Search

Purpose In IOERT breast treatments, a shielding disk is frequently used to protect the underlying healthy structures. The disk is usually composed of two materials, a low-Z material intended to be oriented towards the beam and a high-Z material. As tissues are repositioned around the shield before treatment, the disk is no longer visible and its correct alignment with respect to the beam is guaranteed. This paper studies the dosimetric characteristics of four possible clinical positioning scenarios of the shielding disk. A new alignment method for the shielding disk in the beam is introduced. Finally, it suggests a new design for the shielding disk. Methods As the first step, the IOERT machine “Mobetron 1000” was modeled by using Monte Carlo simulation, tuning the MC model until an excellent match with the measured PDDs and profiles was achieved. Four possible shielding disk positioning scenarios were considered, determining the dosimetric impact. Furthermore, in our center, to prevent beam misalignment, we have developed a shielding disk equipped with guiding rods. Having ascertained a correct alignment between the disk and the beam, we can propose a new internal design of the shielding disk that can improve the dose distribution with a better coverage of the treated area. Results All MC simulations were performed with a 12 MeV beam, the maximum energy of Mobetron 1000 and a 5.5 cm diameter flat tip applicator, this applicator being the most clinically used. The simulations were compared with measurements performed in a water phantom and showed good results within 2.2% of root mean square difference (RMSD). The misplacement positions of the shielding disk have dosimetric impacts in the treatment volume and a small translation could have a significant influence on healthy tissues. The D-scenario is the worst which could happens when the shielding disk is flipped upside down, giving up to 144% dose instead of 90% at the surface of the Pb/Al shielding disk. A new shielding design used, together with our alignment tool, is able to give a more homogeneous dose in the target area. Conclusions The accuracy of shielding disk position can still be problematic in IOERT dosimetry. Any method that can ascertain the good alignment between the shielding disk and the beam is beneficial for the dose distribution and is a prerequisite for an optimized shield internal design that could improve the coverage of the treated area and the protection of healthy tissues.

The activation of concrete is a real problem from the point of view of waste management. Because of the complexity of the issue, Monte Carlo (MC) codes have become an essential tool to its study. But various codes or even nuclear models exist in MC. MCNPX and PHITS have already been validated for shielding studies but GEANT4 is also a suitable solution. In these codes, different models can be considered for a concrete activation study. The Bertini model is not the best model for spallation while BIC and INCL model agrees well with previous results in literature.

The performance of the energy degrader in terms of beam properties directly impacts the design and cost of cyclotron-based proton therapy centers. The aim of this study is to evaluate the performances of different existing and novel degrader materials. The quantitative estimate is based on detailed GEANT4 simulations that analyze the beam-matter interaction and provide a determination of the beam emittance increase and transmission. Comparisons between existing (aluminum, graphite, beryllium) and novel (boron carbide and diamond) degrader materials are provided and evaluated against semi-analytical models of multiple Coulomb scattering. The results showing a potential in emittance reduction for novel materials are presented and discussed in detail., SCOPUS: cp.p, info:eu-repo/semantics/published

International audience; This paper describes a multi-scale approach for the modelling of the degradation of model cement pastesusing reactive transport. It specifically aims at incorporating chemistry-transport feedback results from apore-scale approach into a continuum description. Starting from a numerical representative elementaryvolume of the model cement paste, which was built according to extensive experimental dedicatedchacarterizations, this paper provides three separate descriptions of two different degradations: leachingand carbonation. First, 2D pore-scale simulations are performed and predict degradation depths in verygood agreement with experiments. Second, 3D pore scale descriptions of how the microstructre evolvesprovides accurate description of the evolution of transport properties through degradation. Finally, thoselatter results are incorporated as a feedback law between porosity and effective diffusion coefficient intoa 1D continuum approach of reactive transport. This paper provides pore-scale explanations of whyreactive transport modelling has encountered mitigated success when applied to cementitious materials,especially during carbonation or degradations consisting of precipitation reactions. In addition to that,different degradation modellings are in very good agreement with experimental observations.

The energy loss function (ELF) is a key parameter for the calculations of energy losses undergone by electrons in matter. It is often the only input in the calculations performed within the models based on the semi-classical dielectric response theory. Its perfect knowledge is thus of primordial importance. To evaluate the ELF, it is usual to consider as a model an expansion in Drude-Lindhard (DL) type oscillators with fixed values of the strength, width, energy and dispersion for the various oscillators. However, for materials that are characterized by a single sharp oscillator as aluminium or indium, it has been shown [Phys. Rev. B 46 (1992) 2486] that the damping parameter that corresponds in the DL model to the width of the oscillator increases for decreasing incident electron energy. To emphasize this effect, we perform in this work systematic calculation of the ELF for an indium target and for incident electron energies between 200 and 2000-eV. The ELF is determined by comparing REELS (reflection electron energy loss spectroscopy) experimental inelastic electron scattering cross sections with cross sections calculated within the semi-classical dielectric response model, which is implemented in the QUEELS-e(k,ω)-REELS software (Quantitative analysis of Electron Energy Losses at Surfaces) [Surf. Interface Anal. 36 (2004) 824]. We also perform measurements and calculations for varying incident and exit angles of the electron, namely for angles to the surface normal between 15o and 75o, to check the validity of our results for all geometries. Our results show that the damping parameter of indium in the DL model for electron energy of 200-eV is three times larger than for energy of 2000-eV. Copyright © 2014 John Wiley & Sons, Ltd.

