Showing papers by "Pierre-and-Marie-Curie University published in 1997"

ELF: The Electron Localization Function

Abstract: The chemical bond is always considered from different points of view, depending or on the chemical and physical aspects to be examined. In both cases, description of the chemical bond are chosen that are appropriate for the particular research or application. Therefore, there are significant differences in the understanding as what constitutes chemical bonding. This is acceptable in practice but proves to be a hindrance for true interdisciplinarity. The concept of the chemical bond offer a firm basis upon which to forge links not only within chemistry but also to all related sciences. The general desire and the growing necessity for interdisciplinary collaboration requires a careful treatment of these concepts and, if possible, a tightening and standardization to a level that is widely acceptable and beneficial. In the age of tremendously fast development of computers and computer science, we believe that the electron localization function (ELF) provides a new description of the chemical bond for almost all classes of compounds. Its graphical language earns it the ultimate qualification for enhanced interdisciplinarity.

Warming Early Mars with Carbon Dioxide Clouds That Scatter Infrared Radiation

Abstract: Geomorphic evidence that Mars was warm enough to support flowing water about 3.8 billion years ago presents a continuing enigma that cannot be explained by conventional greenhouse warming mechanisms. Model calculations show that the surface of early Mars could have been warmed through a scattering variant of the greenhouse effect, resulting from the ability of the carbon dioxide ice clouds to reflect the outgoing thermal radiation back to the surface. This process could also explain how Earth avoided an early irreversible glaciation and could extend the size of the habitable zone on extrasolar planets around stars.

A role for Cajal–Retzius cells and reelin in the development of hippocampal connections

Abstract: DURING development of the nervous system, specific recognition molecules provide the cues necessary for the formation of neural connections. In some regions, guiding cues for axonal pathfinding and target selection are provided by specific cells that exist only transiently during development, such as the floorplate or the cortical subplate1–4. In the hippocampus, distinct groups of fibres innervate different layers5. We have tested the hypothesis that transient neurons in the hippocampus6,7 provide positional information for the targeting of these fibres. Here we report that ablation of Cajal–Retzius cells in organotypic slice cultures of hippocampus prevented the ingrowth of entorhinal but not of commissural afferents. Experiments inhibiting Reelin (an extracellular matrix protein expressed by Cajal–Retzius cells) and analysis of reeler mutant mice showed dramatic abnormalities in the development of entorhinal afferents. Thus Cajal–Retzius cells and reelin are essential for the formation of layer-specific hippocampal connections.

Recent extensions of Gurson's model for porous ductile metals

Abstract: This paper is devoted to two distinct extensions of Gurson’s (1977) famous model for plastic voided metals. Gurson’s work was based on an approximate limit-analysis of a typical elementary volume in a porous material, namely a hollow sphere subjected to conditions of arbitrary homogeneous boundary strain rate. The first extension envisaged consists in considering a more general geometry, namely a spheroidal volume containing some spheroidal confocal cavity. The aim here is to incorporate void shape effects into Gurson’s model. The second extension again considers a hollow sphere, but now subjected to conditions of inhomogeneous boundary strain rate. The goal is to account for possible strong variations of the macroscopic mechanical fields at the scale of the representative cell (i.e. of the void spacing), as encountered near crack tips.

Developmental stability and fitness: a review.

Abstract: Developmental stability reflects the ability of individuals to undergo stable development of their phenotype under a range of environmental conditions. Developmental instability is measured in terms of fluctuating asymmetry or phenodeviance. A negative relationship between developmental instability and fitness has figured as a prominent untested assumption in the literature. A review of available information from the literature on the relationship between developmental instability and various fitness components such as growth, fecundity, and longevity suggests that there indeed is a general negative relationship. Symmetrical individuals do generally have faster growth, higher fecundity, and better survival than do more asymmetrical individuals. These differences appear partially to arise from lower competitive ability and higher risks of predation and parasitism of asymmetrical individuals compared with more symmetrical conspecifics. The relationship between developmental stability and fitness ma...

Methanol oxidation as a catalytic surface probe

Abstract: The goal of this review is to present some aspects of the use of a test reaction, i.e., methanol oxidation, as a tool to characterize oxidation catalysts. The selectivity pattern and the formation rates of the reaction products are used to characterize both structural (dispersion) as well as chemical properties (acid-base and redox) on supported oxide catalysts, especially for molybdenum-based systems supported on silica and vanadia on titanium oxide. This highly sensitive technique which gives information on the catalytically active sites at the molecular level characterizes a catalyst at work and is particularly well-adapted to the study of supported catalysts.

