Observation of the π0 → e+e− decay

We have observed for the first time the very rare π⁰ → e⁺e^? decay. The tagged π⁰'s come from K⁺ → π⁺π⁰ decays in flight. We find the branching ratio to be (2.23_?1.1^+2.4) × 10^?7 (90% C.L.). This value is well above the QED unitary limit and comes in rough agreement with a second-order electromagnetic process mediated by vector mesons.     

The use of QCD in hadronic cascades between heavy quark-antiquark bound states

We argue that a perturbative calculation of the spectra and rates of hadronic transitions between heavy quark-antiquark bound states makes sense within QCD provided suitable spin-parity projections are performed. Applications are made to various kinds of cascades. We obtain the correct ψ′ → J/ψππ mass spectrum. The cascades involving the emission of at least three gluons appear very strongly suppressed as compared to those allowing two-gluon emission. Application to the ? family is made.     

Unexpected formation of an azetidine-carborane derivative by dehydration of N-(1,12-dicarba-closo-dodecaboran-1-yl)formamide. The first X-ray structure of a 2,3-bis(imino)azetidine

Efforts to dehydrate (1,12-dicarba-closo-dodecaboran(12)-1-yl)formamide (a = 6.685(2) A, b = 12.877(4) A, c = 12.547(4) A, alpha = gamma = 90 degrees, beta = 90.724(11) degrees, V = 1080.8(6) A(3), Z = 4) resulted in the formation of a series of unexpected products. Addition of the Burgess reagent to the formamide, for example, led to the isolation of the corresponding methyl carbamate (a = 11.529(8) A, b = 11.529(8) A, c = 11.402(12) A, alpha = beta = gamma = 90 degrees, V = 1516(2) A(3), Z = 4), while treatment with triphosgene, a well-known dehydrating agent, resulted in the formation of a highly unusual 2,3-bis(p-carboranylimino)azetidine derivative. This particular compound, in the presence of Re(I), was hydrolyzed to give the corresponding amide, which is the first example of a 2,3-bis(imino)azetidine that has been characterized crystallographically (a = 38.496(13) A, b = 11.920(4) A, c = 27.523(10) A, beta = 127.050(5) degrees, V = 10079(6) A(3), Z = 8).     

Impact of coated calcium carbonate nanofillers and annealing treatments on the microstructure and gas barrier properties of poly(lactide) based nanocomposite films

Amorphous poly(lactide) (PLA) and nanocomposite films were prepared from melt‐blending with precipitated calcium carbonate nanofillers (PCC). Nanocomposites based on uncoated PCC (PCC‐UT), stearic acid coated PCC (PCC‐S), and poly(ε‐caprolactone) coated PCC (PCC‐P) were investigated for an inorganic content fixed to 8 wt %. Using coated nanofillers allowed preserving both PLA average molar mass and thermal stability while enhancing the nanofiller dispersion state. Poly(ε‐caprolactone) was identified as the best coating for optimized morphology and thermal properties. Maxwell law accurately described the increase in oxygen barrier properties observed for the nanocomposites based on PCC‐S. A modified Maxwell law was proposed to take account of the additional increase in barrier properties evidenced for the PLA/PCC‐P nanocomposites and assigned to the particularly strong compatibility between PCL and PLA. Different annealing conditions were investigated to respectively study the impact of physical ageing and PLA crystallization on gas permeability. Different extents of physical ageing did not significantly modify the oxygen transport properties. However, a high permeability decrease was observed for the semicrystalline nanocomposites with respect to the amorphous reference PLA film. Finally, the gain in barrier properties was shown to result from both contribution of the nanofillers and the crystalline phase. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 649–658     

Surface modification of calcium carbonate nanofillers by fluoro- and alkyl-alkoxysilane: Consequences on the morphology,thermal stability and gas barrier properties of polyvinylidene fluoride nanocomposites

Nano-sized precipitated silica coated calcium carbonate fillers (PCC-Si) were modified by an alkyl- and a fluoro-alkoxysilane derivative, respectively. PCC-Si surface modification was characterized by elemental analysis, Fourier transform infrared analysis and X-ray photoelectron spectroscopy. The modification conditions used for surface treatment led to a similar grafting density (around 3.2 μmol m^?2) for the two alkoxysilane derivatives. A significant decrease of filler hydrophilicity was observed after the alkoxysilane treatment. Nanocomposites prepared by melt mixing the modified fillers (10 wt.%) with polyvinylidene fluoride (PVDF) depicted no color change, an enhanced filler dispersion state with an homogeneous dispersion of very small filler aggregates (less than 150 nm diameter size), an increase of the thermal stability at high temperature and no change of the PVDF crystalline morphology. The oxygen permeability decrease measured on the nanocomposite prepared from the perfluorooctyltriethoxysilane modified filler was in good agreement with Maxwell law. The permeability increase evidenced for the nanocomposite based on the octyltriethoxysilane modified PCC suggested the formation of weak interfaces in this system.     