Monte Carlo codes have become an essential tool for studying the radiation protection of particle accelerator facilities such as a proton therapy center. The MCNPX code is well adapted for shielding study but the GEANT4 toolkit is also a suitable solution. Benchmark simulations – secondary-particle production and attenuation – and shielding calculations for a proton therapy center show that both codes provide results in good agreement.

Purpose During last decades many radiotherapy machines, treatment planning systems (TPS) and complicated treatment techniques (VMAT, IMRT, SBRT) have been developed. Fast dose calculation algorithms are used for radiotherapy, applying many approximations during dose calculation to minimize calculation time. The main objective of this presentation is to present how to implement Monte Carlo in clinical daily quality control of TPS and machine performance. Methods EGSnrc ⧹ BEAMnrc were used to create a model of the Agility (Elekta) by using the data provided by the manufacturer. Validation of the MC model was done using the methodology of Pena[1]. Comparison of standard treatment plans between MC treatment planning system and measurements was necessary to verify moving parts of the machine. Converting RTplan DICOM files to MC input files and converting MC dose matrix file to a DICOM file were done using the Moderato platform [2]. Impacts of variance reduction techniques, voxel size and simulating volume were well investigated to minimize calculation time. Homemade MATLAB code was used to extract the data from machine log files which was used as input for MC simulation. Results Validation of MC model was within 2% when comparing to measurement for all PDDs, profiles and output factors of different fields sizes. Good agreement was obtained between TPS algorithm and MC for most of the cases, but small differences were found for small lung lesions, as MC and TPS do not use same cut off energy of secondary electrons. TPS and MC plans using machine log files have a good agreement for all patients. 30 min as max of calculation time and 1% uncertainty for all planning treatment volumes (PTVs) are respected for all patients. The system is currently being extended to use it with other external radiotherapy machines like Clinac® (Varian) and Gamma Knife (Elekta) for double calculation and delivered dose verification. Conclusions Monte Carlo has been used to validate TPS plans as well as performance of the external radiotherapy machine (Agility (Elekta)). Beside to measurements, MC as independent quality control tool is important for complicated treatment techniques.

Deacetoxycephalosporin C synthase from Streptomyces clavuligerus catalyses the conversion of the five-membered penicillin ring to the unsaturated six-membered cephem ring of deacetoxycephalosporin C. The effects on enzyme activity of the penicillin substrate sidechain and various cofactors were investigated using a continuous spectrophotometric assay. The conversion of penicillin G to phenylacetyl-7-aminodeacetoxycephalo sporanic acid (G-7-ADCA) was confirmed, and further details of the reaction were elucidated. The conversion of ampicillin to cephalexin was faster than that of acetyl-6-APA to acetyl-7-ADCA kcat = 0.120 +/- 0.001 s(-1) versus 0.035 +/- 0.001 s(-1), but they had similar Km values: 4.86 +/- 0.12 and 3.28 +/- 0.26 mM, respectively. Amoxycillin and penicillin V were also converted at low levels. Conversion was not detected for penicillanate, 6-aminopenicillanate, carbenicillin, temocillin, ticarcillin or benzylpenicilloic acid, suggesting that the enzyme has a relatively strict selectivity for the sidechain of the penicillin substrate.

The WENDI-II rem meter is one of the most popular neutron dosemeters used to assess a useful quantity of radiation protection, namely the ambient dose equivalent. This is due to its high sensitivity and its energy response that approximately follows the conversion function between neutron fluence and ambient dose equivalent in the range of thermal to 5 GeV. The simulation of the WENDI-II response function with the Geant4 toolkit is then perfectly suited to compare low- and high-energy hadronic models provided by this Monte Carlo code. The results showed that the thermal treatment of hydrogen in polyethylene for neutron, Journal Article, SCOPUS: ar.j, info:eu-repo/semantics/published

Treatments delivered by proton therapy are affected by uncertainties on the range of the beam within the patient, requiring medical physicists to add safety margins on the penetration depth of the beam. To reduce these margins and deliver safer treatments, different projects are currently investigating real-time range control by imaging prompt gammas emitted along the proton tracks in the patient. This study reports on the feasibility, development and test of a new concept of prompt gamma camera using a slit collimator to obtain a one-dimensional projection of the beam path on a scintillation detector. This concept was optimized, using the Monte Carlo code MCNPX version 2.5.0, to select high energy photons correlated with the beam range and detect them with both high statistics and sufficient spatial resolution. To validate the Monte Carlo model, spectrometry measurements of secondary particles emitted by a PMMA target during proton irradiation at 160 MeV were realized. An excellent agreement with the simulations was observed when using subtraction methods to isolate the gammas in direct incidence. A first prototype slit camera using the HiCam gamma detector was consequently prepared and tested successfully at 100 and 160 MeV beam energies. Results confirmed the potential of this concept for real-time range monitoring with millimetre accuracy in pencil beam scanning mode for typical clinical conditions. If we neglect electronic dead times and rejection of detected events, the current solution with its collimator at 15 cm from the beam axis can achieve a 1-2 mm standard deviation on range estimation in a homogeneous PMMA target for numbers of protons that correspond to doses in water at the Bragg peak as low as 15 cGy at 100 MeV and 25 cGy at 160 MeV assuming pencil beams with a Gaussian profile of 5 mm sigma at target entrance.

