Showing papers by "Centre national de la recherche scientifique published in 1979"

Extreme ultraviolet observations from Voyager 1 encounter with Jupiter

Abstract: The global hydrogen Lyman alpha, helium (584 angstroms), and molecular hydrogen band emissions from Saturn are qualitatively similar to those of Jupiter, but the Saturn observations emphasize that the H(2) band excitation mechanism is closely related to the solar flux. Auroras occur near 80 degrees latitude, suggesting Earth-like magnetotail activity, quite different from the dominant Io plasma torus mechanism at Jupiter. No ion emissions have been detected from the magnetosphere of Saturn, but the rings have a hydrogen atmosphere; atomic hydrogen is also present in a torus between 8 and 25 Saturn radii. Nitrogen emission excited by particles has been detected in the Titan dayglow and bright limb scans. Enhancement of the nitrogen emission is observed in the region of interaction between Titan's atmosphere and the corotating plasma in Saturn's plasmasphere. No particle-excited emission has been detected from the dark atmosphere of Titan. The absorption profile of the atmosphere determined by the solar occultation experiment, combined with constraints from the dayglow observations and temperature information, indicate that N(2) is the dominant species. A double layer structure has been detected above Titan's limb. One of the layers may be related to visible layers in the images of Titan.

Isobaric volume and enthalpy recovery of glasses. II. A transparent multiparameter theory

Abstract: A multiordering parameter model for glass-transition phenomena has been developed on the basis of nonequilibrium thermodynamics. In this treatment the state of the glass is determined by the values of N ordering parameters in addition to T and P; the departure from equilibrium is partitioned among the various ordering parameters, each of which is associated with a unique retardation time. These times are assumed to depend on T, P, and on the instantaneous state of the system characterized by its overall departure from equilibrium, giving rise to the well-known nonlinear effects observed in volume and enthalpy recovery. The contribution of each ordering parameter to the departure and the associated retardation times define the fundamental distribution function (the structural retardation spectrum) of the system or, equivalently, its fundamental material response function. These, together with a few experimentally measurable material constants, completely define the recovery behavior of the system when subjected to any thermal treatment. The behavior of the model is explored for various classes of thermal histories of increasing complexity, in order to simulate real experimental situations. The relevant calculations are based on a discrete retardation spectrum, extending over four time decades, and on reasonable values of the relevant material constants in order to imitate the behavior of polymer glasses. The model clearly separates the contribution of the retardation spectrum from the temperature-structure dependence of the retardation times which controls its shifts along the experimental time scale. This is achieved by using the natural time scale of the system which eliminates all the nonlinear effects, thus reducing the response function to the Boltzmann superposition equation, similar to that encountered in the linear viscoelasticity. As a consequence, the system obeys a rate (time) -temperature reduction rule which provides for generalization within each class of thermal treatment. Thus the model establishes a rational basis for comparing theory with experiment, and also various kinds of experiments between themselves. The analysis further predicts interesting features, some of which have often been overlooked. Among these are the impossibility of extraction of the spectrum (or response function) from experiments involving cooling from high temperatures at finite rate; and the appearance of two peaks in the expansion coefficient, or heat capacity, during the heating stage of three-step thermal cycles starting at high temperatures. Finally, the theory also provides a rationale for interpreting the time dependence of mechanical or other structure-sensitive properties of glasses as well as for predicting their long-range behavior.

Elemental and Sr isotope variations in basic lavas from Iceland and the surrounding ocean floor

Abstract: Major and trace element data are used to establish the nature and extent of spatial and temporal chemical variations in basalts erupted in the Iceland region of the North Atlantic Ocean. The ocean floor samples are those recovered by legs 38 and 49 of the Deep Sea Drilling Project. Within each of the active zones on Iceland there are small scale variations in the light rare earth elements and ratios such as K/Y: several central complexes and their associated fissure swarms erupt basalts with values of K/Y distinct from those erupted at adjacent centres; also basalts showing a wide range of immobile trace element ratios occur together within single vertical sections and ocean floor drill holes. Although such variations can be explained in terms of the magmatic processes operating on Iceland they make extrapolations from single basalt samples to mantle sources underlying the outcrop of the sample highly tenuous.

Sequence, structure and activity of phosphoglycerate kinase: a possible hinge-bending enzyme.

Abstract: The fitting of sequenced peptides to a high-resolution X-ray map of phosphoglycerate kinase has yielded the complete sequence and structure of the horse muscle enzyme. Metal ADP and ATP substrates are bound to one of the two widely separated domains in an environment that seems unsuitable for phosphoglycerate binding. The most plausible binding site for the phosphoglycerate substrate is on the other domain about 10 A from the ATP, which implies the possibility of a large scale hinge-bending of the domains to bring the two substrates together in a water-free environment for catalysis.

