New ru(II) complexes containing oxazoline ligands as epoxidation catalysts. Influence of the substituents on the catalytic performance

The synthesis of a family of new Ru complexes containing the facial tridentate ligand with general formula [Ru(II)(T)(D)(X)](n+) (T = trispyrazolylmethane (tpm); D = ((4S,4'S)-(-)-4,4',5,5'-tetrahydro-4,4'-bis(1-methylethyl)-2,2'-bioxazole) (iPr-box-C) or N-(1-hydroxy-3-methylbutan-(2S)-(-)-2-yl)-(4S)-(-)-4-isopropyl-4,5-dihydrooxazole-2-carbimidate (iPr-box-O); X = Cl, H(2)O) has been described. All complexes have been spectroscopically characterized in solution through (1)H NMR and UV-vis techniques, and the redox properties of complexes have also been studied by means of cyclic voltammetry (CV). Furthermore, the chloro complexes presented here have been characterized in the solid state through monocrystal X-ray diffraction analysis. The oxazolinic iPr-box-C ligand undergoes a Ru-assisted hydrolysis reaction generating the corresponding amidate anionic ligand iPr-box-O, that keeps coordinated to the Ru metal center and that produces a strong σ-donation effect over it. The reactivity of the Ru-OH(2) complexes described in this paper together with other similar ones, previously synthesized by us, has been tested with regard to the epoxidation of different olefins. Complexes [Ru(II)(R-box-C)(tpm)OH(2)](BF(4))(2), R = Bz, 3'c/iPr, 3c, show high stereoselectivity in the epoxidation of cis-β-methylstyrene, with the exclusive formation of the cis-epoxide. However, there is a significant difference in regioselectivity between the two catalysts in the epoxidation of 4-vinylcyclohexene; complex 3'c leads to the regioselective oxidation at the ring alkene position, whereas complex 3c leads to the oxidation at the terminal position. Computational calculations indicate only small energy differences between the two possible products of 4-vinylcyclohexene epoxidation, but the energy barriers for the interaction of the catalytic systems with the alkene groups of 4-vinylcyclohexene agree with the reactivity differences found for the two catalysts having isopropyl or benzyl as substituent of the oxazole ligand. Computed local Fukui functions help to explain the observed reactivity trends.     

Table salt and other alkali metal chloride oligomers: structure, stability, and bonding

We have investigated table salt and other alkali metal chloride monomers, ClM, and (distorted) cubic tetramers, (ClM)(4), with M = Li, Na, K, and Rb, using density functional theory (DFT) at the BP86/TZ2P level. Our objectives are to determine how the structure and thermochemistry (e.g., Cl-M bond lengths and strengths, oligomerization energies, etc.) of alkali metal chlorides depend on the metal atom and to understand the emerging trends in terms of quantitative Kohn-Sham molecular orbital (KS-MO) theory. The analyses confirm the high polarity of the Cl-M bond (dipole moment, VDD, and Hirshfeld atomic charges). They also reveal that bond overlap derived stabilization (approximately -26, -20, and -8 kcal/mol), although clearly larger than in the corresponding F-M bonds, contributes relatively little to the (trend in) bond strengths (-105, -90, and -94 kcal/mol) along M = Li, Na, and K. Thus, the Cl-M bonding mechanism resembles more closely that of the even more ionic F-M bond than that of the more covalent C-M or H-M bonds. Tetramerization causes the Cl-M bond to expand, and it reduces its polarity.     

