution of water masses are focused on, such as the transition between North Atlantic and South Atlantic Central Waters at 20°–25°N, the properties and anticyclonic circulation of the Rockall Channel mode water, and the northern boundary of the large-scale Mediterranean Water plume at about 50°N. An eastward transport of about 11×10 6 m 3 s −1 is found to enter the eastern boundary layer at densities lower than 27.25 and feed southward alongshore currents. The relation of the incoming transport to the water mass distribution and its eventual splitting into several outflowing components are discussed. Apart from the downward entrainment of upper water known to occur in the Gulf of Cadiz, there is no sign of the so-called “eastern boundary ventilation” mechanism in the central water density range. Yet a significant transport is found to escape the winter mixed layer toward the interior of the subtropical gyre, suggesting the horizontal southward currents across the sloping bottom of the mixed layer to be the main cause of ventilation at the eastern boundary.

Characterization of recombinant plant cinnamate 4-hydroxylase produced in yeast. Kinetic and spectral properties of the major plant P450 of the phenylpropanoid pathway.

Abstract: Helianthus tuberosus cinnamate 4-hydroxylase (CYP73 or CA4H), a member of the P450 superfamily which catalyses the first oxidative step of the phenylpropanoid pathway in higher plants by transforming cinnamate into p-coumarate, was expressed in the yeast Saccharomyces cerevisiae. The PCR-amplified CA4H open reading frame was inserted into pYeDP60 under the transcriptional control of a galactose-inducible artificial promoter. Engineered S. cerevisiae strains producing human P450 reductase or normal or overproduced amounts of yeast P450 reductase were transformed to express recombinant CA4H. When grown on galactose, yeast cells produced CA4H holoprotein bound to the endoplasmic reticulum membrane as judged from the reduced iron/carbon monoxide difference spectrum centered at 452 nm and from typical cinnamate 4-hydroxylase activity upon coupling with the different P450 reductases and NADPH. Some CA4H protein was found also addressed to the yeast mitochondria but as a low-activity form. The spectral and kinetic characterizations of the yeast-produced CA4H in different redox protein environments are presented using both assays on yeast microsomal fractions and bioconversions on living cells. Results indicate that the microsomal system constituted by the overexpressed yeast P450 reductase and CA4H is characterized by a 1:1 coupling between NADPH oxidation and cinnamate hydroxylation and by one of the highest turnover numbers reported for an NADPH-dependent P450 reaction. Based on spectral perturbation and inhibition studies, coumarate appeared to have no detectable affinity for the enzyme. A possible geometry of the substrate recognition pocket is discussed in the light of these data.

Bifurcation Effects in Ductile Metals With Nonlocal Damage

Abstract: The purpose of this paper is to investigate some bifurcation phenomena in a porous ductile material described by the classical Gurson (1977) model, but with a modified, nonlocal evolution equation for the porosity. Two distinct problems are analyzed theoretically: appearance of a discontinuous velocity gradient in a finite, inhomogeneous body, and arbitrary loss of uniqueness of the velocity field in an infinite, homogeneous medium. It is shown that no bifurcation of the first type can occur provided that the hardening slope of the sound (void-free) matrix is positive. In contrast, bifurcations of the second type are possible; nonlocality does not modify the conditions of first occurrence of bifurcation but does change the corresponding bifurcation mode, the wavelength of the latter being no longer arbitrary but necessarily infinite. A FE study of shear banding in a rectangular mesh deformed in plane strain tension is finally presented in order to qualitatively illustrate the effect of finiteness of the body; numerical results do evidence notable differences with respect to the case of an infinite, homogeneous medium envisaged theoretically.

V region-mediated selection of autoreactive repertoires by intravenous immunoglobulin (i.v.Ig).

Abstract: In addition to its use as substitute therapy for primary and secondary immunodeficiencies, intravenous immunoglobulin (i.v.Ig) is increasingly being used as an immunomodulating therapy in the treatment of patients with a variety of autoimmune and systemic inflammatory disorders (Schwartz 1990, Dietrich et al. 1992b, Dwyer 1992, Ronda et al. 1993). The use of i.v.Ig in these situations is supported by a few randomized clinical trials and a large number of uncontrolled and smaller studies. Of relevance to this chapter are that the reported beneficial effects of i.v.Ig include those in autoantibody-mediated autoimmune diseases (e.g., autoimmune cytopenias, anti-factor VIII autoimmune disease) as well as diseases in which autoaggressive T cells are primarily involved in the pathogenesis (e.g., autoimmune uveitis) (Imbach et al. 1981, Bussel et al. 1983, Sultan et al. 1984, McGuire et al. 1987, LeHoang et al. 1994). Where it is a feature of the disease, successful outcome of i.v.Ig therapy is associated with an improvement in the patient's systemic inflammatory condition. Modulation of B-cell and Tcell functions and of cytokine production has further been observed in animal models of autoimmune diseases following administration of human i.v.Ig or of normal homologous IgG (Forsgren et al. 1991, Rossi et al. 1991a, Saoudi et al. 1993, Hentati et al. 1994). The design of trials to establish the efficacy and appropriate therapeutic sched-

Studies of columns of beads under external vibrations.

Abstract: As a toy model for dissipative granular materials, we investigate a one-dimensional column of beads undergoing external vibrations. The analysis is both experimental and numerical. We display the crossover from a condensed to a fluidized state of the column; the parameters are the agitation, the number of beads, and the momentum restitution coefficient. We find clustered states for high dissipation and/or a large number of beads. Our experimental findings support the appearance of a fluidized regime at low dissipation and of a Feigenbaum-type bifurcation scenario at high dissipation.

Synthesis of chiral and racemic functional polymers from glycidol and thioglycidol

Abstract: Racemic and optically active protected glycidols (as 1-ethoxyethyl ethers) are readily polymerized with anionic initiators (e.g., CsOH). Polymers with molecular weights up to 30 000 are obtained without cleavage of the protective group. Oligomers (degree of polymerization [DPn] 5 to 10) are prepared using an aluminium complex derived from a Schiff's base. The protected thioglycidol (synthesized from the protected glycidol) leads to very high-molecular-weight polymers (150 000–320 000) using anionic or anionic coordinated initiators. Copolymers of protected glycidol with ethylene oxide are obtained with KOH as initiator. Regeneration of hydroxyl groups in polymers is carried out with acidic cleavage. Treatment of protected polyglycidol with formic acid leads first to the formate of polyglycidol. Then, after saponification, pure polyglycidol is obtained. The use of aqueous hydrochloric acid leads directly to the free polymer.