Carrier Protein-Free Enzymatic Biaryl Coupling in Arylomycin A2 Assembly and Structure of the Cytochrome P450 AryC**

The arylomycin antibiotics are potent inhibitors of bacterial type I signal peptidase. These lipohexapeptides contain a biaryl structural motif reminiscent of glycopeptide antibiotics. We herein describe the functional and structural evaluation of AryC, the cytochrome P450 performing biaryl coupling in biosynthetic arylomycin assembly. Unlike its enzymatic counterparts in glycopeptide biosynthesis, AryC converts free substrates without the requirement of any protein interaction partner, likely enabled by a strongly hydrophobic cavity at the surface of AryC pointing to the substrate tunnel. This activity enables chemo-enzymatic assembly of arylomycin A2 that combines the advantages of liquid- and solid-phase peptide synthesis with late-stage enzymatic cross-coupling. The reactivity of AryC is unprecedented in cytochrome P450-mediated biaryl construction in non-ribosomal peptides, in which peptidyl carrier protein (PCP)-tethering so far was shown crucial both in vivo and in vitro.     

Quantitative liquid jet instability measurement system using asymmetric magnification and digital image processing

A new technique for measuring the growth of instabilities on the surface of liquid jets flowing into gas is demonstrated. A collimated beam of white light illuminates the jet from behind, forming a shadow image. A pair of cylindrical lenses are arranged to provide different magnifications in the streamwise and cross-stream directions. A number of streamwise diameters and one cross-stream diameter are thus captured with maximum resolution in a single image on a charge-coupled device (CCD) electronic camera. A short-duration spark is used to freeze the jet motion. A mask representing the theoretical edge-response of the imaging system is digitally convolved with the cross-stream gray scale data to obtain sub-pixel resolution of the jet edge profile. The method is demonstrated using the well-known capillary jet instability and a ratio of streamwise to cross-stream magnifications of 40. Well-resolved single images show the development of the instability from small perturbations through the formation of the first drop. The system forms an accurate automated method of measuring the development of liquid jet instabilities. It can readily be applied to practical problems including liquid jet atomization.List of symbols a undisturbed jet radius - k nondimensional wavenumber (= 2a/) - Q gas-to-liquid density ratio - r ₀ mean jet radius, from initial region of image - R Reynolds number (= 2Ua/) - U mean jet velocity - We Weber number - z streamwise coordinate, origin at jet orifice - temporal growth rate - _s measured spatial growth rate - nondimensional temporal growth rate - r absolute value of height of peaks or troughs relative to r ₀ - r ₁ height of first extremum in a particular record - instability wavelength - liquid viscosity - liquid density - surface tension of liquid-gas interface     

Water-based route to colloidal Mn-doped ZnSe and core/shell ZnSe/ZnS quantum dots

Relatively monodisperse and highly luminescent Mn(2+)-doped zinc blende ZnSe nanocrystals were synthesized in aqueous solution at 100 °C using the nucleation-doping strategy. The effects of the experimental conditions and of the ligand on the synthesis of nanocrystals were investigated systematically. It was found that there were significant effects of molar ratio of precursors and heating time on the optical properties of ZnSe:Mn nanocrystals. Using 3-mercaptopropionic acid as capping ligand afforded 3.1 nm wide ZnSe:Mn quantum dots (QDs) with very low surface defect density and which exhibited the Mn(2+)-related orange luminescence. The post-preparative introduction of a ZnS shell at the surface of the Mn(2+)-doped ZnSe QDs improved their photoluminescence properties, resulting in stronger emission. A 2.5-fold increase in photoluminescence quantum yield (from 3.5 to 9%) and of Mn(2+) ion emission lifetime (from 0.62 to 1.39 ms) have been observed after surface passivation. The size and the structure of these QDs were also corroborated by using transmission electron microscopy, energy dispersive spectroscopy, and X-ray powder diffraction.     

The Brønsted Acid Catalyzed,Enantioselective Vinylogous Mannich Reaction

The first catalytic, enantioselective vinylogous Mannich reaction of acyclic silyl dienolates is reported. A second‐generation 2,2′‐dihydroxy‐1,1′‐binaphthyl (BINOL)‐based phosphoric acid has been developed and further optimized as an enantioselective organocatalyst. Upon protonation of the imines, chiral contact ion pairs are generated in situ and attacked highly diastereoselectively by the nucleophile. γ‐Substituted silyl dienolates that lead to more highly substituted Mannich products with a second stereogenic center in good diastereoselectivity have been employed in these reactions. The reaction path has been elucidated with NMR spectroscopy and mass spectrometry, which suggest that the protic reaction medium found to be optimal in these reactions serves to trap the cationic silicon species as silanol. A crystal structure of a phosphoric acid bound imine was obtained that provides insight into the binding mode and a rationale for the stereochemical course of the reaction.     