A sensitive confirmatory method for aflatoxins in maize based on liquid chromatography/electrospray ionization tandem mass spectrometry

A liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) method for measurement of aflatoxins B1, B2, G1, and G2 in maize is described. Aflatoxins (AFs) were extracted from 1 g samples by using tri-portions of acetonitrile/water (80:20, v/v) (10 + 7 + 7 mL), and 2/5 of the extract diluted to 500 mL by water was cleaned up with a 100 mg Carbograph-4 cartridge. After the addition of the internal standard AFM1, the final extract was analyzed by LC/ESI-MS/MS in positive ion mode using multiple reaction monitoring with a triple-quadrupole instrument. A C(18) column thermostatted at 45 degrees C with a mobile phase gradient of acetonitrile/water with 2 mmol/L ammonium formate was used. Although the matrix suppression effect was negligible, quantitation was achieved by an external calibration procedure using matrix-matched standard solutions to improve accuracy. Sample recoveries at four spiking levels ranged from 81 to 101% (relative standard deviation (RSD) 相似文献   

Reaction of Aplysia limacina metmyoglobin with hydrogen peroxide

Myoglobin (Mb) from gastropod mollusc Aplysia limacina shows only 20% sequence homology to the 'prototype' sperm whale Mb but exhibits a typical Mb fold and can reversibly bind oxygen. An intriguing feature of aplysia Mb is that it lacks the distal histidine and displays a ligand stabilisation based on an arginine. Here we report the reaction of aplysia metMb with hydrogen peroxide studied by optical and electron paramagnetic resonance (EPR) spectroscopies. Two electron oxidation of the protein by H2O2 results in formation of two intermediates typical for this class of reactions, the oxoferryl haem state and a globin-bound free radical. An unusual characteristic of the aplysia Mb reaction is formation, prior to haem oxidation, of an optically distinct compound with an EPR spectrum typical of the low spin Fe3+ haem state. This compound is interpreted as the complex between H2O2 and the ferric haem state (Compound), formed prior to cleavage of the dioxygen bond. We conclude that H2O2 is singly deprotonated in Compound which can thus be notated as [Fe3+--OOH]. A new low spin ferric haem state has been observed over the period of Compound decay, and hypotheses have been formulated as to its identity and role. The location of the protein bound radical observed in aplysia Mb is discussed in light of the fact that the protein does not have any tyrosine residues, the most common site of free radical formation in the haem protein/peroxide systems. All intermediates of the reaction are kinetically characterised.     

Liquid chromatography/tandem mass spectrometry determination of organophosphorus flame retardants and plasticizers in drinking and surface waters

A liquid chromatography/electrospray ionization tandem mass spectrometric method for analyzing organophophorus flame retardants and plasticizers in drinking and environmental waters was developed. Five alkyl phosphates, three chlorinated alkyl phosphates, two aryl phosphate and triphenylphosphine oxide were selected for this study. These compounds were extracted from water samples by a hydrophilic polymeric solid-phase extraction cartridge. Accuracy and precision were evaluated analyzing 0.5 L of water samples spiked at concentrations of 10 and 100 ng/L for drinking water and at 300 and 1000 ng/L for river water. Except for trimethyl phosphate, analyte recoveries were better than 80%, and were not dependent on the type of aqueous matrix in which they were dissolved. At the spike levels considered, within-day precision was between 3 and 12% for tap water and between 4 and 14% for river water, and estimated method quantification limits ranged from 0.2 to 3.9 ng/L. A short survey conducted by analyzing some river water samples (River Tiber) ascertained the presence of ten organophosphorus compounds at concentration levels ranging from a few nanograms per liter to 323 ng/L for tris(2-butoxyethyl) phosphate.     

Quantification of bromine in flame-retardant coatings by radiofrequency glow discharge–optical emission spectrometry

