On the mechanism of the P2-Na0.70CoO2→O2-LiCoO2 exchange reaction—Part I: proposition of a model to describe the P2-O2 transition

A model was proposed to describe the exchange reaction of sodium by lithium in P2 crystals. The exchange consists on the formation of nucleation centers and then on the growth of O2 domains in P2 crystals from these nucleation centers. Octahedral environments for lithium ions are obtained when one slab over two glides by (2/3,1/3,0) or by (1/3,2/3,0) in P2 structure. The existence of two different gliding vectors should lead to stacking faulted structures that can be simulated using DIFFaX software. The comparison of simulated and experimental XRD patterns for O2-LiCoO₂ (ex-Na_0.7CoO₂) has shown that in that case the growth of the O2 domains in the P2 crystals is faster than the formation of nucleation centers.     

Crystal packing mediates enantioselective ligand recognition at the peripheral site of acetylcholinesterase

Recently, alkylene-linked heterodimers of tacrine (1) and 5-amino-5,6,7,8-tetrahydroquinolinone (2, hupyridone) were shown to exhibit higher acetylcholinesterase (AChE) inhibition than either monomeric 1 or 2. Such inhibitors are potential drug candidates for ameliorating the cognitive decrements in early Alzheimer patients. In an attempt to understand the inhibition mechanism of one such dimer, (RS)-(+/-)-N-9-(1,2,3,4-tetrahydroacridinyl)-N'-5-[5,6,7,8-tetrahydro-2'(1'H)-quinolinonyl]-1,10-diaminodecane [(RS)-(+/-)-3] bisoxalate, the racemate was soaked in trigonal Torpedo californica AChE (TcAChE) crystals, and the X-ray structure of the resulting complex was solved to 2.30 A resolution. Its structure revealed the 1 unit bound to the "anionic" subsite of the active site, near the bottom of the active-site gorge, as seen for the 1/TcAChE complex. Interestingly, only the (R)-enantiomer of the 2 unit was seen in the peripheral "anionic" site (PAS) at the top of the gorge, and was hydrogen-bonded to the side chains of residues belonging to an adjacent, symmetry-related AChE molecule covering the gorge entrance. When the same racemate was soaked in orthorhombic crystals of TcAChE, in which the entrance to the gorge is more exposed, the crystal structure of the corresponding complex revealed no substantial enantiomeric selectivity. This observation suggests that the apparent enantiomeric selectivity of trigonal crystals of TcAChE for (R)-3 is mainly due to crystal packing, resulting in preferential binding of one enantiomeric inhibitor both to its "host" enzyme and to its neighbor in the asymmetric unit, rather than to steric constraints imposed by the geometry of the active-site gorge.     

Simple metal clusters

The response of alkali cluster ions to an optical excitation is investigated for two different photon energy domains. Below the ionization potential giant resonances in the photoabsorption cross-section are observed for closed shell species. Above the ionization potential, the ionization process competes with the photofragmentation process. The number of valence electrons determines both the behavior of the photoabsorption spectrum and the evolution of the ionization cross-section with the cluster size. The stability of the clusters against an excess of charge is examined through the observation of an asymmetric fission of Na _n ⁺⁺ . Experimental results are discussed in term of an electrostatic model giving an estimate of the critical size of stability and of the height of the coulombic barrier.     

Experimental and computational studies of ring inversion of 1,4-benzodiazepin-2-ones: implications for memory of chirality transformations

We recently reported the enantioselective syntheses of quaternary 1,4-benzodiazepin-2-ones via memory of chirality. The success of this method depends on formation of conformationally chiral enolates that racemize very slowly under the reaction conditions. As a prelude to undertaking experimental and computational studies on the racemization of these enolates, we have studied the ring-inversion process of the parent 1,4-benzodiazepin-2-ones. In this paper, we use dynamic and 2D-EXSY NMR to characterize inversion barriers. Using DFT calculations, we reproduce the experimental results with high accuracy (within 1-2 kcal/mol). Structural parameters obtained from DFT calculations provide valuable insights into the important effect of the N1 substituent on the ring-inversion barrier and shed light on the mechanism of the memory of chirality method. These measurements and calculations provide a foundation for future studies of benzodiazepine enolates and will be valuable in the design of new memory of chirality reactions.     

