Corrosion à haute température et à haute pression de composites thermostructuraux à matrice séquencée SICF/[SI,C,B]M et CF/[SI,C,B]M pour applications aéronautique

Les composites thermostructuraux à matrice séquencée SiCf/[Si,C,B]m et Cf/[Si,C,B] m sont destinés à remplacer les super-alliages dans l’architecture des parties chaudes des moteurs aéronautiques. Leur résistance à l’oxydation/corrosion a été étudiée à partir (i) de vieillissements dans des environnements à haute température et à haute pression, (ii) d’essais mécaniques post-vieillissement à température ambiante, (iii) d’analyses morphologiques post-mortem par le biais de Microscopies Optique et Electronique à Balayage et (iv) de caractérisations physico-chimiques en Diffraction des Rayons X, Microspectrométrie Raman, Spectroscopie Infra-Rouge à Transformée de Fourier, Spectroscopie de Photons X, Spectroscopie d’Electrons Auger et Microanalyse par Spectrométrie de Rayons X. Les essais de corrosion sont menés à 600 et à 1200°C, pendant des durées pouvant atteindre un millier d’heures dans des mélanges Air/H2O(g) (90/10, 80/20 et 50/50) s’écoulant entre 5 et 10 cm.s-1, de la pression atmosphérique jusqu’à 1 MPa. Les influences des différentes conditions expérimentales et des paramètres propres aux matériaux sont mises en évidence et discutées. En outre, des modes d’initiation et de propagation de la corrosion sont proposés pour chacun des deux matériaux et pour chacune des deux températures étudiées. Enfin, une approche théorique, basée sur les équations de diffusion de Stephan-Maxwell et des calculs thermodynamiques, est développée et permet de prévoir l’ordre de grandeur des vitesses de dégradation du SiC et du B4C dans des environnements à haute température et à haute pression. Le modèle analytique prend en compte les phénomènes d’inter-diffusion des espèces gazeuses dans la couche limite située à la surface du carbure oxydé et a été validé à partir de données expérimentales extraites de cette étude et de la littérature.Multi-layered ceramic matrix composites SiCf/[Si,C,B]m and Cf/[Si,C,B] m are potential candidates to replace the current nickel-based alloys in hot sections of aeronautic turbine engines. Their oxidation/corrosion behavior was investigated from (i) exposures in high temperature - high pressure environments, (ii) post-exposure mechanical tests at room temperature, (iii) post-exposure observations using Optical and Scanning Electron Microscopies, (iv) characterization techniques such as X-Ray Diffraction, Raman Microspectrometry, Infra-Red Spectroscopy, XRay Photoelectron Spectroscopy, Auger Electron Spectroscopy and Electron Beam Micro-Analysis. Corrosion tests are performed at 600 and 1200°C, for exposure durations up to 1000 hours in Air/Steam (90/10, 80/20 and 50/50) gas mixtures flowing from 5 to 10 cm.s-1, with the total pressure ranging from atmospheric pressure to 1 MPa. The effects of the environmental conditions and material parameters are highlighted and discussed. In addition, initiation and propagation modes of the corrosion are proposed for both of the materials and each studied temperatures. Finally, a theoretical approach, based on the Stephan-Maxwell diffusion equations and a thermodynamic study, is developed and allows to estimate the degradation rates of SiC and B4C in high temperature - high pressure environments. The analytical model takes into account the interdiffusion phenomena of the gaseous species in the boundary layer located at the surface of the oxidized carbide and is validated using experimental results from this study and literature data

Mycothiol biosynthesis is essential for ethionamide susceptibility in Mycobacterium tuberculosis

Spontaneous mutants of Mycobacterium tuberculosis that were resistant to the anti-tuberculosis drugs ethionamide and isoniazid were isolated and found to map to mshA, a gene encoding the first enzyme involved in the biosynthesis of mycothiol, a major low-molecular-weight thiol in M. tuberculosis. Seven independent missense or frameshift mutations within mshA were identified and characterized. Precise null deletion mutations of the mshA gene were generated by specialized transduction in three different strains of M. tuberculosis. The mshA deletion mutants were defective in mycothiol biosynthesis, were only ethionamide-resistant and required catalase to grow. Biochemical studies suggested that the mechanism of ethionamide resistance in mshA mutants was likely due to a defect in ethionamide activation. In vivo, a mycothiol-deficient strain grew normally in immunodeficient mice, but was slightly defective for growth in immunocompetent mice. Mutations in mshA demonstrate the non-essentiality of mycothiol for growth in vitro and in vivo, and provide a novel mechanism of ethionamide resistance in M. tuberculosis

Examining the role of protein structural dynamics in drug resistance in Mycobacterium tuberculosis

