A simple model for a two-stage chemical reaction with diffusion

We analyse the evolution of a two-stage chemical reaction betweentwo neighbouring plumes of reactants. Under the assumption thatthe plumes are approximately Gaussian we derive a system ofordinary differential equations for the total amount, the centroidand the variance of each reactant. We compare the solution ofthese equations with full numerical simulation of the reaction.Excellent agreement is obtained, with solution of the near-Gaussianmodel requiring considerably less computational effort thanthe full simulations. Of key importance is the yield of thereaction, and we discuss this feature in particular.     

Imaging cytoplasmic cAMP in mouse brainstem neurons

Background  

cAMP is an ubiquitous second messenger mediating various neuronal functions, often as a consequence of increased intracellular Ca²⁺ levels. While imaging of calcium is commonly used in neuroscience applications, probing for cAMP levels has not yet been performed in living vertebrate neuronal tissue before.     

New ab initio study of the spectroscopy of HCNH+

Highly correlated ab initio computations have been performed to investigate the rovibrational energy levels and the rotational transitions of the HCNH⁺ cation and its isotopologues. Using explicitly correlated coupled-cluster methodology and core-valence basis sets, the six-dimensional potential energy surface of the electronic ground state of this ion has been generated. The resulting analytical representation has been used in the full-dimensional variational calculations of the rovibrational energy levels. For all of the isotopologues, the fundamental modes are found to be in close agreement with the available experimental data, and reliable predictions are given for those modes that are not observed experimentally. The rotational transitions, up to the J = 10 value of the total angular momentum quantum number, have also been calculated. The rotational transitions obtained computationally are compared with the values deduced from laboratory experiments or interstellar media detections. Some of these results are predictive, especially those for DCNH⁺ and H¹³CNH⁺. Finally, we provide a theoretical simulation of the rotational spectra for these species at 5 K, 7 K, and 10 K.     

Determining a 'safe' high-mass limit in matrix-assisted laser desorption/ionisation time-of-flight mass spectra of coal derived materials with reference to instrument noise

Three methods for determining a 'safe' estimate for high-mass limits of MALDI spectra of coal derived liquids were explored, using a sample of coal-tar pitch and its pyridine-insoluble fraction. Co-addition of increasing numbers of single-shot spectra (10, 30, 50 and 100 pulses) showed visually observable reductions in noise levels, consistent with robust and statistically meaningful signals. Three separate types of post-acquisition calculation were used to identify high-mass limits of the spectra. (i) A literature method indicated high-mass limits similar to those observed visually-as a shift from baseline at the highest masses, nearly 350 000 u for the coal tar pitch and about 390 000 u for its pyridine insoluble fraction. (ii) Comparing instrument signal with pre-selected multiples of the standard deviation, upper mass estimates of between 40-60 000 u for the coal-tar pitch and about 95 000 u for its pyridine-insoluble fraction were found. (iii) Calculation of the slope was used to identify 'lift-off' of the spectrum from baseline. The angle between the smoothed spectrum and the baseline was matched to a pre-selected value (e.g. 0.5 degrees and 1 degrees ). However, the arbitrary specification of the key parameter did not establish this last method on a firm basis. The choice of a criterion for estimating high-mass limits of MALDI spectra remains a semi-quantitative procedure; a reasonably conservative high-mass limit may be estimated by comparison of signal with five times the standard deviation. However, evaluation of size exclusion chromatograms of the present samples using polystyrene standards suggests that molecular mass distributions of pitch samples arrived at by MALDI mass spectrometry are, at least partly, determined by the limitations of available instruments. Copyright 1999 John Wiley & Sons, Ltd.     

Kinetic Study on Cephamycin C Degradation

Cephamycin C (CepC) is a β-lactam antibiotic that belongs to the cephalosporin class of drugs. This compound stands out from other cephalosporins for its greater resistance to β-lactamases, which are enzymes produced by pathogenic microorganisms that present a major mechanism of bacterial resistance to β-lactam antibiotics. Cephamycin C is produced by the bacterium Streptomyces clavuligerus. Knowledge about the stability of the compound under different values of pH is important for the development of the process of production, extraction, and purification aimed at obtaining higher yields. Therefore, the stability of cephamycin C under different pH levels (2.2, 6.0, 7.0, 7.6, and 8.7) at 20 °C was evaluated in this study. Ultrafiltered broth from batch fermentations of S. clavuligerus was used in the trials. The results indicated that cephamycin C is a more stable compound than other β-lactam compounds such as penicillin and clavulanic acid. A higher degradation rate was observed at very acidic or basic pH levels, while this rate was lower at quasi-neutral pH levels. After 100 h of trial, the initial CepC showed 46 % degradation at pH 2.2, 71 % degradation at pH 8.7, and varied from 15 to 20 % at quasi-neutral pH levels.     

Local Observation of Field Polarity Dependent Flux Pinning by Magnetic Dipoles

A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below T(c) at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.     

LDPE protected by secondary amines of dehydroabietic methyl ester derivatives

The thermal degradation of low density polyethylene in the presence of several esters—methyl-12N-(phenyl)amino-14-amino-dehydroabietate (A 8), methyl-12N-(2-methoxyphenyl)amino-14-amino-dehydroabietate (A 9), methyl-12N-(4-methoxyphenyl)amino-14-amino-dehydroabietate (A 11) and methyl-12N-(phenyl)amino-14-nitro-dehydroabietate (A 12)—was investigated by chemiluminescence at three temperatures: 180, 200, and 220 °C. Kinetic parameters that depict the stability of polymers—temporal characteristics (oxidation induction times, half-period of oxidation and maximum oxidation time), oxidation rates and activation energies—were calculated from the dependencies of chemiluminescence intensity on thermal degradation time. The stabilization effectiveness of the compounds tested and other antioxidants, namely 4010 (amine compound) and Irganox 1010 and 1076 (hindered phenols), is presented for comparison. Experimental data have demonstrated that two members of the studied series, A 8 and A 11, are more efficient than good commercial hindered phenols.