The evolution of coloniality: the emergence of new perspectives

Abstract: The evolution of group living remains an outstanding question in evolutionary ecology. Among the most striking forms of group living are the enormous assemblages of breeders that occur in many colonial marine birds and mammals, with some colonies containing more than a million individuals breeding in close contact. Coloniality is an evolutionary puzzle because individuals pay fitness costs to breed in high densities. Despite numerous potential benefits proposed to overcome these costs, we still lack a general framework to explain coloniality. Several new hypotheses involving breeding habitat and mate selection create promising approaches for studying this enigma.

Reintroduction: challenges and lessons for basic ecology

Abstract: Ecology is a subject where theoretical predictions are often difficult to test experimentally in the field. To address this challenge, the Ecological Society of America suggested exploiting large-scale environmental management decisions in a scientific way. This ‘adaptive management' constitutes one of the purposes of the Sustainable Biosphere Initiative. Meanwhile, in the current context of the biodiversity crisis, translocations and particularly reintroductions of threatened species are becoming more numerous. It is time for ecologists and wildlife managers to collaborate on these unique opportunities for large-scale studies.

A Bayesian approach to introducing anatomo-functional priors in the EEG/MEG inverse problem

Abstract: We present a new approach to the recovering of dipole magnitudes in a distributed source model for magnetoencephalographic (MEG) and electroencephalographic (EEG) imaging. This method consists in introducing spatial and temporal a priori information as a cure to this ill-posed inverse problem. A nonlinear spatial regularization scheme allows the preservation of dipole moment discontinuities between some a priori noncorrelated sources, for instance, when considering dipoles located on both sides of a sulcus. Moreover, we introduce temporal smoothness constraints on dipole magnitude evolution at time scales smaller than those of cognitive processes. These priors are easily integrated into a Bayesian formalism, yielding a maximum a posteriori (MAP) estimator of brain electrical activity. Results from EEG simulations of our method are presented and compared with those of classical quadratic regularization and a now popular generalized minimum-norm technique called low-resolution electromagnetic tomography (LORETA).

Parameters of photosynthesis: Definitions, theory and interpretation of results

Abstract: A global assessment of carbon flux in the world ocean is one of the major undertakings of the Joint Global Ocean Flux Study (JGOFS). This has to be undertaken using historical in situ data of primary productivity. As required by the temporal and spatial scales involved in a global study, it can be conveniently done by combining, through appropriate models, remotely sensed information (chlorophyll a, temperature) with basic information about the parameters related to the carbon uptake by phytoplanktonic algae. This requires a better understanding as well as a more extended knowledge of these parameters which govern the radiative energy absorption and utilization by algae in photosynthesis. The measurement of the photosynthetic response of algae [the photosynthesis (P) versus in-adiance (£) curves], besides being less shiptime consuming than in situ primary production experiments, allows the needed parameters to be derived and systematically studied as a function of the physical, chemical and ecological conditions. The aim of the present paper is to review the significance of these parameters, especially in view of their introduction into models, to analyze the causes of their variations in the light of physiological considerations, and finally to provide methodological recommendations for meaningful determinations, and interpretation, of the data resulting from /"versus £ determinations. Of main concern are the available and usable irradiance, the chlorophyll a-specific absorption capabilities of the algae, the maximum light utilization coefficient (a), the maximum quantum yield (4>m), the maximum photosynthetic rate (Pm) and the light saturation index (£k). The potential of other, non-intrusive, approaches, such as the stimulated variable fluorescence, or the sun-induced natural fluorescence techniques is also examined.

Search for neutral heavy leptons produced in Z decays

Abstract: Weak isosinglet Neutral Heavy Leptons (m) have been searched for using data collected by the DELPHI detector corresponding to 3:3 106 hadronic Z0 decays at LEP1. Four separate searches have been performed, for short-lived m production giving monojet or acollinear jet topologies, and for long-lived m giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio BR(Z0 ! m) of about 1:310−6 at 95% confidence level for m masses between 3.5 and 50 GeV/c2. Outside this range the limit weakens rapidly with the m mass. The results are also interpreted in terms of limits for the single production of excited neutrinos.