Hard X-ray photoelectron spectroscopy (HXPS) using X-rays in the 1.5–15 keV energy range generated by synchrotron sources becomes an increasingly important analysis technique due to its potential for bulk sensitive measurements. However, besides their high energy, another characteristic of photons generated by synchrotron sources is their linear polarization while X-rays from Al K α or Mg K α for instance are unpolarized. This difference implies a possible variation in total path travelled by the photoelectrons generated by the X-rays inside the medium and consequently a modification of the resulting spectrum shape. We show the influence of the polarization on the partial intensity distributions, namely the number of electrons escaping after n inelastic scattering events, for photoelectron with energies of 0.5, 1, 2, 3, 4 and 5 keV and originating from Si 1s 1/2 , Cu 1s 1/2 , Cu 2p 3/2 , Au 4d 3/2 and Au 4f 7/2 subshells. Moreover, we point out the influence of the dipole approximation leading to an underestimation of the partial intensity distributions due to the neglect of the forward–backward asymmetry of the angular photoelectron distribution.

Surface excitations significantly influence the measured peak intensities in elastic peak electron spectroscopy. They are characterised by the surface excitation parameter (SEP) defined as the change in excitation probability of an electron caused by the presence of the surface in comparison with an electron moving in an infinite medium. It is thus important to have a large database of SEP values or to have the possibility to determine it with a user-friendly software. Recently, Novak developed the programme Software for Electron Solid Inelastic Interaction Parameter Calculations (SESINIPAC) within the model of Tung, Chen, Kwei and Chou, which allows to determine inelastic mean free path, differential inelastic mean free path, SEP and differential SEP for various energy loss function models and dispersion relations with as only input the energy loss function of the material. Using SESINIPAC, we calculate SEP for 27 different types of materials (metals, semiconductors and insulators) and for various angles and energies. We compare these results with those obtained previously with the software Quantitative Analysis of Electron Energy Losses at Surfaces (QUEELS), which uses the Yubero-Tougaard model. We show that the dependence on angle of emission and energy is quite similar for the two models. However, the absolute values calculated with SESINIPAC are generally larger than those calculated with QUEELS, and the mean relative difference is 20% for metals and semiconductors but exceeds 50% for insulators. Copyright © 2012 John Wiley & Sons, Ltd.

The genusChryseobacteriumand other genera belonging to the familyFlavobacteriaceaeinclude organisms that can behave as human pathogens and are known to cause different kinds of infections. Several species ofFlavobacteriaceae, includingChryseobacterium indologenes, are naturally resistant to β-lactam antibiotics (including carbapenems), due to the production of a resident metallo-β-lactamase. AlthoughC. indologenespresently constitutes a limited clinical threat, the incidence of infections caused by this organism is increasing in some settings, where isolates that exhibit multidrug resistance phenotypes (including resistance to aminoglycosides and quinolones) have been detected. Here, we report the identification and characterization of a new IND-type variant from aC. indologenesisolate from Burkina Faso that is resistant to β-lactams and aminoglycosides. The levels of sequence identity of the new variant to other IND-type metallo-β-lactamases range between 72 and 90% (for IND-4 and IND-5, respectively). The purified enzyme exhibited N-terminal heterogeneity and a posttranslational modification consisting of the presence of a pyroglutamate residue at the N terminus. IND-6 shows a broad substrate profile, with overall higher turnover rates than IND-5 and higher activities than IND-2 and IND-5 against ceftazidime and cefepime.

this study focuses on evaluating the self-efficacy first and second degrees trainees (n = 1 337) at the end of training in IUFM Nord-Pas de Calais in June 2006. It is based on Bandura’s sociocognitive theory (2002) and takes inspiration from active and located approaches of skills (Masciotra et Jonnaert, 2006). It reflects an original survey methodology, its internal validation using the usual tests of consistency and reliability (Cronbach alpha, item-test correlations) and presents results that reveal both the differences but also convergences in the perception of their skills by trainees PE2 and PLC2. The study concludes by offering perspectives of analysis in latent variables which are particularly relevant when assessing by nature intangible skills with measurement scales associated with statements built as indicators of evaluated skills., Cette étude porte sur l’évaluation du sentiment d’autoefficacité de stagiaires des premier et second degrés (n = 1 337) en fin de formation à l’IUFM Nord-Pas de Calais en juin 2006. Elle se fonde sur la théorie sociocognitive de Bandura (2002) et s’inspire des approches actives et situées de la compétence (Masciotra et Jonnaert 2006). Elle rend compte d’une méthodologie d’enquête originale, de sa validation interne à l’aide des tests usuels de cohérence/fiabilité (alpha de Cronbach, corrélations item-test) et présente des résultats qui révèlent à la fois des différences mais aussi des convergences dans la perception de leurs compétences par les stagiaires PE2 et PLC2. L’étude s’achève en proposant des pistes d’analyse en variables latentes particulièrement pertinentes lorsqu’il s’agit d’évaluer des compétences par nature intangibles à l’aide d’échelles de mesure associées à des énoncés construits, Perrault Bruno, Dubus Alain, Brassart Dominique-Guy. Évaluation du sentiment d’efficacité professionnelle de professeurs novices par la méthode des saynètes valuées. In: Spiral-E. Revue de recherches en éducation, supplément électronique au n°43, 2009. Supplément au n° 43 : Documents officiels et travail enseignant. pp. 1-21.