Antimicrobial resistance represents a growing global health problem. The emergence of novel resistance mechanisms necessitates the development of alternative approaches to investigate the molecular fundamentals of resistance, leading ultimately to new strategies for counteracting them. To gain deeper insight into antibiotic-target interactions, the binding of the frontline anti-tuberculosis drug isoniazid (INH) to a target enzyme, InhA, from Mycobacterium tuberculosis was studied using ultrafast two-dimensional infrared (2D-IR) spectroscopy and molecular simulations. Comparing wild-type InhA with a series of single point mutations, it was found that binding of the INH-NAD inhibitor to susceptible forms of the enzyme caused increased vibrational coupling between residues located in the Rossmann fold co-factor binding site of InhA, reducing dynamic fluctuations. The effect correlated with biochemical assay data, being markedly reduced in the INH-resistant S94A mutant and absent in the biochemically-inactive P193A control. Molecular dynamics simulations and calculations of inter-residue couplings indicate that the changes in coupling and dynamics are not localised to the co-factor binding site, but permeate much of the protein. We thus propose that the resistant S94A mutation circumvents subtle changes in global structural dynamics caused by INH upon binding to the wild-type enzyme that may impact upon the formation of important protein-protein complexes in the fatty acid synthase pathway of M. tuberculosis

Ultralong C100 Mycolic Acids Support the Assignment of Segniliparus as a New Bacterial Genus

Mycolic acid-producing bacteria isolated from the respiratory tract of human and non-human mammals were recently assigned as a distinct genus, Segniliparus, because they diverge from rhodococci and mycobacteria in genetic and chemical features. Using high accuracy mass spectrometry, we determined the chemical composition of 65 homologous mycolic acids in two Segniliparus species and separately analyzed the three subclasses to measure relative chain length, number and stereochemistry of unsaturations and cyclopropyl groups within each class. Whereas mycobacterial mycolate subclasses are distinguished from one another by R groups on the meromycolate chain, Segniliparus species synthesize solely non-oxygenated α-mycolates with high levels of cis unsaturation. Unexpectedly Segniliparus α-mycolates diverge into three subclasses based on large differences in carbon chain length with one bacterial culture producing mycolates that range from C58 to C100. Both the overall chain length (C100) and the chain length diversity (C42) are larger than previously seen for mycolic acid-producing organisms and provide direct chemical evidence for assignment of Segniliparus as a distinct genus. Yet, electron microscopy shows that the long and diverse mycolates pack into a typical appearing membrane. Therefore, these new and unexpected extremes of mycolic acid chemical structure raise questions about the modes of mycolic acid packing and folding into a membrane

The Mycobacterium Tuberculosis FAS-II Dehydratases and Methyltransferases Define the Specificity of the Mycolic Acid Elongation Complexes

BACKGROUND: The human pathogen Mycobacterium tuberculosis (Mtb) has the originality of possessing a multifunctional mega-enzyme FAS-I (Fatty Acid Synthase-I), together with a multi-protein FAS-II system, to carry out the biosynthesis of common and of specific long chain fatty acids: the mycolic acids (MA). MA are the main constituents of the external mycomembrane that represents a tight permeability barrier involved in the pathogenicity of Mtb. The MA biosynthesis pathway is essential and contains targets for efficient antibiotics. We have demonstrated previously that proteins of FAS-II interact specifically to form specialized and interconnected complexes. This finding suggested that the organization of FAS-II resemble to the architecture of multifunctional mega-enzyme like the mammalian mFAS-I, which is devoted to the fatty acid biosynthesis. PRINCIPAL FINDINGS: Based on conventional and reliable studies using yeast-two hybrid, yeast-three-hybrid and in vitro Co-immunoprecipitation, we completed here the analysis of the composition and architecture of the interactome between the known components of the Mtb FAS-II complexes. We showed that the recently identified dehydratases HadAB and HadBC are part of the FAS-II elongation complexes and may represent a specific link between the core of FAS-II and the condensing enzymes of the system. By testing four additional methyltransferases involved in the biosynthesis of mycolic acids, we demonstrated that they display specific interactions with each type of complexes suggesting their coordinated action during MA elongation. SIGNIFICANCE: These results provide a global update of the architecture and organization of a FAS-II system. The FAS-II system of Mtb is organized in specialized interconnected complexes and the specificity of each elongation complex is given by preferential interactions between condensing enzymes and dehydratase heterodimers. This study will probably allow defining essential and specific interactions that correspond to promising targets for Mtb FAS-II inhibitors

Disruption of key NADH-binding pocket residues of the Mycobacterium tuberculosis InhA affects DD-CoA binding ability