There is an increasing concern regarding the toxicity and environmental distribution and impact of brominated organic compounds employed as flame retardants. Thus, present interest in searching for new analytical techniques and methods allowing a rapid, simple and reliable detection of those compounds in materials and wastes potentially containing such flame retardants is not surprising. The feasibility of using radiofrequency glow discharge plasma spectrometry coupled with optical emission spectrometry (rf-GD-OES) as a rapid and simple tool to directly analyse bromine-containing flame-retardant polymeric layers is investigated here. Polymeric layers for calibration were made by mixing appropriate amounts of tetrabromobisphenol A, bisphenol A, phloroglucinol and diphenylmethane-4,4′-diisocyanate in tetrahydrofuran. The corresponding blanks (polymers without tetrabromobisphenol A) were also prepared. Detection of bromine was investigated both in the visible (at 470.48 nm) and in the near-infrared (at 827.24 nm) regions, using a charge-coupled device for detection. Discharge parameters affecting the emission intensity of bromine were first optimized (in argon and helium as possible plasma gases) and the analytical performance characteristics were then evaluated. The best detection limit (0.044% Br) was achieved measuring Br I 827.24 nm in a He discharge, using a forward power of 70 W and a pressure of 45 Torr. The linearity range extended up to 27% Br. Finally, the applicability of the rf-GD-OES method proposed to the quantitative analysis of bromine in solid materials coated with flame-retardant commercial paints was successfully demonstrated. Figure Flame Retardants     

Determination of aflatoxins in olive oil by liquid chromatography-tandem mass spectrometry

A liquid chromatography-tandem mass spectrometric with electrospray ionization (LC/ESI-MS/MS) method for determining the four naturally occurring aflatoxins (AFs) B1, B2, G1, and G2 in olive oil is proposed. AFs were extracted from oil sample by means of matrix solid phase dispersion (MSPDE), utilizing C18 as dispersing material. No further purification step, such as lipid removal, was performed. Aflatoxin M1, the hepatic metabolite of AFB1, was employed as internal standard. Olive oil extract was analyzed by LC/ESI-MS/MS in positive ionization mode, with multireaction monitoring acquisition. Due to a signal suppression ranging between 4 and 23%, quantitation was performed by matrix-matched calibration curves. The regression line coefficients of determination were above 0.9991. Sample recoveries ranged from 92 to 107%, with relative standard deviations below 13% for spiking levels between 0.5 and 5 ng g⁻¹; method quantification limits ranged between 0.04 and 0.12 ng g⁻¹. The developed LC/ESI-MS/MS method, although not as sensitive as LC coupled to fluorescence detection, is rapid, selective, accurate and precise, thus it can be used as confirmatory assay. The MSPDE appears suitable for application to other oleaginous matrices and for multiresidue investigation.     

Properties of aromaticity indices based on the one-electron density matrix

Proper normalization of two previously published indices yields aromaticity measures that, when computed within the Hückel molecular orbital (HMO) approximation, closely match the topological resonance energies per pi electron of aromatic annulenes and their ions. The normalized indices, which quantify aromaticity of individual rings in polycyclic systems, are equally applicable to homocyclic and heterocyclic compounds and can be readily computed from 1-matrices calculated at any level of electronic structure theory. However, only the index ING, derived from the Giambiagi formula, produces proper ordering of aromaticities of heterocyclic compounds, provided it is calculated from all-electron wavefunctions in conjunction with the atoms in molecule (AIM) partitioning. Its values are shown to be strongly affected by electron correlation effects. Because of its apparent inability to distinguish between anti- and nonaromatic systems, ING should only be employed for aromatic species.     

Mapping of possible prion protein self-interaction domains using peptide arrays

Background  

The common event in transmissible spongiform encephalopathies (TSEs) or prion diseases is the conversion of host-encoded protease sensitive cellular prion protein (PrP^C) into strain dependent isoforms of scrapie associated protease resistant isoform (PrP^Sc) of prion protein (PrP). These processes are determined by similarities as well as strain dependent variations in the PrP structure. Selective self-interaction between PrP molecules is the most probable basis for initiation of these processes, potentially influenced by chaperone molecules, however the mechanisms behind these processes are far from understood. We previously determined that polymorphisms do not affect initial PrP^C to PrP^Sc binding but rather modulate a subsequent step in the conversion process. Determining possible sites of self-interaction could elucidate which amino acid(s) or amino acid sequences contribute to binding and further conversion into other isoforms. To this end, ovine – and bovine PrP peptide-arrays consisting of 15-mer overlapping peptides were probed with recombinant sheep PrP^C fused to maltose binding protein (MBP-PrP).