The backward and forward electron emission yields γB and γF have been calculated by Monte Carlo simulations for helium (He++, He+ or He0) ions incident on thin amorphous carbon foils with energies around the electronic stopping power maximum (0.2–2 MeV). Besides the direct excitation of target electrons by the incident projectile, we have taken into account the different charge changing processes (He++ ↔ He+ ↔ He0) undergone by the helium ion in the target. We discuss in particular the connection between the electron emission yield γ and the electronic stopping power (dE/dx)e. We compare our results with previously published experimental results.

Ion induced electron emission (IIEE) is usually described as a three-step process, i.e. electron excitation by the incident projectile, electron transport (and multiplication) and electron escape through the potential barrier at the surface. In many cases, the first step of the process has been carefully described. The second step of the process, i.e. electron transport and multiplication, has often been treated in a very rough way, a simple decreasing exponential law being sometimes used. It is precisely the aim of the present work to show the importance of a correct description of electron transport and multiplication in a theoretical calculation of IIEE. A short overview of the electron transport models developed for IIEE is given in this work. The so-called “Infinite medium slowing-down model” often used in recent works is evaluated by means of Monte Carlo simulations. In particular, the importance of considering correctly the semi-infinite character of the medium and the boundary condition at the vacuum-medium interface is discussed. Quantities like the electron escape depth are also briefly discussed. This evaluation has been performed in the particular case of protons ( 25 keV E H + 3 MeV , without charge exchange) incident on polycrystalline aluminium targets.

7 páginas, 5 figuras.-- et al., Charge changing processes (electron capture and loss) of light ions moving inside metals contribute significantly to the electronemission at intermediate and low impact velocities. An improved method for the description of screening of the moving ion in the electron gas developed by Nagy et al. [Nucl. Instrum. Methods 1996;B115:58.] and Lifshitz et al. [Phys. Rev. A 1998;57:200] will be used to determine the velocity-dependent bound-state wave functions and binding energies related to the moving projectile. These wave functions and binding energies are the basic ingredients for the calculation of the electron capture and loss rates, which determine the equilibrium charge state fractions of the different projectile species, as well as for the calculation of the electron excitation during the charge transfer processes. Explicit calculations were performed for Mg. We compare the contribution of electronemission by decay of plasmons generated during the charge transfer processes with that of plasmon excitation by fast secondary electrons generated by binary ion–electron collisions. The total plasmon-mediated electron yield, obtained by summing the contributions of different mechanisms of each projectile species, weighted with the respective charge state fraction, will be compared with available experimental data.

The forward and backward electron emission yields γ F and γ B have been calculated by Monte Carlo simulation for protons (H + ) and hydrogen atoms (H 0 ) (with energies between 25 keV and 5 MeV) incident on thin amorphous carbon foils. Direct electron excitations by the incident projectiles as well as electron excitations resulting from charge exchange processes undergone by H + or H 0 have been taken into account. For the latter, Auger and Shell processes have been considered. Subsequent electron transport has been considered in order to calculate the forward and backward electron emission yields γ F and γ B .

The forward ( γ F ) and backward ( γ B ) electron emission yields have been measured for protons incident on thin carbon foils for incident energies between 2 and 7 MeV as a function of the target thickness. Comparisons with theoretical results obtained by Monte Carlo simulations are presented. In particular, the Meckbach factor R γ = γ F / γ B is discussed.

Deacetoxycephalosporin C synthase (DAOCS) from Streptomyces clavuligerus catalyses the oxidative ring expansion of the penicillin nucleus into the nucleus of cephalosporins. The reaction requires dioxygen and 2-oxoglutarate as co-substrates to create a reactive iron-oxygen intermediate from a ferrous iron in the active site. The active enzyme is monomeric in solution. The structure of DAOCS was determined earlier from merohedrally twinned crystals where the last four C-terminal residues (308-311) of one molecule penetrate the active site of a neighbouring molecule, creating a cyclic trimeric structure in the crystal. Shortening the polypeptide chain from the C terminus by more than four residues diminishes activity. Here, we describe a new crystal form of DAOCS in which trimer formation is broken and the C-terminal arm is free. These crystals show no signs of twinning, and were obtained from DAOCS labelled with an N-terminal His-tag. The modified DAOCS is catalytically active. The free C-terminal arm protrudes into the solvent, and the C-terminal domain (residues 268-299) is rotated by about 16 degrees towards the active site. The last 12 residues (300-311) are disordered. Structures for various enzyme-substrate and enzyme-product complexes in the new crystal form confirm overlapping binding sites for penicillin and 2-oxoglutarate. The results support the notion that 2-oxoglutarate and dioxygen need to react first to produce an oxidizing iron species, followed by reaction with the penicillin substrate. The position of the penicillin nucleus is topologically similar in the two crystal forms, but the penicillin side-chain in the new non-twinned crystals overlaps with the position of residues 304-306 of the C-terminal arm in the twinned crystals. An analysis of the interactions between the C-terminal region and residues in the active site indicates that DAOCS could also accept polypeptide chains as ligands, and these could bind near the iron.