Characterization of Elementary Chemical Processes by Catastrophe Theory

Abstract: The topological analysis of the electron localization function (ELF) provides a convenient theoretical framework to characterize chemical bonds. This method does not rely on the particular approximations that are made in actual quantum chemical calculations of the electronic structure. In principle, it can be applied to exact wave functions as well as to experimental electron densities. Introduction of a control space, such as a set of reaction pathways, allows extension of the analysis to chemical reactions. The study of the bifurcations occurring during such processes is of particular interest for their classification and their qualitative description. This is achieved with the help of Rene Thom's catastrophe theory. The following examples are discussed: the ammonia inversion, the breaking of the ethane C−C bond, and the breaking of the dative bond in NH3BH3. The types of catastrophe and their unfolding have been determined for each of these processes. As by-products, nonempirical definitions of covale...

pH-Responsive Gels of Hydrophobically Modified Poly(acrylic acid)

Abstract: pH-responsive gels of hydrophobically modified (HM) weak polyacid were prepared from acrylic acid and n-alkyl acrylates (n = 8, 12, 18). The HM gels obtained bear up to 20 mol % of n-alkyl acrylate units randomly distributed along the network chains. The pH-driven swelling of these gels upon ionization in an aqueous medium was studied. The effect of the fraction and of the side chain length of n-alkyl acrylate groups on the equilibrium degree of swelling was examined. It was shown that the swelling transition shifts to alkaline pH with increasing hydrophobicity of the gel. This was explained by the stabilization of the collapsed state of the gel by hydrophobic aggregation of n-alkyl side chains. The formation of such aggregates, which break down in the course of gel ionization, was confirmed by the fluorescent probe method with pyrene as a probe and by NMR spectroscopy. Potentiometric titration data of HM poly(acrylic acid) (PAA) gels and of the corresponding linear copolymers evidence that the introducti...

The use of conspecific reproductive success for breeding patch selection in terrestrial migratory species

Abstract: Classical models of breeding habitat selection rarely deal with the question of information gathering for patch quality assessment. In this paper, we present two models comparing the fitness outcomes of behavioural strategies based on conspecific reproductive success as a cue to assess local environmental quality before selecting a new breeding habitat. The models deal with two phases of the life-cycle of a territorial migratory species: recruitment to a breeding population (model 1) and breeding site fidelity of subsequent breeding attempts (model 2). The first model shows that prospecting breeding patches before recruiting is the best strategy if the environment is predictable and contains a low proportion of good patches, even if it implies losing a breeding opportunity. The second model shows that dispersing after a breeding attempt according to the patch’s breeding success rather than the individual’s own success is the best strategy if the environment is patchy. These results underline the importance of studying the spatio-temporal variations of factors affecting reproductive success when considering the importance of habitat selection strategies based on conspecifics. Moreover, they allow the understanding of individual behaviour patterns observed in natural populations and their potential consequences at the metapopulation level.

The Tim core complex defines the number of mitochondrial translocation contact sites and can hold arrested preproteins in the absence of matrix Hsp70-Tim44

Abstract: Preprotein import into mitochondria is mediated by translocases located in the outer and inner membranes (Tom and Tim) and a matrix Hsp70-Tim44 driving system. By blue native electrophoresis, we identify an approximately 90K complex with assembled Tim23 and Tim17 as the core of the inner membrane import site for presequence-containing preproteins. Preproteins spanning the two membranes link virtually all Tim core complexes with one in four Tom complexes in a stable 600K supercomplex. Neither mtHsp70 nor Tim44 are present in stoichiometric amounts in the 600K complex. Preproteins in transit stabilize the Tim core complex, preventing an exchange of subunits. Our studies define a central role for the Tim core complexes in mitochondrial protein import; they are not passive diffusion channels, but can stably interact with preproteins and determine the number of translocation contact sites. We propose the hypothesis that mtHsp70 functions in protein import not only by direct interaction with preproteins, but also by exerting a regulatory effect on the Tim channel.

Annual variability of phytoplankton and bacteria in the subtropical North Pacific Ocean at Station ALOHA during the 1991–1994 ENSO event

