Îlot de stabilité

L’îlot de stabilité est un ensemble hypothétique de nucléides transuraniens qui présenteraient une période radioactive très supérieure à celle des isotopes voisins. Ce concept est issu du modèle en couches du noyau atomique, dans lequel les nucléons sont vus comme des objets quantiques qui se répartissent dans le noyau en niveaux d'énergie de façon similaire aux électrons dans les atomes : lorsqu'un niveau d'énergie est saturé de nucléons, cela confère une stabilité particulière au noyau. Il existerait ainsi des « nombres magiques » de protons et de neutrons qui assureraient une grande stabilité aux noyaux qui en sont composés ; les noyaux ayant à la fois un « nombre magique » de protons et un « nombre magique » de neutrons sont dits « doublement magiques ». L'îlot de stabilité serait essentiellement constitué de nucléides ayant un nombre magique de neutrons, voire qui seraient doublement magiques. On remarque qu'il existe déjà un îlot de stabilité constitué par l'uranium 238, l'ura

Properties of Ir4 clusters in the gas phase and on oxide substrates

Vladan Stevanovic

Results of a theoretical study on the properties of Ir4 clusters in the gas–phase and on oxide surfaces are presented. The work is based on density functional theory (DFT) within the generalized gradient approximation (GGA) and ultrasoft pseudopotentials. Properties of a small particle such as Ir4 cluster are entirely determined by its geometry. The already known result that the most stable form of Ir4 in the gas–phase is the square structure which is significantly more stable than the butterfly and tetrahedron is confirmed. This result is in contradiction with experiments which indicate that the oxide supported Ir4 adopts a tetrahedral configuration. It is shown in this thesis that the chemical environment has a strong influence on the relative stability of Ir4 clusters. On MgO(100) surface, the square isomer remains the most stable Ir4 structure, well separated in energy from the other two. Moreover, the tetrahedron is heavily distorted by the interaction with the surface oxygen. Presence of point defects (neutral and charged O vacancies) affects the energy ordering making tetrahedron and square very close in energy, but the structural distortion of the tetrahedron is even bigger and the predicted data do not correspond to experiments. On TiO2(110) the tetrahedron and square structures become degenerate and the butterfly becomes the least stable isomer. Moreover, structural distortions are very small, in agreement with experimental data. It is shown that the TiO2 surface influences the relative stability of the three isomers through a particularly strong electrostatic field. Interactions of Ir4 with H, C and O atoms as well as with CO molecules have been studied. Adsorption of a single C atom strongly influences the relative stability of the three isomers. Upon C adsorption, the butterfly becomes the most stable gas–phase isomer while on both surfaces the tetrahedron is the most probable structure. Adsorption of a single H or O atom does not produce the same effect. The interaction with CO molecules is also important given the experimental procedure used for producing supported Ir4 clusters. It is shown that on MgO(100), CO dissociation is as probable as the competing process CO desorption justifying the presence of carbon adatoms on Ir4 clusters which brings theoretical predictions in better agreement with experimental data.

EPFL2009

Modeling and analysis of ethanol production in Escherichia coli

Yannick Mettraux

The ethanol production for biofuels is meant to be increased in the next years, but the yield could be improved. The goal of this study is to build a model of Escherichia coli KO11 to perform a rational design to improve the yield of ethanol production. The model was written in Cell Designer. The kinetic equations were picked up from the literature, with their kinetic parameters. All parameters of the equations couldn’t be directly extracted from the literature, and the initial metabolite concentrations couldn’t be found in the literature, due to thermodynamical infeasibility or incompleteness of data. Flux balance analysis was used to determine the steady state fluxes and a thermodynamical analysis determined the metabolites concentration. Together these data allows computations of missing paramaters. The metabolite time course was governed by mass-balance equations and the system could be solved numerically. The main results are the enzyme maximal rates and the thermodynamically feasible initial concentrations. The next step was to correct the model for stability. Differents ways were explored to obtain stability of the model, but they failed

2010

Properties of Ir4 clusters in the gas phase and on oxide substrates

Vladan Stevanovic

EPFL2009

Involvement of the THO complex subunit Thp2 in telomere stability

Properties of Ir4 clusters in the gas phase and on oxide substrates

Modeling and analysis of ethanol production in Escherichia coli

Modeling and analysis of ethanol production in Escherichia coli

Involvement of the THO complex subunit Thp2 in telomere stability

Properties of Ir4 clusters in the gas phase and on oxide substrates