Prebiotic synthesis of sequential peptides on the Hadean beach by a molecular engine working with nitrogen oxides as energy sources

Addressing the still open question of the prebiotic origin of sequential macromolecules (peptides, nucleic acids) on the primitive Earth, we describe a molecular engine (the primary pump), which works at ambient temperature and continuously generates, elongates and complexifies sequential peptides. This new scenario is based on a cyclic reaction sequence, whose keystep is the activation of amino acids into their N‐carboxyanhydrides (NCA) through nitrosation by NOx. This process could have taken place on tidal beaches; it requires a buffered ocean, emerged land and a nitrosating atmosphere. With the help of geochemical studies and computer simulations of atmosphere photochemistry, we show that the primitive Earth during the Hadean may have satisfied all these requirements. © 2001 Society of Chemical Industry.     

Comportement chaotique des filtres récursifs adaptatifs: application à la compression des signaux téléphoniques

Adaptive filtering gives simple iterative methods for extracting a useful signal by linearly filtering an observation correlated with this useful signal. When the observation contains past values of the filter output—as is the case for a recursive structure— the adaptive filter is a non linear operator which can generate complex behaviours even though adaptation is a standard derivative of the gradient algorithm. Such behaviours are observed in a particular case : theArma adaptive prediction with a sinusoidal input. A slow adaptation speed involves a quasi-periodic behaviour due to the locally unstable character of the predictor. This phenomenon called selfstabilization persists as the adaptation speed increases. Its study is more complicated : the behaviour becomes chaotic. As an illustration, the digitization of telephone signals throughArma adaptive prediction shows the necessity of controlling the qualitative behaviour of the adaptive filtering solutions.     

Effect of Molybdenum on the Structure and on the Crystallization of SiO2–Na2O–CaO–B2O3 Glasses

Crystallization of the poorly durable Na₂MoO₄ phase able to incorporate radioactive cesium must be avoided in SiO₂–Al₂O₃–B₂O₃–Na₂O–CaO glasses developed for the immobilization of Mo-rich nuclear wastes. Increasing amounts of B₂O₃ and MoO₃ were added to a SiO₂–Na₂O–CaO glass, and crystallization tendency was studied. Na₂MoO₄ crystallization tendency decreased with the increase of B₂O₃ concentration whereas the tendency of CaMoO₄ to crystallize increased due to preferential charge compensation of BO₄⁻ entities by Na⁺ ions. ²⁹Si MAS NMR showed that molybdenum acts as a reticulating agent in glass structure. Trivalent actinides surrogate (Nd³⁺) were shown to enter into CaMoO₄ crystals formed in glasses.     

Processing of CaLa2S4 infrared transparent ceramics: A comparative study of HP and FAST/SPS techniques

The densification of CaLa₂S₄ (CLS) powders prepared by combustion method was investigated by the use of Field-Assisted Sintering Technique (FAST) and Hot Pressing (HP). CLS powders were sintered using FAST at 1000°C at different pressures and heating rates and sintered by HP under 120 MPa from 800°C to 1100°C for 6 hours with a heating rate of 10°C/min. Comparison of both techniques was further realized by use of the same conditions of pressure, dwell time, and heating rate. Complementary techniques (XRD, SEM-EDS, density measurements, FTIR spectroscopy) were employed to correlate the sintering processes/parameters to the microstructural/compositional developments and optical transmission of the ceramics. Both sintering techniques produce ceramics with submicrometer grain size and relative density of about 99%. Nevertheless, HP is more suitable to densify CLS ceramics without fragmentation and also reach higher transmission than FAST. Transmission of 40%–45% was measured out of a possible maximum of 69% based on the Fresnel losses in the 8-14 μm window when HP is applied at 1000°C for 6 hours under 120 MPa. In both techniques, ceramics undergo reduction issues that originate from graphitic sintering atmosphere.     

Adjoint Monte Carlo neutron transport using cross-section probability table representation

The probability table representation of cross-sections is generally used to deal with neutron interactions in the unresolved energy range. In the frame of neutron transport methods, the capability of the probability table representation of cross-sections on the whole neutron energy range has been mentioned by Cullen (1974) and it has been already demonstrated for the Monte Carlo transport calculations by Zheng et al. (1998). Such an advantage is also illustrated here with a simple neutron propagation configuration dealt with the TRIPOLI-4 Monte Carlo transport code.     

Nouveau dispositif d’annulation d’écho utilisant un filtre interpolateur

This paper presents a new over-sampling echocanceller structure in which a simple structured interpolator is introduced and replaces half the number of echo-cancellers used in a classical structure. If dis-symmetrical equipments are authorized at the extremities of the transmission system (different carrier frequencies), the cascade of an echo-canceller and an interpolator reaches much better performances than those of a classical system. This is due to the interpolator which reduces the noise of the gradient algorithm governing the echo-cancellers evolution. Using such an interpolator decreases the computational complexity in very attractive proportions (up to 44%). Computer simulations confirm theoretical developments and let appreciate the good robustness of the depicted system.