It is well known that the charge state of incident ions has an influence on the stopping power as well as on the electron emission characteristics. We have studied here electron emission for protons and hydrogen atoms incident on polycrystalline Al targets for impact velocities up to 2v0 (v0 is the Bohr velocity). Besides the direct excitation of target electrons by the incident projectile, we have considered different charge changing processes that give rise to additional electron excitation, hence contributing to electron emission. These processes are also responsible for the charge state dynamics of the moving projectile. We compare our results with previous results and in particular with the predictions of the simple model proposed by Ghosh and Khare [Phys. Rev. 125 (1962) 1254].

Stopping power d E /d x and electron emission yield γ are known to be strongly influenced by electron capture and loss processes. In this work we consider proton impact on thin Al foils for impact velocities below two atomic units. We investigate the effect of the charge changing processes on the forward/backward electron emission yields, γ F and γ B as well as on the specific yields, Λ F and Λ B ( Λ = γ /(d E /d x )). The dependence of the emission yields and of the specific yields as a function of the target thickness is discussed in terms of the charge changing processes.

For incident protons with velocities around 1 a.u., electron capture and loss processes are known to play an important role. In particular, electron emission as well as electronic stopping power are strongly influenced by the charge changing processes. Several different electron capture and loss processes have been identified [Solid State Phys. 43 (1990) 229], i.e. Auger valence band processes, shell processes and resonant coherent processes. In the latter, the incident projectile undergoes a periodic excitation due to the periodic crystalline potential so that an electron can be lost or captured. In the literature, several different choices have been made for the crystalline potential. It is precisely the aim of the present work to review and compare the various potentials used in the literature and to show the influence of this choice on the resonant coherent electron capture and loss cross sections.

When a monoenergetic electron beam bombards a solid target, some electrons are backscattered without energy loss. This elastic electron backscattering effect plays an important role in many experimental techniques, like low-energy electron diffraction, scanning electron microscopy and others. Recently, the elastic peak has been used for the experimental determination of electron inelastic mean free paths (IMFPs) in the solid. This experimental determination (which is now considered as the most reliable [J. Phys. Chem. Ref. Data 28 (1999) 19] consists in comparing the measurement to a model calculation in which the IMFP is a parameter. In most cases for the model calculation, a simple non-analog Monte Carlo simulation (where the inelastic events are considered as absorptions and taken into account by a weight) is used. In many cases, long computational times are needed, especially when the solid angle of the electron detector is small. In this work, we introduce a new weighted Monte Carlo algorithm, which combines several techniques for variance reduction. Results of extensive numerical tests are presented, demonstrating the increased effectiveness of the new algorithm.

Charge changing processes are known to have a strong influence on ion-induced electron emission characteristics. However, up to now, only a few theoretical models of electron emission incorporate electron capture and loss cross sections. For incident protons with velocities around 1 a.u., a thorough theoretical model of the various charge changing processes undergone by the proton is necessary for a correct description of the electron emission induced by the projectile. Indeed, all the electrons excited in the various processes have to be taken into account. In this work, we consider conduction band Auger capture and loss processes. In particular, we consider plasmon assisted processes, which are not negligible with respect to electron–hole pair assisted Auger processes for electron capture.

Inside an IBA proton therapy centre, secondary neutrons are produced due to nuclear interactions of the proton beam with matter mainly inside the cyclotron, the beam line, the treatment nozzle and the patient. Accurate measurements of the neutron ambient dose equivalent H*(10) in such a facility require the use of a detector that has a good sensitivity for neutrons ranging from thermal energies up to 230 MeV, such as for instance the WENDI-2 detector. WENDI-2 measurements have been performed at the Westdeutsches Protonentherapiezentrum Essen, at several positions around the cyclotron room and around a gantry treatment room operated in two different beam delivery modes: Pencil Beam Scanning and Double Scattering. These measurements are compared with Monte Carlo simulation results for the neutron H*(10) obtained with MCNPX 2.5.0 and GEANT4 9.6., SCOPUS: ar.j, info:eu-repo/semantics/published

