Catalytic dehydration of isopropanol on neodymium-Y and X-zeolites and the effect of γ-irradiation

Four different samples of NdX, NdY, NdNH₄X and NdNH₄Y-zeolites were prepared by ionexchange methods. DTA and XRD analyses have been carried out for the samples. The thermally activated zeolites were irradiated by 1.5 and 10.0 Mrad -rays. The catalytic activities of these samples were tested in dehydration of isopropanol. The results of DTA indicated that all samples showed endothermic peaks at about 215 °C related to the release of physically adsorbed water and exothermic peaks at 850–950 °C indicating the collapses of the zeolite. The X-ray analysis revealed that the exchange of sodium by neodymium or ammonium followed by neodymium ions did not change the crystal structure but some decrease in the crystallinity was observed. The catalytic activities of these zeolites were measured in dehydration of isopropanol as a function of temperature. It was found that the activity of the prepared Nd-zeolites depends on the crystallinity of zeolites and on the condensation products formed on catalyst surface. However, the irradiated samples exhibited higher catalytic activities in isopropanol conversion than the unirradiated ones. The observed higher activity for irradiated samples was attributed to the increase of the number of acidic centers responsible for dehydration of alcohols. These centers were formed as a result of the formation of tricoordinate aluminium atoms in -irradiated zeolites.     

Evaluating precursor-directed biosynthesis towards novel erythromycins through in vitro studies on a bimodular polyketide synthase

Background: Modular polyketide synthases (PKSs) catalyse the biosynthesis of complex polyketides using a different set of enzymes for each successive cycle of chain extension. Directed biosynthesis starting from synthetic diketides is a potentially valuable route to novel polyketides. We have used a purified bimodular derivative of the erythromycin-producing polyketide synthase (DEBS 1-TE) to study chain extension starting from a variety of diketide analogues and, in some cases, from the alternative acyl-CoA thioester substrates.Results: Chain initiation in vitro by DEBS 1-TE module 2 using a synthetic diketide analogue as a substrate was tolerant of significant structural variation in the starter unit of the synthetic diketide, but other changes completely abolished activity. Interestingly, a racemic β-keto diketide was found to be reduced in situ on the PKS and utilised in place of its more complex hydroxy analogue as a substrate for chain extension. The presence of a diketide analogue strongly inhibited chain initiation via the loading module. Significantly higher concentrations of diketide N-acetylcysteamine analogues than their corresponding acyl-CoA thioesters are required to achieve comparable yields of triketide lactones.Conclusions: Although a broad range of variation in the starter residue is acceptable, the substrate specificity of module 2 of a typical modular PKS in vitro is relatively intolerant of changes at C-2 and C-3. This will restrict the usefulness of approaches to synthesise novel erythromycins using synthetic diketides in vivo. The use of synthetic β-keto diketides in vivo deserves to be explored.     

Comparison of partial digestion procedures for determination of Ca, Cr, Cu, Fe, K, Mg, Mn, Na, P, and Zn in milk by inductively coupled plasma-optical emission spectrometry

A fast procedure was developed for determination of Ca, Cr, Cu, Fe, K, Mg, Mn, Na, P, and Zn in milk samples. This procedure consisted of a partial digestion with hydrochloric acid on a hot plate. The results obtained were compared with 3 other digestion procedures (dry ashing and 2 microwave digestions). All the procedures showed similar precision levels, with coefficients of variation <10% for most analyzed elements. Accuracy was evaluated by using certified reference materials, and the values obtained were within the confidence intervals for these products. The results obtained were not considered statistically different. The partial digestion on a hot plate with HCl can be very practical for laboratories with relatively large numbers of sample analyses.     

The mechanism of 1,2-addition of disilene and silene: hydrogen halide addition

The mechanism of 1,2-addition reactions of HF and HCl to Si=Si, Si=C, and C=C bonds has been investigated by ab initio quantum chemical methods. Geometries and relative energies of the stationary points and all the transition states were determined by using the MP2/6-311++G(d,p), B3LYP/6-311++G(d,p), and CBS-Q levels of theory. The investigated reactions can be characterized by two main thermodynamic profiles. The type in which the reagent molecule attacks a carbon atom is moderately exothermic with a high activation barrier. The second type in which a hydrogen halide attacks a silicon is strongly exothermic with a low activation energy. At the early stage of all the reactions a weakly bonded initial complex is found which indicates that the initial step of all the reactions is an electrophilic attack of hydrogen halide. The geometry and charge distribution of the transition state of the reactions indicate two main types of mechanism. If silicon is attacked, the halogen-silicon bond formation precedes the H-Y bond breaking. If, however, carbon is attacked, the first step is always an ionic dissociation of the hydrogen halide and a carbenium ion formation, which is stabilized by the C-Y bond formation in the final step of the reaction. The reaction diagrams and proposed mechanisms explain the experimentally found regioselectivity well.     