Morphological and biochemical maturation of neurones cultured in the absence of glial cells

Abstract: Cultures of isolated neurones offer possibilities for investigating their properties directly in the absence of glial cells. Furthermore, such a culture provides a suitable model for the study of the biochemical and functional maturation of neuronal cells in a well defined molecular environment. Culture techniques of isolated neurones have been developed for cells originating from the peripheral nervous system1, especially sympathetic ganglion cells2–5. Neurones from the central nervous system (CNS) have proved more difficult to maintain in culture in the absence of other cell types6. It is only recently that hippocampal neurones from rat embryos have been successfully maintained in culture7. We now describe a method for obtaining a pure culture of dispersed neurones from the telencephalon of 8-day-old chick embryos. This method is derived from a technique previously developed in our laboratory8. In those culture conditions, the neuronal cells were obtained by dissociation of cerebral hemispheres from 5–7-day-old chick embryos. The cells remained isolated for only 48 h after plating. Beginning with the 3rd day in vitro, the cells clumped together and formed large aggregates in which it was quite difficult to identify the cell types. We have improved those culture conditions and have succeeded in cultivating neurones which remained well dispersed for the whole period of culture.

A 30,000-yr isotope climatic record from Antarctic ice

Abstract: Simple glaciological conditions at Dome C in east Antarctica have made possible a more detailed and accurate interpretation of an ice core to 950 m depth spanning some 32,000 yr than that obtained from earlier ice cores. Dated events in comparable marine core has enabled the reduction of accumulation rate during the last ice age to be estimated. Climatic events recorded in the ice core indicate that the warmest Holocene period in the Southern Hemisphere occurred at an earlier date than in the Northern Hemisphere.

Phase diagram of the Kondo lattice

Abstract: We discuss the phase diagram of the Kondo lattice: we generalize to the lattice a method proposed by Yoshimori and Sakurai for the single-impurity Kondo problem; this method transforms the Kondo exchange interaction into a fictitious $s\ensuremath{-}f$ hybridization, and gives a resonance of width ${T}_{K}$ at the Fermi level. We study the Kondo phase and compare its energy with the energy of the magnetic phase. The Kondo state is stable when the exchange interaction is larger than a critical value; this state is insulating when the conduction band is half filled.

Disappearance of pink-pigmented Globigerinoides ruber at 120,000 yr BP in the Indian and Pacific Oceans

Abstract: The planktonic foraminiferal species Globigerinoides ruber with pink-pigmented tests occupied a worldwide warm-water belt during much of the Pleistocene. This variety was exterminated from the Indian and Pacific Oceans at about 120,000 yr BP, based on the oxygen isotope stratigraphy in 11 Indo-Pacific deep-sea cores, but it continued to live on to the present in the North and South Atlantic Ocean and the Mediterranean Sea. The disparate Atlantic and Indo-Pacific records reflect faunal provincialism that has been developing in the two regions since the Pliocene.

Variance of the distributions of energy levels and of the transition arrays in atomic spectra

Abstract: Formulas are derived for the mean values and variances of the energy distributions of the levels of an atomic configuration and of the radiative transitions between the levels of two configurations (in intermediate coupling). The variance ${\ensuremath{\sigma}}^{2}$ of the distribution of the eigenstate energies belonging to a given configuration is considered first: ${\ensuremath{\sigma}}^{2}$ is expressed as a linear combination of squares and cross products of the usual Slater electrostatic and spin-orbit radial integrals. It is shown how this expression can be used to check the numerical matrices of energy-integral coefficients. Then expressions are derived for the mean value and for the variance of the weighted distribution of the transition energies between two configurations (the weight of each transition being its strength) in the $n{l}^{N+1}\ensuremath{-}n{l}^{N}{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ and $n{l}^{N}{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}\ensuremath{-}n{l}^{N}{n}^{\ensuremath{'}\ensuremath{'}}{l}^{\ensuremath{'}\ensuremath{'}}$ cases. This derivation is based on the second-quantization formalism. An extension is made to the case of complementary configurations. For transitions $n{l}^{N+1}\ensuremath{-}n{l}^{N}{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$, an explicit formula is obtained for the shift between the mean energy of the transition array and the difference of the mean energies of the configurations. Numerical tables of the angular coefficients appearing in ${\ensuremath{\sigma}}^{2}$ are given for most cases where $l$, ${l}^{\ensuremath{'}}$, ${l}^{\ensuremath{'}\ensuremath{'}}\ensuremath{\le}3$. The main application presented here concerns highly ionized spectra of molybdenum, with transitions between $3{d}^{N+1}$ and $3{d}^{N}4p$, $3{d}^{N}4f$, $3{d}^{N}5p$, and $3{d}^{N}5f$. The agreement between experimental and theoretical (ab initio) mean wave numbers and variances is good. A discussion of the physical conditions of applicability of the results to experimental situations is given.