Deacetoxycephalosporin C synthase is an iron(II) 2-oxoglutaratedependent oxygenase that catalyzes the oxidative ring-expansion of penicillin N to deacetoxycephalosporin C. The wild-type enzyme is only able to efficiently utilize 2-oxoglutarate and 2-oxoadipate as a 2-oxoacid co-substrate. Mutation of arginine 258, the side chain of which forms an electrostatic interaction with the 5-carboxylate of the 2-oxoglutarate co-substrate, to a glutamine residue reduced activity to about 5% of the wild-type enzyme with 2-oxoglutarate. However, other aliphatic 2-oxoacids, which were not co-substrates for the wild-type enzyme, were utilized by the R258Q mutant. These 2-oxoacids “rescued” catalytic activity to the level observed for the wild-type enzyme as judged by penicillin N and G conversion. These co-substrates underwent oxidative decarboxylation as observed for 2-oxoglutarate in the normal reaction with the wild-type enzyme. Crystal structures of the iron(II)- 2-oxo-3-methylbutanoate (1.5 A), and iron(II)-2-oxo-4-methylpentanoate (1.6 A) enzyme complexes were obtained, which reveal the molecular basis for this “chemical co-substrate rescue” and help to rationalize the co-substrate selectivity of 2-oxoglutaratedependent oxygenases.

A theoretical study of the characteristics of kinetic electron emission induced by He + and He ++ ions (in the MeV-range) incident on polycrystalline aluminium targets is presented in this paper. The models of interactions for the incident projectiles as well as for the excited electrons are described. Especially, the electron excitation induced by incident He + is calculated by taking into account the structure of the composite projectile. Charge exchange processes by the projectiles are also considered. The subsequent electron transport and escape is calculated by means of a Monte Carlo simulation code. The calculated electron emission yield is compared to experimental results. The differences between He + and He ++ are discussed, the role of the electron lost by He + being especially emphasised.

The Poisson-Boltzmann method was used to compute the pKa values of titratable residues in a set of class C β-lactamases. In these calculations, the pKa of the phenolic group of residue Tyr150 is the only one to stand out with an abnormally low value of 8.3, more than one pKa unit lower than the measured reference value for tyrosine in solution. Other important residues of the catalytic pocket, such as the conserved Lys67, Lys315, His314, and Glu272 (hydrogen-bonded to the ammonium group of Lys315), display normal protonation states at neutral pH. pKa values were also computed in catalytically impaired β-lactamase mutants. Comparisons between the relative kcat values and the Tyr150 pKa value in these mutants revealed a striking correlation. In active enzymes, this pKa value is always lower than the solution reference value while it is close to normal in inactive enzymes. These results thus support the hypothesis that the phenolate form of Tyr150 is responsible for the activation of the nucleophilic serine. The possible roles of Lys67 and Lys315 during catalysis are also discussed. Proteins 2000;40:23–28. © 2000 Wiley-Liss, Inc.

In recent years, there has been an increasing interest in the elastic electron backscattering phenomenon partly due to its use in experimental determination of inelastic electron mean-free-paths. A critical review is presented of the current status of theoretical models to describe this phenomenon. Particular stress is put on determination of the accuracy and range of validity of various analytical models by comparison to extensive Monte Carlo simulations.

A Sequence Analysis Method. Data that can be expressed as successive states - temporal or not -- of variable length, depending on the subjects, can be described as "sequences". The analysis of such entities should allow for their automatlc classification in categories as homogeneous as possible, but also the most differentiated possible, while allowing for their interpretation via an ideal-type characterization of the categories. Secondly, such an analysis should allow for differentiated weightings of the states and their separations to avoid the null hypothesis of semantic equivalence of all states and all separations. A mathematical model of accumulated density matrices, integrated into a software program, permits us to attain these objectives slmultaneously and, moreover, results in a method of colored graphic representation of categories of sequences. The data used in this example come from research on the professional trajectories of 520 continuing-education teachers which used an earlier version of this method.

Summary By challenging the efficiency of some of our most useful antimicrobial weapons, bacterial antibiotic resistance is becoming an increasingly worrying clinical problem. A good antibiotic is expected to exhibit a high affinity for its target and to reach it rapidly, while escaping chemical modification by inactivating enzymes and elimination by efflux mechanisms. A study of the behaviour of a b-lactamase-overproducing mutant of Enterobacter cloacae in the presence of several penicillins and cephalosporins showed that the minimum inhibitory concentration (MIC) values for several compounds were practically independent of the sensitivity of the target penicillin binding protein (PBP), even for poor b-lactamase substrates. This apparent paradox was explained by analysing the equation that relates the antibiotic concentration in the periplasm to that in the external medium. Indeed, under conditions that are encountered frequently in clinical isolates, the factor characterizing the PBP sensitivity became negligible. The conclusions can be extended to all antibiotics that are sensitive to enzymatic inactivation and efflux mechanisms and must overcome permeability barriers. It would be a grave mistake to neglect these considerations in the design of future antibacterial chemotherapeutic agents.

Low energy elastic electron backscattering has been used recently as a new tool for surface analysis. The theoretical description of this phenomenon has been mostly made by Monte Carlo simulation. Recently, several analytical approaches have been developed based on the Boltzmann equation for electron transport. However, these approaches do not give a clear image of the elastic electron backscattering phenomenon. It is precisely the aim of this paper to contribute to the study of this phenomenon by Monte Carlo simulation and to propose a very simple expression for the backscattering yield as well as a simple model for the angular distribution of elastically reflected electrons.

