Theoretical study of thermal decomposition of azomethane

Summary.  Thermal one- and two-bond dissociation processes of cis- and trans-azomethane were studied by ab initio computation with DZP and TZ2P basis sets, using the d(N–C) bond lengths as the reaction coordinates. The geometries were optimized at the MP2 level, and the dissociation energies obtained exploiting a single-point, fourth-order M?ller–Plesset calculations [MP4SDTQ/TZ2P]. At this level of theory including zero-point energies, the trans-isomer is by 9.3 kcal/mol more stable than the cis-isomer. The results show that the energetically more favourable one-bond cleavage proceeds without transition state with the predicted bond dissociation energy D ₀ of 47.8 kcal/mol for trans-azomethane and 38.5 kcal/mol for cis-azomethane. With calculated barrier heights the unimolecular dissociation rate constants have been determined by means of the RRKM theory. The second-order saddle points localized for synchronous decomposition pathways lie 13 (trans)-23(cis) kcal/mol above the one-bond dissociation energies [MP2/DZP]. Received May 28, 1996/Final version received November 1, 1996 / Accepted November 1, 1996     

Application of L-Cysteine-Capped ZnS Nanoparticles in the Determination of Nucleic Acids Using the Resonance Light Scattering Method

Nanometer-sized L-cysteine-capped ZnS particles were synthesized by a colloidal aqueous method. The functionalized nanoparticles are water-soluble and suitable for biological applications. In Tris-HCl buffer solution, nucleic acids combine with cysteine-capped nano-ZnS particles by intermolecular forces to form larger nanoparticles. There are two resonance light scattering peaks at 304.5nm and 373.8nm, respectively. The enhanced RLS is related to the concentration of nucleic acids in the range of 0.04 to 1.2µgmL^–1 for calf thymus DNA and 0.2 to 1.0µgmL^–1 for fish sperm DNA. The detection limits (3) are 19ngmL^–1 for calf thymus DNA and 23ngmL^–1 for fish sperm DNA, respectively. Four synthetic samples were analyzed satisfactorily.     

Improved Fluorometric DNA Determination Based on the Interaction of the DNA/Polycation Complex with Hoechst 33258

When DNA is mixed with the cationic polyelectrolyte poly(diallyldimethyl ammonium chloride) (PDDA), the DNA/PDDA complex is formed instantaneously at room temperature. This complex is much more efficient in enhancing the fluorescence of Hoechst 33258 (H 33258) than DNA alone. Based on the interaction of H 33258 with the DNA/PDDA complex, a new fluorescence assay for DNA is described. At pH 7.3 in Tris-HCl buffered solution, the DNA/PDDA complex causes a sharp enhancement in fluorescence intensity of H 33258. Simultanously, the emission maximum wavelength of H 33258 blueshifts from 490nm to 450nm, while the excitation redshifts from 345 to 350nm. The calibration graphs for calf thymus DNA (ctDNA) and herring sperm DNA (hsDNA) are both linear up to 5.0µgmL^–1 when the concentration of H 33258 and PDDA are fixed at 1.5×10^–6 and 1.6×10^–5molL^–1, respectively. The method is specific for native DNA. The 3 detection limits for ctDNA and hsDNA are 1.8 and 5.6ngmL^–1, respectively, i.e. much lower than in the presence of H 33258 alone. Four synthetic samples were determined satisfactorily. This method can also be developed to investigate the formation and the nature of the complexes between DNA and polycations, which have recently been widely applied in some fields such as genetic engineering and gene therapy.     

Agro-industrial waste enzymes: Perspectives in circular economy

According to the Food and Agriculture Organization of the United Nations, approximately 1.3 billion tons of food is wasted each year, equivalent to approximately one-third of world production. Agri-food wastes are the source of proteins, carbohydrates, lipids, and other essential minerals that have been exploited for value-added products by the development of biorefineries and sustainable business as important elements of circular economies. The innovation and materialization of these types of processes, including the use of disruptive technologies on microbial bioconversion and enzyme technology, such as nanotechnology, metabolic engineering, and multi-omics platforms, increase the perspectives on the waste valorization process. Lignocellulolytic enzymes, pectinases, and proteases are mainly used as catalyzers on agri-food waste treatment, and their production in house might be the trend in near future for agro-industrial countries. Another way to transform the agri-food wastes is via aerobic or anaerobic microbial process from fungal or bacterial cultures; these processes are the key to produce waste enzymes.     

Radiation synthesis of n-vinyl 2-pyrrolidone/acrylonitrile interpenetrating polymer networks and their use in uranium recovery from aqueous systems

Interpenetrating Polymer Networks (IPNs) based on Poly n-vinyl 2-pyrrolidone (PVP) and Acrylonitrile (AN) were prepared by irradiating PVP solutions prepared in AN. PVP/AN mixtures were irradiated by ⁶⁰Co-γ rays at room temperature at a dose rate of 0.5 kGy/hour. IPNs were characterized by using FT-IR and Thermal Analysis techniques. The chelating adsorbents containing amidoxime groups were prepared by the reaction of these IPNs with hydroxylamine in aqueous NaOH solution at 50°C. These amidoxime containing adsorbents were used in adsorption studies for the recovery of uranium from aqueous systems. The adsorption capacity of an IPN with equivolume fraction of PVP and amidoximated PAN was found to be 750mg UO₂²⁺/g dry amidoximated IPN.     