Abstract: Time-series data on community structure in the upper 200 m at Station ALOHA in the subtropical North Pacific were collected at approximately monthly intervals from December 1990 through to March 1994 during an extended El Niiio-Southern Oscillation (ENSO) event. Samples were analyzed by flow cytometry to enumerate Prochlorococcus, Synechococcus, picoeucaryotes, 3–20 μm algae, and heterotrophic bacteria, as well as to quantify cellular chlorophyll fluorescence for the autotrophic components. A significant seasonal cycle was evident in cellular chlorophyll fluorescence for each of the autotrophic components, with maxima occurring each winter as a consequence of photoacclimation. Abundance of each picophytoplankton component exhibited temporal variability on both seasonal and interannual scales. Although the magnitude of the seasonal cycles in the abundance was relatively small, the cycles appeared to be out of phase. Typically, abundance maxima of Synechococcus occurred in winter, of picoeucaryotes in spring, and of Prochlorococcus during summer/fall. The different timing in these cycles may explain why the presence of a seasonal pattern in total phytoplankton biomass has been difficult to establish. Abundance of the larger 3–20 μm algae varied over two orders of magnitude during the time series, with no obvious seasonal pattern. The 3–20 μm algae were a small percentage of the total estimated carbon biomass (∼8%). Heterotrophic bacteria were the most numerous of the picoplankton, and the seasonal pattern in their 200-m integrated abundance paralleled Prochlorococcus over the time series. Together, the procaryotes contributed 60–90% of the total estimated microbial carbon. Significant interannual variation in the total 200-m integrated microbial carbon estimates may be related to the effects of the extended ENSO event, which began in 1991.

Fitness loss and germline mutations in barn swallows breeding in Chernobyl

Abstract: The severe nuclear accident at Chernobyl in 1986 resulted in the worst reported accidental exposure of radioactive material to free-living organisms1. Short-term effects on human populations inhabiting polluted areas include increased incidence of thyroid cancer2, infant leukaemia3, and congenital malformations in newborns4. Two recent studies5,6 have reported, although with some controversy7,8, that germline mutation rates were increased in humans and voles living close to Chernobyl, but little is known about the viability of the organisms affected9. Here we report an increased frequency of partial albinism, a morphological aberration associated with a loss of fitness, among barn swallows, Hirundo rustica, breeding close to Chernobyl. Heritability estimates indicate that mutations causing albinism were at least partly of germline origin. Furthermore, evidence for an increased germline mutation rate was obtained from segregation analysis at two hypervariable microsatellite loci, indicating that mutation events in barn swallows from Chernobyl were two- to tenfold higher than in birds from control areas in Ukraine and Italy.

Multifractal Cascade Dynamics and Turbulent Intermittency

Abstract: Turbulent intermittency plays a fundamental role in fields ranging from combustion physics and chemical engineering to meteorology. There is a rather general agreement that multifractals are being very successful at quantifying this intermittency. However, we argue that cascade processes are the appropriate and necessary physical models to achieve dynamical modeling of turbulent intermittency. We first review some recent developments and point out new directions which overcome either completely or partially the limitations of current cascade models which are static, discrete in scale, acausal, purely phenomenological and lacking in universal features. We review the debate about universality classes for multifractal processes. Using both turbulent velocity and temperature data, we show that the latter are very well fitted by the (strong) universality, and that the recent (weak, log-Poisson) alternative is untenable for both strong and weak events. Using a continuous, space-time anisotropic framework, we th...

A meta‐analysis of the heritability of developmental stability

Abstract: The existence of additive genetic variance in developmental stability has important implications for our understanding of morphological variation. The heritability of individual fluctuating asymmetry and other measures of developmental stability have frequently been estimated from parent-offspring regressions, sib analyses, or from selection experiments. Here we review by meta-analysis published estimates of the heritability of developmental stability, mainly the degree of individual fluctuating asymmetry in morphological characters. The overall mean effect size of heritabilities of individual fluctuating asymmetry was 0.19 from 34 studies of 17 species differing highly significantly from zero (P < 0.0001). The mean heritability for 14 species was 0.27. This indicates that there is a significant additive genetic component to developmental stability. Effect size was larger for selection experiments than for studies based on parent-offspring regression or sib analyses, implying that genetic estimates were unbiased by maternal or common environment effects. Additive genetic coefficients of variation for individual fluctuating asymmetry were considerably higher than those for character size per se. Developmental stability may be significantly heritable either because of strong directional selection, or fluctuating selection regimes which prevent populations from achieving a high degree of developmental stability to current environmental and genetic conditions.

Lidar determination of the entrainment zone thickness at the top of the unstable marine atmospheric boundary layer