Tuberculosis (TB) is a global health problem that affects over 10 million people. There is an urgent need to develop novel antimicrobial therapies to combat TB. To achieve this, a thorough understanding of key validated drug targets is required. The enoyl reductase InhA, responsible for synthesis of essential mycolic acids in the mycobacterial cell wall, is the target for the frontline anti-TB drug isoniazid. To better understand the activity of this protein a series of mutants, targeted to the NADH co-factor binding pocket were created. Residues P193 and W222 comprise a series of hydrophobic residues surrounding the cofactor binding site and mutation of both residues negatively affect InhA function. Construction of an M155A mutant of InhA results in increased affinity for NADH and DD-CoA turnover but with a reduction in Vmax for DD-CoA, impairing overall activity. This suggests that NADH-binding geometry of InhA likely permits long-range interactions between residues in the NADH-binding pocket to facilitate substrate turnover in the DD-CoA binding region of the protein. Understanding the precise details of substrate binding and turnover in InhA and how this may affect protein-protein interactions may facilitate the development of improved inhibitors enabling the development of novel anti-TB drugs

Employee involvement : 'how do coal mines in Queensland utilise employee involvement processes?'

Many Australian businesses compete in the global marketplace, and companies seeking a competitive edge in this business environment consider the engagement of their people in the business to be a strategic advantage. This 'engagement of people' strategy utilises participatory or collaborative management practices that can be collectively considered under the umbrella term 'employee involvement' (EI) and considered desirable from both a management and employee perspective. Yet EI appears as an organisational paradox, that is, while management want EI and employees want EI it should be effective and work well. However, often EI does not deliver in full for both management and workers. The Queensland coal mining industry is one such industry that competes in the global marketplace and many companies within that industry seek to improve their competitive positions by directly involving their employees. This investigation looks at how coal mines in Queensland utilise Employee Involvement processes. In doing so the investigation seeks to understand EI as a concept, as well as a practice, and to determine influential factors for effective EI at BHP Billiton Mitsubishi Alliance (BMA) mines - the major coal mining company in the Queensland coal mining industry. This investigation was undertaken using a case study methodology based on in-depth, semi-structured interviews. People were interviewed from various organisational levels at four BMA mines and BMA's corporate office. The investigation findings establish that EI, as a concept, is best understood by its application. Also the key common attributes of EI that were evident are involvement of actual crews, information sharing, the opportunity to influence decisions and that EI in safety management is considered mandatory. BMA does utilises formal EI practices. However, embedded in these formal EI practices are informal EI practices that involve more people and have greater organisational breadth in their acceptance and impact. While EI was recognised as a management initiative, it was management's commitment to establishing and maintaining the supportive environment which fostered an EI program that was more critical for implementing an EI culture than the mechanistic formal EI programs utilised by BMA. In establishing the importance of informal EI practices over more formal EI practices, the role of the supervisor is considered vital in creating a supportive environment that both fosters the employees sense of management commitment and their sense of personal value

Analyse et optimisation d'une classe de systèmes dynamiques hybrides à commutations autonomes

This work is devoted to the study of a particular class of hybrid dynamical systems (h.d.s.) with autonomous switchings, these being generated by a hysteresis phenomenon. More particularly, after having introduced the studied mathematical model, an analysis of this h.d.s. class is realized with notably a study of limit cycles (equations to determine them, stability) and of bifurcations (saddle node, period doubling). Existence of some properties which characterize chaotic systems like the sensitivity to initial conditions is also highlighted with both graphic and computationl (Lyapunov exponent) ways. Finally, a parametric optimization part is proposed in order to improve some system performances. All those theoretical results are applied to a thermal system (thermostat with an anticipative resistance) and to an electronic one (DC/DC converter) with formal calculus (Maple), interval analysis (Proj2D) and numerical simulations (Matlab).Les travaux de recherche de ce mémoire porte sur l'étude d'une classe particulière de systèmes dynamiques hybrides (s.d.h.) à commutations autonomes, ces dernières étant engendrées par un phénomène d'hystérésis. Plus particulièrement, après avoir introduit le système mathématique étudié, une analyse de cette classe de s.d.h. est réalisée avec notamment une étude des cycles limites (équations qui les déterminent, stabilité) et des phénomènes de bifurcations (noeud-selle, doublement de période). L'existence de propriétés caractéristiques des systèmes chaotiques comme la sensibilité aux conditions initiales est également mise en avant de façons graphique et calculatoire (exposant de Lyapunov). Enfin, une partie optimisation paramétrique a aussi été traitée dans le but d'améliorer certaines performances du système. Tous ces résultats théoriques sont appliqués à un système thermique (thermostat à résistance d'anticipation) et à un système électronique (convertisseur statique) à l'aide du calcul forme (Maple), de l'analyse par intervalles (Proj2D) et de simulations numériques (Matlab