Structure électronique de quelques dérivés nitrés insaturés (méthode L.C.A.O. améliorée)

The electronic structure of Nitro-,p- andm-dinitro-,s-trinitro-benzene, nitro-ethylene, mono-, and dinitro-butadien have been calculated by an improved LCAO method.     

Structure, function, and inhibition along the reaction coordinate of CTX-M beta-lactamases

CTX-M enzymes are an emerging group of extended spectrum beta-lactamases (ESBLs) that hydrolyze not only the penicillins but also the first-, second-, and third-generation cephalosporins. Although they have become the most frequently observed ESBLs in certain areas, there are few effective inhibitors and relatively little is known about their detailed mechanism. Here we describe the X-ray crystal structures of CTX-M enzymes in complex with different transition-state analogues and beta-lactam inhibitors, representing the enzyme as it progresses from its acylation transition state to its acyl enzyme complex to the deacylation transition state. As the enzyme moves along this reaction coordinate, two key catalytic residues, Lys73 and Glu166, change conformations, tracking the state of the reaction. Unexpectedly, the acyl enzyme complex with the beta-lactam inhibitor cefoxitin still has the catalytic water bound; this water had been predicted to be displaced by the unusual 7alpha-methoxy of the inhibitor. Instead, the 7alpha-group appears to inhibit by preventing the formation of the deacylation transition state through steric hindrance. From an inhibitor design standpoint, we note that the best of the reversible inhibitors, a ceftazidime-like boronic acid compound, binds to CTX-M-16 with a K(i) value of 4 nM. When used together in cell culture, this inhibitor reversed cefotaxime resistance in CTX-M-producing bacteria. The structure of its complex with CTX-M enzyme and the structural view of the reaction coordinate described here provide templates for inhibitor design and intervention to combat this family of antibiotic resistance enzymes.     

The first rare earth organometallic complex of 1,4,7-trithiacyclononane: a precursor to unique cationic ethylene and alpha-olefin polymerisation catalysts supported by an all-sulfur donor ligand

[Sc([9]aneS3)(CH2SiMe3)3], the first rare earth organometallic complex of 1,4,7-trithiacyclonane, is a precursor to ethylene and alpha-olefin polymerisation catalysts upon activation with BAr(F)3 or [CPh3][BAr(F)4](Ar(F) = C6F5); these are the first cationic rare earth organometallic catalysts supported by an all-sulfur donor ligand.     

Sur la contribution des differents porteurs à la zone de charge d'espace d'un semiconducteur multivallée

We study, in the case of the parabolic approximation, the space charge of a two-valley conduction band semiconductor; the electrons associated to these valleys are supposed to have different effective masses. By using a non-degenerate distribution we analyse the contribution of the different carriers to the space charge and the effect of surface band bending, temperature, effective masses, dope and type of material. We point out that the space charge can be governed by either type of carrier or both. By comparing the results with those given by a single valley model, we show the incidence of the initial hypothesis on the value and evolution of the space charge with temperature and surface band bending. As an exemple, we study briefly n-type GaSb.     

A propos de l'interprétation des variations avec la température du travail de sortie des surfaces de GaSb

We expose the problems raised by the interpretation of the measurement of semiconductor work function variations obtained at different temperatures by the Kelvin condensator method. The variations of the electronic affinity versus temperature are calculated from band theory results.