Electrodeposited lead dioxide coatings

Lead dioxide coatings on inert substrates such as titanium and carbon now offer new opportunities for a material known for 150 years. It is now recognised that electrodeposition allows the preparation of stable coatings with different phase structures and a wide range of surface morphologies. In addition, substantial modification to the physical properties and catalytic activities of the coatings are possible through doping and the fabrication of nanostructured deposits or composites. In addition to applications as a cheap anode material in electrochemical technology, lead dioxide coatings provide unique possibilities for probing the dependence of catalytic activity on layer composition and structure (critical review, 256 references).     

The nonribosomal peptide synthetase enzyme DdaD tethers N(β)-fumaramoyl-l-2,3-diaminopropionate for Fe(II)/α-ketoglutarate-dependent epoxidation by DdaC during dapdiamide antibiotic biosynthesis

The gene cluster from Pantoea agglomerans responsible for biosynthesis of the dapdiamide antibiotics encodes an adenylation-thiolation didomain protein, DdaD, and an Fe(II)/α-ketoglutarate-dependent dioxygenase homologue, DdaC. Here we show that DdaD, a nonribosomal peptide synthetase module, activates and sequesters N(β)-fumaramoyl-l-2,3-diaminopropionate as a covalently tethered thioester for subsequent oxidative modification of the fumaramoyl group. DdaC catalyzes Fe(II)- and α-ketoglutarate-dependent epoxidation of the covalently bound N(β)-fumaramoyl-l-2,3-diaminopropionyl-S-DdaD species to generate N(β)-epoxysuccinamoyl-DAP (DAP = 2,3-diaminopropionate) in thioester linkage to DdaD. After hydrolytic release, N(β)-epoxysuccinamoyl-DAP can be ligated to l-valine by the ATP-dependent ligase DdaF to form the natural antibiotic N(β)-epoxysuccinamoyl-DAP-Val.     

Polyamine-functional sterically stabilized latexes for covalently cross-linkable colloidosomes

Sterically stabilized polystyrene latexes were prepared by aqueous emulsion polymerization using a poly(ethylene imine) (PEI) stabilizer in the presence of 4-vinylbenzyl chloride (4-VBC; 1.0 wt % based on styrene). Partial quaternization of the amine groups on the PEI chains by 4-VBC occurs in situ, hence producing a chemically grafted steric stabilizer. Such 4-VBC-modified PEI chains were grafted more efficiently onto the polystyrene particles than unmodified PEI, as judged by aqueous electrophoresis, XPS, and nitrogen microanalysis. Moreover, partially quaternized PEI gave significantly smaller polystyrene particles than those synthesized in the absence of any PEI stabilizer or those synthesized using unmodified PEI. The partially quaternized PEI-stabilized polystyrene latex proved to be an effective emulsifier at pH 9, forming stable oil-in-water Pickering emulsions when homogenized (12,000 rpm, 2 min, 20 °C) with four model oils, namely, n-dodecane, methyl myristate, isononyl isononanoate, and sunflower oil. The primary and/or secondary amine groups on the PEI stabilizer chains were successfully cross-linked using three commercially available polymeric reagents, namely, tolylene 2,4-diisocyanate-terminated poly(propylene glycol) (PPG-TDI), poly(propylene glycol) diglycidyl ether (PPG-DGE), or poly(ethylene glycol) diglycidyl ether (PEG-DGE). Cross-linking with the former reagent led to robust colloidosomes that survived the removal of the internal oil phase on washing with excess alcohol, as judged by optical microscopy and SEM. PPG-TDI reacted very rapidly with the PEI stabilizer chains, with cross-linking being achieved during homogenization. Well-defined colloidosomes could be formed only by using sunflower oil and isononyl isononanoate with this cross-linker at 20 °C. However, cooling to 0 °C allowed colloidosomes to be formed using n-dodecane, presumably because of the slower rate of cross-linking at this reduced temperature. PPG-DGE proved to be a more generic cross-linker because it formed robust colloidosomes with all four model oils. However, cross-linking was much slower than that achieved using PPG-TDI, with intact colloidosomes being formed only after ～12 h at 20 °C. The PEG-DGE cross-linker allowed cross-linking to be conducted at 20 °C from the aqueous phase (rather from within the oil droplets for the oil-soluble PPG-TDI or PPG-DGE cross-linkers). In this case, well-defined colloidosomes were obtained at 50 vol % with surprisingly little intercolloidosome aggregation, as judged by laser diffraction studies.     

Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

The geometric structures of small cationic rhodium clusters Rh(n)(+) (n = 6-12) are investigated by comparison of experimental far-infrared multiple photon dissociation spectra with spectra calculated using density functional theory. The clusters are found to favor structures based on octahedral and tetrahedral motifs for most of the sizes considered, in contrast to previous theoretical predictions that rhodium clusters should favor cubic motifs. Our findings highlight the need for further development of theoretical and computational methods to treat these high-spin transition metal clusters.     

