On the “Wolfsberg–Helmholz Conjecture” of “Extended‐Hückel Theory”

After having reviewed some pioneer integral approximations closely related to Rüdenberg's expansions of one‐ and two‐electron orbital products, we apply the previously described “Implicit Multi‐Center Integration” techniques on Roothaan's “restricted” Fock‐matrix components over standard atomic orbital bases. The resulting compact forms are very similar to the well‐known “Wolfsberg–Helmholz Conjecture” of “Extended‐Hückel Theory,” which relates the various off‐diagonal matrix elements of “restricted” Fock‐type to their corresponding diagonal counterparts. In this way, a “nonempirical Extended‐Hückel Theory” can be created. © 2012 Wiley Periodicals, Inc.     

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Ohne Zusammenfassung     

(1S,4R,5R,6R)‐6‐Methoxycarbonyl‐4‐pivaloyloxy‐2‐(pivaloyloxymethyl)bicyclo[3.1.0]hex‐2‐ene‐1‐carboxylic acid

The stereochemistry of the title compound, C₂₀H₂₈O₈, a key step in the preparation of analogues of mannostatins, potent inhibitors of α‐mannosidase, has been established. The carboxyl­ic acid group at C1 unexpectedly eclipses the C1—C2 bond. The cyclo­propane ring makes a dihedral angle of 109.4 (2)° with the cyclo­pentene ring.     

Detection of trace radioisotopes in soil,sediment and vegetation by glow discharge mass spectrometry

The possibility for the determination of some radioisotopes of cesium, strontium, plutonium, uranium and thorium by glow discharge mass spectrometry (GDMS) in soils, sediments and vegetations is investigated. The preparation of samples is described as a combination of the use of a conductive host matrix and a secondary cathode in order to decrease the dilution effect of the blending material for the trace level determination and to gain a stable discharge. Effects of interferences arising from the nature of the conductive host matrix and of the secondary cathode on the sensitivity of the method are discussed. The determination of (137)Cs and (90)Sr has been attempted and the results obtained were in agreement with those from other analytical techniques. Accuracy, internal and external precisions have been also evaluated. GDMS is shown to be a helpful technique for the determination of radioisotopes in environmental samples. Radioisotopes can be determined according to the matrix of the sample (e.g. grass), also in presence of isobaric interferences. However, limitations still exist on the application of GDMS.     

Electrophoresis and Sedimentation of Charged Fibers

The electrophoretic translational and rotational velocities of a fiber are determined in the limitsL/d≥≥ 1 and λ/d≥≥ 1, where λ is the double layer thickness andLanddare the fiber length and diameter, respectively. The fiber translates due to the imposed electric field at a rate that increases with increasing double layer thickness. A nonuniformly charged fiber with a net charge dipole rotates until it becomes aligned with the electric field. The results for the translational and rotational velocities are obtained in a semianalytic form. The effect of the deformation of the ion cloud surrounding a charged sedimenting fiber on the fiber's translational and rotational velocities is also determined. The ion cloud retards the sedimentation velocity relative to that of an uncharged fiber with the same orientation. If the fiber possesses both a net charge and a charge dipole, it will rotate into a vertical alignment in which the end with the larger absolute charge is on top.     

Azithromycin extraction from municipal wastewater and quantitation using liquid chromatography/mass spectrometry

Azithromycin is a broad spectrum antimicrobial agent that is approved in the United States for use in humans. Due to azithromycin's low rate of metabolism it is likely to be found in wastewater treatment plants, where its broad spectrum of antimicrobial activity could lead to development of resistance in bacteria. A liquid-liquid extraction using K2CO3 and methyl-t-butyl ether (MTBE) was used to extract azithromycin from 10 ml aliquots of wastewater. Liquid chromatography was performed using a Luna C18(2) (30 mm x 2.0 mm) column with a mass spectrometer friendly mobile phase containing 50:24:2:24 acetonitrile, methanol, tetrahydrofuran, and 0.04 M ammonium hydroxide. The mass spectrometer used an electrospray source with positive ionization and an ion trap detector. A linear standard curve from 5 to 200pg/ml was validated and used to quantitate azithromycin in wastewater.     