Tautomerism in monothio-β-diketones studied by PMR and quantum chemical calculations

It is shown by proton NMR and quantum chemical PCILO calculations that monothio-β-ketones

in solution exist as different isomers depending on substituents, solvent and temperature. In solvents with low dielectric constants the hydrogen-bridged cis enol form predominates, and is in fast equilibrium (in the NMR scale) with nonchelated trans enol forms down to ?100°C at least. Transition enthalpies are of the order of 7–15 kJ mol^?1. In compounds with R₂ = Me the rotation of the Me -CS group around the neighbouring C-C- bond can be detected and the thermodynamic and kinetic parameters estimated. The reason for the slowing down of this rotation, inferred from the line broadening in NMR, is probably the stabilization of a trans isomer by interaction between the protons of the methyl group and the lone pairs of the oxygen atom. There are no indications for the presence of chelated and non-chelated enethiol isomers from the PMR studies and quantum chemical calculations performed.     

Cd2Ge7O16中Tb的长余辉发光特性

本文研究了Cd₂Ge₇O₁₆∶Tb³⁺材料的发光及其长余辉性质。指出Tb³⁺的发光是该离子的 ⁵D₃- ⁷D_J、 ⁵D₄- ⁷D_J两种跃迁产生的；随着掺杂浓度的增加 ⁵D₄- ⁷D_J跃迁增强，发光颜色由蓝变绿。并把该材料的长余辉性质归结为基质结构中有电子陷阱和空穴陷阱。提出余辉机理模型。     

Application of Graphite-Epoxy Composite Electrodes in Differential Pulse Anodic Stripping Voltammetry of Heavy Metals

An analytical study on the use of graphite-epoxy composite (GEC) electrodes for differential pulse anodic stripping voltammetry (DPASV) of heavy metals is presented. This study is accompanied by microscopic observations of the electrode surface before and after the stripping step in comparison to glassy carbon electrode. GEC electrodes show much better accumulation properties and consequently acceptable behaviour which makes them suitable as working electrodes in the DPASV of heavy metals. Lead determination in real water samples in a batch system as well as some preliminary results in a flow-through system are presented. The detection limits in batch measurements were 100ppb for Cd, 10pb for lead and 50ppb for copper. The detection limit for lead in the flow-through system was similar to that in the batch. The results obtained show that these low cost and easy to prepare materials can be of interest in future research concerning stripping techniques of heavy metals and other analytes.     

Spectral and non-spectral interferences in inductively coupled plasma mass-spectrometry

Inductively coupled plasma mass spectrometry of environmental and biological samples is often hampered by spectral and non-spectral interferences. Spectral interferences, caused by the limited resolution of the quadrupole mass spectrometer, can be eliminated in a variety of ways. For their identification inspection of a signal versus carrier gas flowrate is useful. Anion exchange allows the removal of most S and Cl containing compounds, which are at the origin of the majority of spectral interferences. Matrix modification, for example the addition of ethanol and subsequent optimization of the gas flow rates in a number of cases enables the reduction of the interferences to insignificant values. Often a mathematical correction based on isotopic signal ratios can be applied. Non-spectral interferences can be divided in reversible, that is occurring while the sample is being measured, and irreversible matrix effects, that is clogging of the nebulizer and sampling orifices or deposition on the torch or in the ion lens stack. The errors associated with non-spectral interferences can be eliminated by appropriate calibration procedures, adapted sample preparation or limitation of the amount of sample delivered to nebulizer, plasma and sampling devices, for example by the application of flow injection. Applications of all the elimination procedures are described for the analysis of sea-water, estuarine water, soil and sewage extracts, percolate water, urine, serum and wine.     

Molecular cloning and expression of an endo-β-1,4-d-glucanase I (avicelase I) gene from Bacillus cellulyticus K-12 and characterization of the recombinant enzyme

Bacillus cellulyticus K-12 Avicelase (Avicelase I; EC 3.2.1.4) gene (ace A) has been cloned in Escherichia coli by using the vector pT7T3U19 and HindIII-HindIII libraries of the chromosomal inserts. The libraries were screened for the expression of avicelase by monitoring the immunoreaction of the antiavicelase (immunoscreening). Positive clones (Ac-3, Ac-5, and Ac-7) contained the identical 3.5-kb HindIII fragment as determined by restriction mapping and Southern hybridization, and expressed avicelase efficiently and constitutively using its own promoter in the heterologous host. From the immunoblotting analysis, a polypeptide that showed a carboxymethylcellulase (CMCase) activity with an M _r , of 64,000 was detected. The recombinant endo 1,4-β- _d -glucanase I was purified to homogeneity from an intracellular fraction of E. coli by DEAE-Toyopearl M650, Phenyl Toyoperal M650, and TSK gel HW50S chromatography. The enzyme had a monomeric structure, its relative molecular mass being 65 kDa by gel filtration and 64 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The pI was 5.3 and the optimal pH was 4.6, and the enzyme was stable at pH 4.0–10.5. The enzyme had a temperature optimum of 50°C and was stable at 55°C for 48 h, and retained approx 20% of its activity after 30 min at 70°C. It showed high activity toward carboxymethylcellulose (CMC) as well as p-nitrophenyl-β-d-cellobioside, 4-methylumbelliferyl cellobioside, Avicel, filter paper, and some cellooligosaccharides. K _m values for CMC and Avicel were 7.6 and 85.2 mg/mL, respectively, whereas V _max values were 201 and 9.2 μmol · min⁻¹ · mg⁻¹, respectively. Cellotetraose (G4) was preferentially cleaved into cellobiose (G2) and cellopentaose (G5) was cleaved into G2 + cellotriose (G3), whereas cellohexaose (G6) was cleaved into G4 + G2 and, to a lesser extent, into G3 + G3. G3 was not cleaved at all. G2 was the main product of Avicel hydrolysis. G2 inhibited whereas Mg⁺⁺ stimulated the activity of CMCase and Avicelase. Hydrolysis of CMC took place with a rapid decrease in viscosity but a slow liberation of reducing sugars. Based on these results, it appeared that the cellulase should be regarded as endo type, although it hydrolyzed Avicel.