Gas chromatographic-mass spectral studies after methylation of metabolites produced by some anaerobic bacteria in spent media

The gas chromatographic analysis of metabolites such as volatile fatty acids (VFAs) and non-volatile fatty acids (NVFAs) for identification of anaerobic bacteria is now widely performed. Cultures of anaerobes tested for NVFAs as methyl esters were found to contain several unidentified compounds not previously detected and/or reported with methylation procedures. Gas chromatographic-mass spectrometric studies demonstrated that these compounds correspond to the methyl esters of both saturated and unsaturated short-chain fatty acids, and also of 2-hydroxy and 2-oxo acids. The distribution of these acids among different species of anaerobes was determined and their amounts were measured. The effects of supplementing the culture medium with either glucose or amino acids on the production of these acids are described. The use of very polar stationary phases is suggested for a better separation of all NVFAs.     

Regularity of solutions for some variational problems subject to a convexity constraint

We first study the minimizers, in the class of convex functions, of an elliptic functional with nonhomogeneous Dirichlet boundary conditions. We prove C¹ regularity of the minimizers under the assumption that the upper envelope of admissible functions is C¹. This condition is optimal at least when the functional depends only on the gradient [3]. We then give various extensions of this result. In Particular, we consider a problem without boundary conditions arising in an economic model introduced by Rochet and Choné in [4]. © 2001 John Wiley & Sons, Inc.     

Experimental study of thermal mixing layer using variable temperature hot-wire anemometry

The buoyancy effects on the development of the thermal mixing layer downstream from a horizontal separating plate were studied by comparing stable and unstable counter-gradient configurations. In this study, the novel experimental technique called parameterizable constant temperature anemometer, proposed by Ndoye et al. (Meas Sci Technol 21(7):075401, 2010), was improved to make possible the simultaneous measurement of temperature and two velocity components with an x-wire probe. The buoyancy effects on the flow are discussed through the transport equations of turbulent kinetic energy and temperature variance. In view of the low Richardson numbers at stake (Ri _f < 0.03), the buoyancy forces appeared logically to be quantitatively negligible compared to the main driving forces, but such a low-energy forcing mechanism was in fact sufficient in unstable configurations to increase the shear stress and the expansion rate of the mixing layer significantly, both phenomena being associated with enhanced production of turbulence.     

Dynamic mechanical thermal and infrared analyses of polyacrylonitrile “electrografted” onto a metal

Polyacrylonitrile (PAN) films have been “grafted” onto copper electrodes by cathodic polarization and analyzed by infrared spectroscopy and dynamic mechanical thermal analysis (DMTA). The dynamic mechanical response shows two or three transitions depending on the film thickness and the potential deposition. The viscoelastic properties of “ungrafted” PAN films, e.g., solvent cast films of commercially available PAN, are deeply different from those of “electrografted” films. The experimental data support that “ungrafted” chains are paracrystalline in contrast to the “grafted” ones which are essentially amorphous. Moreover, the irreversible transformation of the “grafted” PAN chains observed beyond 225°C is confirmed by Fourier transform infrared (FTIR) analysis and ascribed to an intramolecular cyclization of PAN into polyimine. This reaction occurs rapidly and at a comparatively low temperature with respect to “ungrafted” PAN, which suggests that the “grafted” chain configuration might be predominantly isotactic. The isotacticity and the amorphous structure appear to decrease as the thickness of the PAN film is increased. Literature data and the herein reported observations would suggest a dependence of the amorphous structure of PAN on the chain isotacticity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 543–553, 1998