Binary Au/MWCNT and Ternary Au/ZnO/MWCNT Nanocomposites: Synthesis,Characterisation and Catalytic Performance

Gold nanoparticles of 10–24 and 5–8 nm in size were obtained by chemical citrate reduction and UV photoreduction, respectively, on acid‐treated multiwalled carbon nanotubes (MWCNTs) and on ZnO/MWCNT composites. The shape and size of the deposited Au nanoparticles were found to be dependent upon the synthetic method used. Single‐crystalline, hexagonal gold particles were produced in the case of UV photoreduction on ZnO/MWCNT, whereas spherical Au particles were deposited on MWCNT when the chemical citrate reduction method was used. In the UV photoreduction route, n‐doped ZnO serves as the e^? donor, whereas the solvent is the hole trap. All materials were fully characterised by UV/Vis spectroscopy, scanning electron microscopy, transmission electron microscopy, X‐ray photoelectron spectroscopy, Raman spectroscopy and BET surface analysis. The catalytic activity of the composites was studied for the selective hydrogenation of α,β‐unsaturated carbonyl compound 3,7‐dimethyl‐2,6‐octadienal (citral). The Au/ZnO/MWCNT composite favours the formation of unsaturated alcohols (selectivity=50 % at a citral conversion of 20 %) due to the presence of single‐crystalline, hexagonal gold particles, whereas saturated aldehyde formation is favoured in the case of the Au/MWCNT nanocomposite that contains spherical gold particles.     

Analysis of EU priority polycyclic aromatic hydrocarbons in food supplements using high performance liquid chromatography coupled to an ultraviolet, diode array or fluorescence detector

High performance liquid chromatography coupled to an ultraviolet, diode array or fluorescence detector (HPLC/UV-FLD) has been used to set up a method to detect the 15(+1) EU priority polycyclic aromatic hydrocarbons (PAHs) in food supplements covering the categories of dried plants and plant extracts excluding oily products. A mini validation was performed and the following parameters have been determined: limit of detection, limit of quantification, precision, recovery and linearity. They were in close agreement with quality criteria described in the Commission Regulation (EC) No 333/2007 concerning the PAH benzo[a]pyrene in foodstuffs, except the not fluorescent cyclopenta[c,d]pyrene for which the UV detection leads to a higher limit of detection. Analysis of twenty commercial food supplements covering mainly the class of dried plants was performed to evaluate their PAHs contamination levels and to test the applicability of the method to various plant matrices. Fifty percent of analyzed samples showed concentration exceeding 2 μg kg⁻¹ for one or more PAHs.     

Simple hollow fiber renewal liquid membrane extraction method for pre-concentration of Cd(II) in environmental samples and detection by Flame Atomic Absorption Spectrometry

A hollow fiber renewal liquid membrane (HFRLM) extraction method to determine cadmium (II) in water samples using Flame Atomic Absorption Spectrometry (FAAS) was developed. Ammonium O,O-diethyl dithiophosphate (DDTP) was used to complex cadmium (II) in an acid medium to obtain a neutral hydrophobic complex (ML₂). The organic solvent introduced to the sample extracts this complex from the aqueous solution and carries it over the poly(dimethylsiloxane) (PDMS) membrane, that had their walls previously filled with the same organic solvent. The organic solvent is solubilized inside the PDMS membrane, leading to a homogeneous phase. The complex strips the lumen of the membrane where, at higher pH, the complex Cd-DDTP is broken down and cadmium (II) is released into the stripping phase. EDTA was used to complex the cadmium (II), helping to trap the analyte in the stripping phase. A multivariate procedure was used to optimize the studied variables. The optimized variables were: sample (donor phase) pH 3.25, DDTP concentration 0.05% (m/v), stripping (acceptor phase) pH 8.75, EDTA concentration 1.5 × 10⁻² mol L⁻¹, extraction temperature 40 °C, extraction time 40 min, a solvent mixture N-butyl acetate and hexane (60/40%, v/v) with a volume of 100 μL, and addition of ammonium sulfate to saturate the sample. The sample volume used was 20 mL and the stripping volume was 165 μL. The analyte enrichment factor was 120, limit of detection (LOD) 1.3 μg L⁻¹, relative standard deviation (RSD) 5.5% and the working linear range 2-30 μg L⁻¹.     

Monitoring carbamazepine in surface and wastewaters by an immunoassay based on a monoclonal antibody

The pharmaceutical compound carbamazepine (CBZ) is an emerging pollutant in the aquatic environment and may potentially be used as a wastewater marker. In this work, an enzyme-linked immunosorbent assay (ELISA) for the detection of carbamazepine in surface and sewage waters has been developed. The heterogeneous immunoassay is based on a commercially available monoclonal antibody and a novel enzyme conjugate (tracer) that links the hapten via a hydrophilic peptide (triglycine) spacer to horseradish peroxidase. The assay achieves a limit of detection of 24 ng/L and a quantitation range of 0.05–50 μg/L. The analytical performance and figure of merits were compared to liquid chromatography–tandem mass spectrometry after solid-phase extraction. For nine Berlin surface water samples and one wastewater sample, a close correlation of results was observed. A constant overestimation relative to the CBZ concentration of approximately 30% by ELISA is probably caused by the presence of 10,11-epoxy-CBZ and 2-hydroxy-CBZ in the samples. The ELISA displayed cross-reactivities for these compounds of 83% and 14%, respectively. In a first screening of 27 surface water samples, CBZ was detected in every sample with concentrations between 0.05 and 3.2 μg/L. Since no sample cleanup is required, the assay allowed for the determination of carbamazepine with high sensitivity at low costs and with much higher throughput than with conventional methods.