Three-dimensional representation of metastable peaks at high mass resolution

Isobaric daughter ions formed from a common metastable ion have been analysed at high mass resolution. A three-dimensional representation is used to summarize the data for competitive CO and C₂H₄ losses and competitive CHO˙ and C₂H₅˙ losses from metastable molecular ions of cyclohexanone.     

Integral decomposition in polyhedra

We say that a polyhedronP satisfies weak integral decomposition if whenever an integral vector is the sum ofk vectors inP it is also the sum ofk integral vectors inP. This property is related to rounding results for packing and covering problems. We study the property and two related properties, and give results concerning integral polymatroids, totally unimodular matrices and network flows, pairs of strongly-base-orderable matroids, and branchings in directed graphs.     

The effects of substrate orientation on the mechanism of a phosphotriesterase

While the underlying chemistry of enzyme-catalyzed reactions may be almost identical, the actual turnover rates of different substrates can vary significantly. This is seen in the turnover rates for the catalyzed hydrolysis of organophosphates by the bacterial phosphotriesterase OpdA. We investigate the variation in turnover rates by examining the hydrolysis of three classes of substrates: phosphotriesters, phosphothionates, and phosphorothiolates. Theoretical calculations were used to analyze the reactivity of these substrates and the energy barriers to their hydrolysis. This information was then compared to information derived from enzyme kinetics and crystallographic studies, providing new insights into the mechanism of this enzyme. We demonstrate that the enzyme catalyzes the hydrolysis of organophosphates through steric constraint of the reactants, and that the equilibrium between productively and unproductively bound substrates makes a significant contribution to the turnover rate of highly reactive substrates. These results highlight the importance of correct orientation of reactants within the active sites of enzymes to enable efficient catalysis.     

CVD growth of N-doped carbon nanotubes on silicon substrates and its mechanism

In the present study, we report the chemical vapor deposition (CVD) of nitrogen-doped (N-doped) aligned carbon nanotubes on a silicon (Si) substrate using ferrocene (Fe(C5H5)2) as catalyst and acetonitrile (CH3CN) as the carbon source. The effect of experimental conditions such as temperature, gaseous environment, and substrates on the structure and morphology of N-doped carbon nanotubes arrays is reported. From XPS and EELS data, it was found that the nitrogen content of the nanotubes could be determined over a wide range, from 1.9% to 12%, by adding the addition of hydrogen (H2) to the reaction system. It was also shown by SEM that N-doped carbon nanotube arrays could be produced on Si and SiO2 substrates at suitable temperatures, although at different growth rates. Using these concentrations, it was possible to produce three-dimensional (3D) carbon nanotubes architectures on predetermined Si/SiO2 patterns. The mechanism underlying the effect of nitrogen containing carbon sources on nanotube formation was explored using X-ray photoelectron spectroscopy (XPS).     

Spatiotemporal dynamics of wormlike micelles under shear

Velocity profiles in a wormlike micelle solution (cetyl trimethyl ammonium bromide in D2O) are recorded using ultrasound every 2 s during a startup experiment into the shear-banding regime. The stress relaxation occurs over more than 6 h and corresponds to the very slow nucleation and growth of the high-shear band. Moreover, oscillations of the interface position with a period of about 50 s are observed during the growth process. Strong wall slip, metastable states, and transient nucleation of three-band flows are also reported and discussed in light of previous experiments and theoretical models.