Automatic ignition of the sample vs. use of the bunsen burner in the rapid silver absorbent method for sulfur

Summary Comparable series of sulfur determinations by the silver absorbent method were performed on four test samples. One series was carried out with automatic ignition using a stationary electric sample burner during a constant time period for each analysis; the other series by igniting the sample with a Bunsen burner held in the hand until the last trace of sample disappeared.There was no obviously assured difference in the quality of the analytical figures obtained by the two methods of sample ignition. Statistical analysis of the data, however, disclosed that the automatic burner did actually give a slight improvement in precision over the hand burner although the ultimate limit of accuracy was the same in each case.

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Zusammenfassung Mit vier reinen Verbindungen wurden Reihenbestimmungen des Schwefels durch Verbrennung und Absorption des Schwefeltrioxyds an Silberwolle so ausgeführt, daß eine Reihe von Verbrennungen mit dem von Hand geführten Bunsenbrenner immer einer entsprechenden Reihe von Bestimmungen gegenübergestellt werden konnte, bei der die Verbrennung mit einem ruhenden, automatisch regulierten elektrischen Heizkörper erfolgte.Die Resultate der automatischen Verbrennungen zeigten eine so geringe Verbesserung gegenüber jenen, die durch Verbrennung mit dem handregulierten Bunsenbrenner erhalten wurden, daß der augenscheinliche Unterschied lediglich dem Zufall zugeschrieben werden könnte. Eine statistische Untersuchung zeigte, daß die automatische Verbrennung tatsächlich eine geringfügige Verbesserung der Präzision gibt, ohne jedoch die Genauigkeit der arithmetischen Mittel der Versuchsreihen merkbar zu ändern.

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Résumé On a effectué des déterminations de soufre en séries comparables par la méthode d'absorption par l'argent sur quatre échantillons types. L'une des séries comportait une combustion automatique à l'aide d'un brûleur électrique fixe pendant un temps constant pour chaque analyse. L'autre série comportait l'emploi d'un becBunsen conduit à la main pour la combustion de l'échantillon jusqu'à disparition de ses dernières traces.On n'a constaté aucune différence marquée dans la qualité des résultats analytiques obtenus par l'une ou l'autre méthode de combustion de l'échantillon. L'analyse statistique des résultats obtenus fait cependant apparaître une légère amélioration de la précision lorsque le brûleur automatique est substitué au brûleur à main bien que les écarts extrêmes obtenus soient les mêmes pour chacun des cas.

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To Prof. Dr.Hans Lieb at his 70th birthday.     

Gas-phase interaction of H2S with O2: A kinetic and quantum chemistry study of the potential energy surface

Quantum chemical calculations were carried out to study the interaction of hydrogen sulfide with molecular oxygen in the gas phase. The basic mechanism, the rates of reaction, and the potential energy surface were calculated. Isomers and transition states that connect the reactants with intermediates and products of reaction were identified using the G2 method and B3LYP/6-311+G(3df,2p) functional. Hydrogen abstraction to form HO2 + SH is the dominant product channel and proceeds through a loose transition state well-described at the level of calculation employed. The temperature dependence of the rate coefficient in the range 300-3000 K has been determined on the basis of the ab initio potential energy surface and with variational transition-state theory. The reaction is 169.5 kJ mol(-1) endothermic at 0 K with a rate constant given by 2.77 x 10(5) T(2.76) exp(-19 222/T) cm3 mol(-1) s(-1) and should proceed slowly under atmospheric thermal conditions, but it offers a route to the initiation of H2S combustion at relatively low temperatures.     

One site fits both: A model for the ternary complex of folate + NADPH in R67 dihydrofolate reductase, a D2 symmetric enzyme

R67 dihydrofolate reductase (DHFR) is a novel enzyme that confers resistance to the antibiotic trimethoprim. The crystal structure of R67 DHFR displays a toroidal structure with a central active-site pore. This homotetrameric protein exhibits 222 symmetry, with only a few residues from each chain contributing to the active site, so related sites must be used to bind both substrate (dihydrofolate) and cofactor (NADPH) in the productive R67 DHFR?NADPH?dihydrofolate complex. Whereas the site of folate binding has been partially resolved crystallographically, an interesting question remains: how can the highly symmetrical active site also bind and orient NADPH for catalysis? To model this ternary complex, we employed DOCK and SLIDE, two methods for docking flexible ligands into proteins using quite different algorithms. The bound pteridine ring of folate (Fol I) from the crystal structure of R67 DHFR was used as the basis for docking the nicotinamide-ribose-P_i (NMN) moiety of NADPH. NMN was positioned by both DOCK and SLIDE on the opposite side of the pore from Fol I, where it interacts with Fol I at the pore's center. Numerous residues serve dual roles in binding. For example, Gln 67 from both the B and D subunits has several contacts with the pteridine ring, while the same residue from the A and C subunits has several contacts with the nicotinamide ring. The residues involved in dual roles are generally amphipathic, allowing them to make both hydrophobic and hydrophilic contacts with the ligands. The result is a `hot spot' binding surface allowing the same residues to co-optimize the binding of two ligands, and orient them for catalysis.     