Abstract: The thickness of the entrainment zone at the top of the marine atmosphericboundary layer (MABL) has been documented by an airborne lidar on twoconsecutive days during a cold-air outbreak episode over the Mediterranean.In addition to the lidar observations, in situ turbulent flux measurementsat three levels in the MABL were made by a second aircraft. The flights' tracksare broken down in segments 25–30 km long and the data are filtered for theparametrization of turbulent entrainment in the MABL at scales smaller thana few kilometres. The structural parameters of the entrainment zone aredetermined by lidar from the distributions of the instantaneous MABL topheight. The average values Ph0 and Ph2 of the cumulativeprobability distributions are used to define the bottom and top heights of the entrainment zone h0 and h2, respectively. The parameters h0 andh2 are calculated by reference to a linear vertical buoyancy flux profilein the framework of a first-order jump model. The model is constrained by bothlidar and in situ data to determine Ph0 and Ph2 and so h0and h2. In unstable conditions theaverage fraction Ph0 is estimated to be 6.0 ± 1%. It is shown to beslightly sensitive to the presence of cloud at small cloud fractions.The mean value of the ratio of the inversion level buoyancy flux to the surfacebuoyancy flux ARv is found to range from 0.15 to 0.30 depending on the shearin the MABL. The average value is 0.22 ± 0.05. Our resultsare in good agreement with previous analysis at comparable spatial scales.In purely convective conditions, the value of ARv given by theparametrizations fitted to our results is about 0.10–0.12, a value smallerthan the commonly accepted value of 0.2. When compared to previousparametrization results, our proportionality constant for the mechanicalproduction of turbulent kinetic energy is also found to be scaled down, ingood agreement with large-eddy simulation results. It is suggestedthat mesoscale organized motions in the MABL is the source of thisdifference.

A Simple Model of the Decadal Response of the Ocean to Stochastic Wind Forcing

Abstract: A simple linear model is used to estimate the decadal response of the extratropical ocean to wind stress forcing, assuming a flat bottom, a mean state at rest, and no dissipation. The barotropic fields are governed by a time-dependent Sverdrup balance, the baroclinic ones by the long Rossby wave equation. The ocean is bounded by a coast in the east and a radiation condition is used in the west. At each frequency, the baroclinic response consists of a forced response plus a Rossby wave generated at the eastern boundary. For zonally independent forcing, the response propagates westward at twice the Rossby phase speed. The wind stress is assumed to be stochastic with a white frequency spectrum, so the model represents the continuous excitation of the ocean interior by the weather fluctuations. The model predicts the shape and level of the frequency spectra of the oceanic pressure field and their variation with longitude and latitude. The baroclinic response is spread over a continuum of frequencies,...

Design and Magnetic Properties of a Magnetically Isolated GdIIICuII Pair. Crystal Structures of [Gd(hfa)3Cu(salen)], [Y(hfa)3Cu(salen)], [Gd(hfa)3Cu(salen)(Meim)], and [La(hfa)3(H2O)Cu(salen)] [hfa = Hexafluoroacetylacetonato, salen = N,N‘-Ethylenebis(salicylideneaminato), Meim = 1-Methylimidazole]

Abstract: The crystal structures of three compounds have been determined at room temperature, namely [Gd(hfa)3Cu(salen)] (1), [Y(hfa)3Cu(salen)] (2), and [Gd(hfa)3Cu(salen)(Meim)] (3), and the crystal structure of a fourth compound, [La(hfa)3(H2O)Cu(salen)] (4), has been determined at −100 °C; hfa = hexafluoroacetylacetonato, salen = N,N‘-ethylenebis(salicylideneaminato), and Meim = 1-methylimidazole. [La(hfa)3Ni(salen)] (5), isomorphous with 1, has also been synthesized. 1 crystallizes in the monoclinic system, space group P21/n, with a = 17.292(5) A, b = 22.370(5) A, c = 19.658(6) A, β = 90.07(2)°, and Z = 8. 2 crystallizes in the triclinic system, space group P1, with a = 12.207(3) A, b = 13.019() A, c = 13.011(6) A, α = 82.87(4)°, β = 83.55(3)°, γ = 70.91(3)°, Z = 2. 3 crystallizes in the monoclinic system, space group P21/n, with a = 16.521(9) A, b = 20.381(5) A, c = 12.758(6)°, β = 93.22(6)°, and Z = 4. 4 crystallizes in the monoclinic system, space group P21/n, with a = 12.826(2) A, b = 23.067(8) A, c = 13....

Renormalised solutions of nonlinear parabolic problems with L1 data: existence and uniqueness

Abstract: In this paper we prove the existence and uniqueness of a renormalised solution of the nonlinear problemwhere the data f and u0 belong to L1(Ω × (0, T)) and L1 (Ω), and where the function a:(0, T) × Ω × ℝN → ℝN is monotone (but not necessarily strictly monotone) and defines a bounded coercive continuous operator from the space into its dual space. The renormalised solution is an element of C0 ([ 0, T] L1 (Ω)) such that its truncates TK(u) belong to withthis solution satisfies the equation formally obtained by using in the equation the test function S(u)φ, where φ belongs to and where S belongs to C∞(ℝ) with