Synthesis of L-Rhamnose and N -Acetyl-D-Glucosamine Derivatives Entering in the Composition of Bacterial Polysaccharides by Use of Glucansucrases

Transglucosylation reactions using sucrose as glucosyl donor and either N-acetyl-D-glucosamine, L-rhamnose, or methyl α -L-rhamnopyranoside as acceptors were carried out with recombinant glucansucrases from families 70 and 13 of glycoside-hydrolases. Depending on the enzyme specificity, various carbohydrate structures were synthesized and characterized including α -D-glucopyranosyl-(1 → 6)-N-acetyl-D-glucosamine, α -D-glucopyranosyl-(1 → 4)-N-acetyl-D-glucosamine, α -D-glucopyranosyl-(1 → 1)-β -L-rhamnopyranoside, α -D-glucopyranosyl-(1 → 4)-α -D-glucopyranosyl-(1 → 1)-β -L-rhamnopyranoside, methyl α -D-glucopyranosyl-(1 → 4)-α -L-rhamnopyranoside, and methyl α -D-glucopyranosyl-(1 → 3)-α -L-rhamnopyranoside. Disaccharides were obtained with yields going up to 64%. The structural diversity generated as well as the obtained yields appear to be related to enzyme active site architecture, which can be modulated and improved by enzyme engineering. Several of the obtained disaccharides enter in the composition of surface polysaccharides of pathogenic bacteria, among which is Shigella flexneri. Our results outline the potential of glucansucrases in the chemo-enzymatic synthesis of complex carbohydrates of biological interest whose chemical synthesis may be seen as a limitation.     

Thermodynamics of Complexation of Tetraalkylammonium and Rare-earth Cations with Two Sulfonatocalix[n]arenes (n=4 and 6) in Aqueous Solution

We report results of a microcalorimetry study of the association of inorganic and organic cations with two p-sulfonatocalix[n]arenes (host 1: n = 4; host 2: n = 6) in aqueous solution at 298.15 K. First, we have determined the thermodynamic parameters for the complexation between the host 2 and a series of quaternary ammonium cations. We have evaluated the influence of the pH on the structure and energetics of these organic complexes. We have also reported the association constant and enthalpy of reaction for the complexation of some rare-earth cations (Sm³⁺, Dy³⁺, Y³⁺ and Sc³⁺ cations) with the two hosts. In all cases we have observed the formation of 1:1 complexes.

Whereas the association is driven by a favourable entropy change for the inorganic cations (ΔH > 0 and TΔS>>0), it is controlled by a favourable enthalpy change for the organic cations (ΔH < < 0 and TΔS < 0 or >0). In acidic solution, the complexes formed between host 2 and tetraalkylammonium cations are weaker than those formed with the cyclic tetramer. In neutral solution this effect is not observed. All the results are in line with a conformational change of host 2 with the pH.     

The enhanced cyan fluorescent protein: a sensitive pH sensor for fluorescence lifetime imaging

pH is an important parameter that affects many functions of live cells, from protein structure or function to several crucial steps of their metabolism. Genetically encoded pH sensors based on pH-sensitive fluorescent proteins have been developed and used to monitor the pH of intracellular compartments. The quantitative analysis of pH variations can be performed either by ratiometric or fluorescence lifetime detection. However, most available genetically encoded pH sensors are based on green and yellow fluorescent proteins and are not compatible with multicolor approaches. Taking advantage of the strong pH sensitivity of enhanced cyan fluorescent protein (ECFP), we demonstrate here its suitability as a sensitive pH sensor using fluorescence lifetime imaging. The intracellular ECFP lifetime undergoes large changes (32 %) in the pH 5 to pH 7 range, which allows accurate pH measurements to better than 0.2 pH units. By fusion of ECFP with the granular chromogranin A, we successfully measured the pH in secretory granules of PC12 cells, and we performed a kinetic analysis of intragranular pH variations in living cells exposed to ammonium chloride.     

Nonlinear variants of the TR/BDF2 method for thermal radiative diffusion

We apply the Trapezoidal/BDF2 (TR/BDF2) temporal discretization scheme to nonlinear grey radiative diffusion. This is a scheme that is not well-known within the radiation transport community, but we show that it offers many desirable characteristics relative to other second-order schemes. Several nonlinear variants of the TR/BDF2 scheme are defined and computationally compared with the Crank–Nicholson scheme. It is found for our test problems that the most accurate TR/BDF2 schemes are those that are fully iterated to nonlinear convergence, but the most efficient TR/BDF2 scheme is one based upon a single Newton iteration. It is also shown that neglecting the contributions to the Jacobian matrix from the cross-sections, which is often done due to a lack of smooth interpolations for tabular cross-section data, has a significant impact upon efficiency.     

Pulsed erbium fiber laser with an acetylene-filled photonic crystal fiber for saturable absorption

We investigate the dynamics of an erbium-doped fiber ring laser that is equipped with an intracavity hollow core photonic crystal fiber gas cell. The cell is filled with acetylene as a saturable absorber. We observe cw operation at low pressures, Q switching at intermediate pressure levels, and mode locking at high pressures applied. Moreover, we show that the transition from the cw to the pulsed mode may be exploited for sensitive gas detection.