International audience; The building up of the space charge induced by electron bombardment in an insulating target is due to the stabilization of self-trapped electrons and holes in polaronic traps. For the energies considered, the target charges positively and the secondary electrons emitted at low energies can be attracted back to the surface. This results in a self-regulation effect where the total secondary yield tends to unity and the surface potential stabilizes at a low positive value. This conclusion is checked for various experimental conditions. The electrons landing on the target form a ring of negative charges that progressively spread out on the surface of the sample.

It is well known that the electron emission yield from insulators is larger than from metals. Most of the previous works on electron emission have been on metals, and very few works have been done on insulators, either experimentally or theoretically. We present in this paper a microscopic description of proton induced electron emission from alumina. Elastic and inelastic interactions for the incident protons and excited electrons are considered as well as the charging-up of the target. The characteristics of proton induced electron emission from alumina (especially the yield γ) are calculated by a Monte Carlo simulation method. A comparison with aluminum targets is done, especially with respect to the influence of the electron inelastic mean free path and of the work function.

Charge state effects are known to be important for particle induced electron emission. Indeed, the charge state of the incident beam has a strong effect on the emission yields. Incident neutral hydrogen atoms can induce secondary electrons after an electron loss process. At high impact energies (above 200 keV) target electrons are excited by both the proton and the electron which can be treated in a first approximation as independent “primary particles”. A more precise description should include different microscopic processes: (i) target electron excitation by the undissociated neutral H0; (ii) excitation of the projectile electron by the loss process accompanied by target electron excitation; (iii) excitation of target electrons by the proton and the electron. At high impact energies it is justified to neglect the capture processes. It is the aim of this paper to calculate the electron emission characteristics of aluminum including all these processes. The various microscopic processes are incorporated in a Monte Carlo simulation code.

Structural and biochemical studies of the orf12 gene product (ORF12) from the clavulanic acid (CA) biosynthesis gene cluster are described. Sequence and crystallographic analyses reveal two domains: a C-terminal penicillin-binding protein (PBP)/beta-lactamase-type fold with highest structural similarity to the class A beta-lactamases fused to an N-terminal domain with a fold similar to steroid isomerases and polyketide cyclases. The C-terminal domain of ORF12 did not show beta-lactamase or PBP activity for the substrates tested, but did show low-level esterase activity towards 3'-O-acetyl cephalosporins and a thioester substrate. Mutagenesis studies imply that Ser173, which is present in a conserved SXXK motif, acts as a nucleophile in catalysis, consistent with studies of related esterases, beta-lactamases and d-Ala carboxypeptidases. Structures of wild-type ORF12 and of catalytic residue variants were obtained in complex with and in the absence of clavulanic acid. The role of ORF12 in clavulanic acid biosynthesis is unknown, but it may be involved in the epimerization of (3S,5S)-clavaminic acid to (3R,5R)-clavulanic acid.

Serine beta-lactamases contribute widely to the beta-lactam resistance phenomena. Unfortunately, the intimate details of their catalytic mechanism remain elusive and subject to some controversy even though many "natural" and "artificial" mutants of these different enzymes have been isolated. This paper is essentially focused on class C beta-lactamases, which contain a Tyr (Tyr150) as the first residue of the second conserved element, in contrast to their class A counterparts, in which a Ser is found in the corresponding position. We have modified this Tyr residue by site-directed mutagenesis. On the basis of the three-dimensional structure of the Enterobacter cloacae P99 enzyme, it seemed that residues Glu272 and His314 might also be important. They were similarly substituted. The modified enzymes were isolated and their catalytic properties determined. Our results indicated that His314 was not required for catalysis and that Glu272 did not play an important role in acylation but was involved to a small extent in the deacylation process. Conversely, Tyr150 was confirmed to be central for catalysis, at least with the best substrates. On the basis of a comparison of data obtained for several class C enzyme mutants and in agreement with recent structural data, we propose that the phenolate anion of Tyr150, in conjunction with the alkyl ammonium of Lys315, acts as the general base responsible for the activation of the active-site Ser64 during the acylation step and for the subsequent activation of a water molecule in the deacylation process. The evolution of the important superfamily of penicillin-recognizing enzymes is further discussed in the light of this proposed mechanism.

It has been evidenced experimentally that, for proton impact on a thick solid target, the ratio Λ of the backward electron yield γB and the electronic stopping power ( d E d x) e is approximately constant over a large energy range (a few keV to several MeV). The macroscopic theoretical models (Sternglass 1957; Schou 1980; …) assume constancy of this ratio. However, from a microscopic point of view, there is no obvious reason for that. We present in this paper a microscopic calculation (microscopic cross sections and Monte Carlo simulation method for the transport) for proton induced electron emission from aluminium and amorphous carbon. The electron yields and the factors Λ from thick and thin (forward and backward emissions) targets are calculated and compared to experimental results.