The deposition of nanostructured β-PbO2 coatings from aqueous methanesulfonic acid for the electrochemical oxidation of organic pollutants

Highly crystalline, nanostructured, three-dimensional β-PbO₂ coatings were successfully obtained by galvanostatic deposition from baths containing aqueous lead(II) and methanesulfonic acid (CH₃SO₃H). This constitutes a much more environmentally friendly methodology compared to plating of β-PbO₂ in HNO₃. The deposits exhibited high quality and good adherence. The crystallite size was in the range 20–30 nm and AFM imaging revealed very uniform, rough deposits (i.e., 255–275 nm rms). The oxidative destruction of Methyl Orange azo dye was studied by electrochemical advanced oxidation processes (EAOPs). An electro-Fenton process with a high surface area carbon-felt cathode performed better than the single anodic oxidation. Rapid and complete decolorisation was achieved following pseudo first-order kinetics. The stability of the β-PbO₂ electrodes during the electrolyses was also demonstrated.     

Highly concentrated catalytic asymmetric allylation of ketones

[reaction: see text] We report the catalytic asymmetric allylation of ketones under highly concentrated reaction conditions with a catalyst generated from titanium tetraisopropoxide and BINOL (1:2 ratio) in the presence of isopropanol. This catalyst promotes the addition of tetraallylstannane to a variety of ketones to produce tertiary homoallylic alcohols in excellent yield (80-99%) with high enantioselectivities (79-95%). The resulting homoallylic alcohols can also be epoxidized in situ using tert-butyl hydroperoxide (TBHP) to afford cyclic epoxy alcohols in high yield (84-87%).     

Purification and properties of a β-galactosidase with potential application as a digestive supplement

Functional-based screening of crude β-galactosidase activities from 42 yeast strains resulted in the selection of a single enzyme of potential interest as a digestive supplement. β-Galactosidase produced by Kluyveromyces marxianus DSM5418 was purified to homogeneity by a combination of gel filtration, ion-exchange, and hydroxylapatite chromatographies. The denatured (123 kDa) and native molecular masses (251 kDa) suggest that the enzyme is a homodimer. The optimum pH and temperature of the purified enzyme were 6.8 and 37°C, respectively. The unpurified β-galactosidase in particular displayed a high level of stability when exposed to simulated intestinal conditions in vitro for 4 h. Matrix-assisted laser desorption ionization mass sectrometry analysis revealed that the enzyme's trypsin-generated peptide mass fingerprint shares several peptide fragment hits with β-galactosidases from Kluyveromyces lactis. This confirms the enzyme's identity and indicates that significant sequence homology exists between these enzymes.     

Analysis and deconvolution of dimethylnaphthalene isomers using gas chromatography vacuum ultraviolet spectroscopy and theoretical computations

An issue with most gas chromatographic detectors is their inability to deconvolve coeluting isomers. Dimethylnaphthalenes are a class of compounds that can be particularly difficult to speciate by gas chromatography – mass spectrometry analysis, because of their significant coelution and similar mass spectra. As an alternative, a vacuum ultraviolet spectroscopic detector paired with gas chromatography was used to study the systematic deconvolution of mixtures of coeluting isomers of dimethylnaphthalenes. Various ratio combinations of 75:25; 50:50; 25:75; 20:80; 10:90; 5:95; and 1:99 were prepared to test the accuracy, precision, and sensitivity of the detector for distinguishing overlapping isomers that had distinct, but very similar absorption spectra. It was found that, under reasonable injection conditions, all of the pairwise overlapping isomers tested could be deconvoluted up to nearly two orders of magnitude (up to 99:1) in relative abundance. These experimental deconvolution values were in agreement with theoretical covariance calculations performed for two of the dimethylnaphthalene isomers. Covariance calculations estimated high picogram detection limits for a minor isomer coeluting with low to mid-nanogram quantity of a more abundant isomer. Further characterization of the analytes was performed using density functional theory computations to compare theory with experimental measurements. Additionally, gas chromatography – vacuum ultraviolet spectroscopy was shown to be able to speciate dimethylnaphthalenes in jet and diesel fuel samples.     

Apparent two-dimensional behavior of TiO2 nanotubes revealed by light absorption and luminescence

Optical absorption and photoluminescence (PL) properties of colloidal TiO(2) nanotubes, produced by the alkali hydrothermal method, were studied at room temperature in the range 300-700 nm. Nanotubes having an internal diameter in the range 2.5-5 nm have very similar optical properties, in contrast to the expected behavior for quasi-1-D systems. This is explained by the complete thermal smearing of all 1-D effects, due to the large effective mass of charge carriers in TiO(2), resulting in an apparent 2-D behavior of TiO(2) nanotubes.     

Chemoselective palladium-catalyzed deprotonative arylation/[1,2]-Wittig rearrangement of pyridylmethyl ethers

Control of chemoselectivity is one of the most challenging problems facing chemists and is particularly important in the synthesis of bioactive compounds and medications. Herein, the first highly chemoselective tandem C(sp³)–H arylation/[1,2]-Wittig rearrangement of pyridylmethyl ethers is presented. The efficient and operationally simple protocols enable generation of either arylation products or tandem arylation/[1,2]-Wittig rearrangement products with remarkable selectivity and good to excellent yields (60–99%). Choice of base, solvent, and reaction temperature play a pivotal role in tuning the reactivity of intermediates and controlling the relative rates of competing processes. The novel arylation step is catalyzed by a Pd(OAc)₂/NIXANTPHOS-based system via a deprotonative cross-coupling process. The method provides rapid access to skeletally diverse aryl(pyridyl)methanol core structures, which are central components of several medications.