Inter-laboratory comparison of methods used for analysing polycyclic aromatic hydrocarbons (PAHs) in soil samples

Summary In view of the investigation of contaminated model sites in Baden-Württemberg an inter-laboratory comparison was organized for the methods used for analysing polycyclic aromatic hydrocarbons (PAHs). The laboratories performing chemical and physical analyses at the model sites participated at the request of the Landesanstalt für Umweltschutz, Baden-Württemberg. This inter-laboratory comparison was meant as a first laboratory evaluation, and not as the search for the best method available. Furthermore, it was to show the difficulties in comparing results from different laboratories, which have analysed identical samples. The results of the inter-laboratory comparison between the nine laboratories showed a reproducibility coefficient of variation, for the total of the 16 PAHs according to the EPA, between 48.8 and 61.1% in the concentration range of 5 to 55 mg/kg in the soil samples. One of the laboratories reported significantly lower values compared with the true value. This was probably due to an ill-suited (sample) preparation method. Although the results of the inter-laboratory comparison are, in general, satisfactory, the reproducibility can be improved by standardizing the analytical method for the determination of PAHs in solid environmental samples.     

Sampling and analysis of nanoparticles with cold fibre SPME device

A new approach is described to capture nano‐size aerosols on internally‐cooled micro tubing of the solid‐phase microextraction (SPME) device followed by convenient introduction of the collected analytes into analytical instrument. Particles were generated using an aerosol formation by homogeneous nucleation of an organic vapor, and subsequent growth to nano‐size particles by coagulation of decanedioic acid, bis[2‐ethylhexyl] ester (DEHS). The approach was validated by using carbon dioxide‐cooled micro tubing to collect the nanosize DEHS particles followed by analyses on GC‐flame ionization detector (FID). Particle size ranged from 150 to 590 nm. Temperature difference between the SPME device and DEHS particles mixture created a temperature gradient and resulted in thermophoretic effect that was determining the extraction rate. SPME device was cooled to as low as –75°C, while the DEHS remained close to room temperature. Several aspects of nanoparticle sampling were tested to demonstrate the principle of the sampling approach. These included the effects of thermal gradient, sample flow rate, sampling time, CO₂ delivery mode (constant coolant delivery vs. constant temperature), and particle size. Results were normalized to measure particulate concentrations using direct sampling with PTFE filters. Nanoparticle extractions of DEHS mass were proportional to sampling time. Normalized mass of DEHS extracted increased with increase in temperature gradient and with increase of the cross flow velocity. Preliminary results indicate that the variation of heat transfer boundary layer caused by the variation in the cross flow velocity produce self‐compensating effect at constant coolant delivery, indicating that this approach could be used for field determinations including the time‐weighted average sampling of nanoparticles. Thus, it may be possible to develop simple device based on this concept for field applications.     

Generation of free radicals by emodic acid and its [D-Lys6]GnRH-conjugate

In an attempt to develop an efficient chemotherapeutic agent targeted at malignant cells that express receptors to gonadotropin releasing hormone (GnRH) we coupled [D-Lys6]GnRH covalently to an emodin derivative, i.e. emodic acid (Emo) to yield [D-Lys6(Emo)]GnRH. Emodin is a naturally occurring anthraquinone which is widely used as a laxative and has other versatile biological activities. Physico-chemical studies employing electron paramagnetic resonance and electrochemistry of the conjugate as well as the (Emo) moiety showed that these compounds could be easily reduced either chemically, photochemically or enzymatically to their corresponding semiquinones. In the presence of oxygen the semiquinones generated reactive oxygen species (ROS), mainly superoxide and hydroxyl radicals, which were detected by the spin trapping method. Moreover, upon irradiation with visible light these compounds produced ROS and a highly reactive excited triplet state of Emo, which by itself may cause the oxidation of certain electron acceptors such as amino acids and bases of nucleic acids. Thus, [D-Lys6]GnRH-photosensitizer conjugates may be potentially used for targeted photodynamic chemotherapy aimed at treating cancer cells that carry GnRH receptors. These conjugates may also induce cytotoxicity in the dark similar to common conventional chemotherapeutic agents. The peptidic moiety, [D-Lys6]GnRH, was found to be stable toward highly reactive ROS generated either from enzymatic reduction or upon photoirradiation. The physico-chemical properties of Emo were only marginally influenced by the peptidic [D-Lys6]GnRH carrier.     

Strained heterocyclic systems. VII. The mass spectral fragmentations of dihydrocycloalka[b]quinolines and quinoline N-Oxides

High resolution mass spectra of compounds 1-6 were investigated. The quinolines ( 1-3 ) all exhibit a prominent (M-1)⁺ peak and subsequent loss of HCN. These processes are consistent with azatropylium ion intermediates. The N-oxides ( 4-6 ) all exhibit major peaks at (M-16)⁺; the abundance of the latter is related to the geometry of the molecule. The four-membered ring compounds (3 and 6) give more complex spectra, which reflect the influence of the fused strained ring.