Pulse radiolysis study on the initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene

Initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene have been studied by means of microsecond pulse radiolysis. For styrene, absorption bands caused by the monomer cation radical St⁺? appear at 630 and 350 nm in a mixture of isopentane and n-butyl chloride at about ?165°C. In parallel with the decay of St⁺?, three absorption bands appear in the near-infrared (IR) region, and at 600 and 450 nm. The IR and 600 nm bands are assigned to the associated dimer cation radical St₂⁺?, and the 450 nm band to the bonded dimer cation radical St-St⁺?. The kinetic behavior of these species shows that reaction of St⁺? with styrene monomer forms both St₂⁺? and St-St⁺?. With the decay of St-St⁺?, another absorption band appears at 340 nm, and the lifetime of this band is relatively long. The 340 nm band may be due to carbonium ions of the growing polystyrene. For α-methylstyrene, the monomer cation radical (at 690 and 350 nm), the associated dimer cation radical (in the near-IR region and at 620 nm) and the bonded dimer cation radical (at 480 nm) behave in a manner similar to that of the corresponding styrene species. The absorption band caused by carbonium ions of growing poly(α-methylstyrene) appears at 340 nm.     

Evaluating the performance of an inexpensive,commercially available,NFC-powered and smartphone controlled potentiostat for electrochemical sensing

To move electrochemical sensing technology from the laboratory into point-of-care (POC)-based settings, electrochemical platforms must be cost-effective, portable, and user-friendly. Here, we present the comparison of a near-field communication (NFC) potentiostat with a traditional potentiostat. The NFC potentiostat is roughly the size of a credit card and is wirelessly powered by a smartphone with a user-friendly application for electrochemical analysis. The proposed system is portable, affordable, and provides analytical capabilities comparable to traditional benchtop potentiostats. Different redox probes were investigated, including ferricyanide, ruthenium hexamine, and hydroquinone, highlighting the ability of the NFC potentiostat to measure a variety of target analytes. In addition, the NFC potentiostat can perform several electrochemical techniques, such as cyclic voltammetry, differential pulse voltammetry, chronoamperometry and square wave voltammetry. The combination of affordable screen-printed carbon electrodes (SPCE)s with the portable, inexpensive NFC potentiostat highlights this platform‘s promise towards POC electrochemical sensing applications, particularly for medical diagnostics in rural or low-income areas.     

Modular Oxime Formation by a trans-AT Polyketide Synthase

Modular trans-acyltransferase polyketide synthases (trans-AT PKSs) are enzymatic assembly lines that biosynthesize complex polyketide natural products. Relative to their better studied cis-AT counterparts, the trans-AT PKSs introduce remarkable chemical diversity into their polyketide products. A notable example is the lobatamide A PKS, which incorporates a methylated oxime. Here we demonstrate biochemically that this functionality is installed on-line by an unusual oxygenase-containing bimodule. Furthermore, analysis of the oxygenase crystal structure coupled with site-directed mutagenesis allows us to propose a model for catalysis, as well as identifying key protein-protein interactions that support this chemistry. Overall, our work adds oxime-forming machinery to the biomolecular toolbox available for trans-AT PKS engineering, opening the way to introducing such masked aldehyde functionalities into diverse polyketides.     

Intramolecular charge-transfer fluorescence of 1-phenyltridecamethylbicyclo[2.2.2]octasilane

1-Phenyltridecamethylbicyclo[2.2.2]octasilane (1) is prepared by the reaction of the corresponding 1-chlorotridecamethylbicyclo[2.2.2]octasilane with phenyllithium in the presence of N,N,N',N'-tetramethylethylenediamine and the molecular structure of is determined by X-ray crystallography. Phenyloligosilane shows dual fluorescence even in non-polar hexane though the TICT-like mechanism is not applicable.