With peptide substrates, the penicillin-sensitive dd-peptidases exhibit a strict specificity for d-Ala-d-Xaa C-termini. Only glycine is tolerated as the C-terminal residue, but with a significantly decreased activity. These enzymes also hydrolyse various ester and thiolester analogues of their natural substrates. Some of the thiolesters whose C-terminal leaving group exhibited an l stereochemistry were significantly hydrolysed by some of the studied enzymes, particularly by the Actinomadura R39 dd-peptidase. By contrast, the strict specificity for a d residue in the penultimate position was fully retained. The same esters and thiolesters also behaved as substrates for β-lactamases. In this case, thiolesters exhibiting l stereochemistry in the C-terminal position could also be hydrolysed, mainly by the class C and class D enzymes. But, more surprisingly, the class C Enterobacter cloacae P99 β-lactamase also hydrolysed thiolesters containing an l residue in the penultimate position, sometimes more efficiently than the d isomer.

Dans cette etude, nous proposons un ensemble de methodes pour l'analyse morphologique de milieux pulverulents. Ces methodes sont evaluees a partir d'applications a des poudres d'UO 2 . Elles sont basees sur le modele des feuilles mortes, qui simule un processus de masquage, et ne necessitent pas de segmentation d'images. Nous avons constate qu'il est preferable d'eliminer l'approche de type segmentation pour les structures complexes, et d'operer directement sur les images a niveaux de gris. Ces nouveaux algorithmes sont compares aux methodes traditionnelles pour mesurer une granulometrie par ouvertures.

The side chains of residues Thr299 and Thr301 in the Streptomyces R61 DD-peptidase have been modified by site-directed mutagenesis. These amino acids are part of a beta-strand which forms a wall of the active-site cavity. Thr299 corresponds to the second residue of the Lys-Thr(Ser)-Gly triad, highly conserved in active-site beta-lactamases and penicillin-binding proteins (PBPs). Modification of Thr301 resulted only in minor alterations of the catalytic and penicillin-binding properties of the enzyme. No selective decrease of the rate of acylation was observed for any particular class of compounds. By contrast, the loss of the hydroxy group of the residue in position 299 yielded a seriously impaired enzyme. The rates of inactivation by penicillins were decreased 30-50-fold, whereas the reactions with cephalosporins were even more affected. The efficiency of hydrolysis against the peptide substrate was also seriously decreased. More surprisingly, the mutant was completely unable to catalyse transpeptidation reactions. The conservation of an hydroxylated residue in this position in PBPs is thus easily explained by these results.

The role of the highly conserved Lys315 residue in the catalytic mechanism of a class C beta-lactamase has been probed by site-directed mutagenesis. Lys315 has been replaced by a histidine in the Enterobacter cloacae 908R beta-lactamase, thus introducing a tritratable group to probe the role of the positive charge, and by a glutamine. The effects of these mutations have been studied on the kinetics of penicillin G and cephalothin turnover and on the pre-steady-state kinetics with carbenicillin at different pH. Results showed that substrate binding was not impaired by the mutations, so that an interaction with the substrate-free carboxylate in the Henri-Michaelis complex could be ruled out. Lys315 must have a catalytic role as shown by the decreased acylation and deacylation rates observed with the mutant enzymes. The mutants exhibited a lower activity at acidic pH, and this observation could be correlated with a decreased affinity for (3-aminophenyl)boronate, a compound devoid of free carboxylate which binds to the active site and forms an adduct mimicking the tetrahedral intermediate. This suggested that Lys315 was somehow involved in accelerating the nucleophilic substitutions along the reaction pathway. The study was extended to modified substrates where the free carboxylate had been esterified. Neither acylation nor deacylation seemed severely impaired with these compounds, showing that the interaction between the enzyme and the substrate-free carboxylate did not play a major role in catalysis.

A «double competence» second cycle cursus in computer science and education brings together learnings bound to both topics. It aims to turn out professionals in educative sofware and multimedia, along with holders of resource centers. The idea of this cursus comes from the double constatation of software industry’s unability to ensure a pertinent and good quality production in that field, and necessity to build the professional competence of people involved in production, experimentation and validation of educative use products on the first basis of an educative identity. This cursus anticipates upon social demand which appears only by indirect ways. Promoving and integrating it into more classical studies systemes raises many difficulties and questions., Un second cycle de «double compétence» en informatique et Sciences de l’Éducation comporte en parallèle des enseignements relevant de ces deux disciplines. Il vise à former des professionnels de la production en logiciel et multimédia éducatifs, ainsi que des animateurs de centres de ressources. A l’origine de cette formation, on trouve le constat de l’incapacité de l’industrie du logiciel à assurer une production pertinente et de qualité dans le domaine, ainsi que la nécessité de construire la qualification professionnelle des personnes impliquées dans la production, l’expérimentation et la validation des produits à usage éducatif d’abord sur la base de l’identité d’agent éducatif. Cette filière anticipe sur une demande sociale qui ne se manifeste que de manière indirecte. Sa promotion et son intégration à des ensembles de formation plus classiques soulèvent des difficultés et des interrogations., Dubus Alain. Un essai de formation professionnalisante pour un métier qui n’existe pas encore : le second cycle de double compétence en informatique et Sciences de l’Éducation. In: Spirale. Revue de recherches en éducation, n°13, 1994. L’université et les savoirs professionnels, sous la direction de Raymond Bourdoncle. pp. 183-198.