Saccade size control in reading: Evidence for the linguistic control hypothesis

Abstract: In Experiment 1, it is shown that during reading the eye makes larger saccades near long words than near short words. The effects are reduced when the subject’s peripheral vision is diminished by the use of a moving “window” centered on the subject’s fixation point, outside of which letters are replaced by Xs. In Experiment 2, it is shown that even if linguistic predictions are kept constant, the eye tends to make longer jumps when approaching THE than when approaching a three-letter verb. This “THE-skipping” effect is weaker if THE is compared with an auxiliary (HAD, WAS, or ARE) than if it is compared with a less frequently occurring verb (ATE, RAN, MET). It follows that knowledge of the lexicon can combine with information from peripheral vision fast enough to influence saccade size from moment to moment.

The Role of Vision in the Control of Posture During Linear Motion

Abstract: Publisher Summary This chapter reviews some findings concerning the role of vision in the control of posture that have been obtained in recent years. It describes some results obtained in laboratories concerning the effect of vision on postural control during linear horizontal or vertical motion of either body or visual surround. Gains and phases obtained by measuring either linear vection or body pitch induced by sinusoidal motion of visual scenes are compared. These curves seem to confirm the general statement that vision contributes to postural stabilization in the low frequency range of body movements. However, new experimental findings are described in the chapter that shows successively: (1) Direction-specific influences of visual surround motion when visual and body motion are combined, demonstrating also the enhanced effect of vision when active postural tasks are used. (2) The action of vision on the early motor responses to postural perturbation. (3) A strong decrease of early motor responses during free fall in the monkey, which is shown to occur with visual stabilization within 50–100 msec from the onset of the fall. These results are interpreted as implying a role of vision in the general setting of the parameters of motor responses during posture and movement.

Evolutionary diversification of structure and function in the family of intracellular calcium-binding proteins

Abstract: The maximum parsimony method was used to reconstruct the genealogical history of the family of intracellular calcium-binding proteins represented by six major present-day lineages, three of which--calcium dependent modulator protein, heart and skeletal muscle troponin Cs, and alkali light chains of myosin--were found to share a closer kinship with one another than with the other lineages. Similarly, parvalbumins and regulatory light chains of myosin were depicted as more closely related, whereas the branch of intestinal calcium-binding protein proved to have the most distant separation. The computer-generated amino acid sequence for the common ancestor of these six lineages described a four domain protein in which each domain of approximately 40 amino acid residues had a mid-region. 12 residue segment that bound calcium and had properties most resembling those of the calcium dependent modulator protein. It could then be deduced that parvalbumins evolved by deletion of domain I, inactivation of calcium-binding properties in domain II, and acquisition of increased affinity for Ca++ and Mg++ in domains III and IV. Regulatory light chains of myosin lost the cation binding property from three domains, retaining it in I, whereas alkali light chains of myosin lost this ability from each of the four domains. In skeletal muscle troponin C all domains retained their calcium-binding activity; however, like parvalbumins, domains III and IV acquired high affinity properties. Cardiac troponin C lost its binding activity from domain I but otherwise resembled the skeletal muscle form. Finally, intestinal calcium-binding protein evolved by deletion of domains III and IV. Positive selection could be implicated in these evolutionary changes in that the rate of fixation of mutations substantially increased in the mid portions of those domains which were loosing calcium-binding activity. Likewise, when the cation binding sites were changing from low to high affinity, an accelerated rate of fixed mutations was observed. Once this new functional parameter was selected these regions showed a remarkable conservatism, as did those binding sites which were maintaining the lower affinity. Moreover even in sequence regions not directly involved in cation binding, the lineage of troponin C because very conservative over the past 300 million years, perhaps becuase of the necessity for maintaining specific interfaces in order for the molecule to interact with troponin I and T in a functional thin myofilament. A similar phenomenon was observed in domain II of the regulatory light chains of the myosin lineage suggesting a possible binding site with the heavy chain of myosin.

Structure and sequence of the multihaem cytochrome c3.

Abstract: The molecular structure of cytochrome c3 isolated from Desulfovibrio desulfuricans has been solved on the basis of its crystallographic determination at 2.5 A resolution and of an essentially complete sequence. The molecule consists of a single polypeptide chain wrapped around a very compact core of four non-parallel haems which present a relatively high degree of exposure to the solvent. Alignment of the amino acid sequences of cytochrome c3 from several species suggests that the structure reported here is characteristic of the cytochrome c3 group.