In-lens cryo-high resolution scanning electron microscopy: methodologies for molecular imaging of self-assembled organic hydrogels.

The micro- and nanoarchitectures of water-swollen hydrogels were routinely analyzed in three dimensions at very high resolution by two cryopreparation methods that provide stable low-temperature specimens for in-lens high magnification recordings. Gemini surfactants (gS), poly-N-isopropylacrylamides (p-NIP Am), and elastin-mimetic di- (db-E) and triblock (tb-E) copolymer proteins that form hydrogels have been routinely analyzed to the sub-10-nm level in a single day. After they were quench or high pressure frozen, samples in bulk planchets were subsequently chromium coated and observed at low temperature in an in-lens field emission SEM. Pre-equilibrated planchets (4-40 degrees C) that hold 5-10 microl of hydrogel facilitate dynamic morphological studies above and below their transition temperatures. Rapidly frozen samples were fractured under liquid nitrogen, low-temperature metal coated, and observed in-lens to assess the dispersion characteristics of micelles and fragile colloidal assemblies within bulk frozen water. Utilizing the same planchet freezing system, the cryoetch-HRSEM technique removed bulk frozen water from the hydrogel matrix by low-temperature, high-vacuum sublimation. The remaining frozen solid-state sample faithfully represented the hydrogel matrix. Cryo- and cryoetch-HRSEM provided vast vistas of hydrogels at low and intermediate magnifications whereas high magnification recordings and anaglyphs (stereo images) provided a three-dimensional prospective and measurements on a molecular level.     

Reactions of CF3-enones with ethyl nitroacetate and nitromethane: synthesis of CF3-γ-nitroketones

Reactions of α,β-unsaturated CF₃-ketones with nitromethane and ethyl nitroacetate have been investigated. We found that α,β-unsaturated trifluoromethylketones react with ethyl nitroacetate in the presence of calcinated potassium fluoride to form two classes of 1,4-conjugated addition products: CF₃-γ-nitroketones and 6,6,6-trifluoro-2-nitro-5-oxohexanoates in nearly quantitative yields. The products obtained are precursors for CF₃-pyrrolidine synthesis.     

Determination of thiabendazole in fruit juices by a new monoclonal enzyme immunoassay

A competitive, indirect enzyme-linked immunosorbent assay (ELISA) for thiabendazole has been developed and applied to the analysis of fruit juices spiked with this fungicide. The immunoassay is based on a new monoclonal antibody derived from a hapten functionalized at the nitrogen atom in the 1-position of the thiabendazole structure. To our knowledge, such a structure has not been previously used to obtain antibodies to thiabendazole. The I50 value and the detection limit of the ELISA for standards were 0.2 and 0.05 ng/mL, respectively. Fruit juices were analyzed by diluting samples in assay buffer, without extraction or cleanup. Samples were not even centrifuged or filtered to remove fruit pulp. Under these conditions, the immunoassay was able to accurately determine thiabendazole down to 1 ng/mL in orange and grapefruit juices, down to 5 ng/mL in banana juice, and down to 20 ng/mL in apple and pear juices. Sensitivity differences of the ELISA were caused by the minimum dilution required by each juice to minimize matrix effects: 1/10 for orange and grapefruit juices, 1/50 for banana juice, and 1/100 for apple and pear juices. In an attempt to further increase the sensitivity of the immunoassay for matrixes showing the strongest interferences, apple and pear juices spiked with thiabendazole at low levels (1-20 ng/mL) were extracted with ethyl acetate before analysis. This simple procedure entailed a significant reduction of matrix effects, which in fact allowed us to determine accurately as low as 5 ng/mL thiabendazole in apple and pear juices. Irrespective of whether samples were analyzed by the direct dilution method or after extraction, the simplicity, sensitivity, and sample throughput of this monoclonal immunoassay makes it a very convenient method for the routine monitoring of thiabendazole residues in fruit juices.     

A mixed‐valence chloride‐bridged (pincer)IrIII–(diene)IrI complex

The title compound, [2,6‐bis(di‐tert‐butylphosphino)phenyl‐1κ³P,C¹,P′]di‐μ‐chlorido‐1:2κ⁴Cl:Cl‐(2η⁴‐cycloocta‐2,5‐diene)hydrido‐1κH‐diiridium(I,III) hexane hemisolvate, [Ir₂(C₈H₁₂)(C₂₄H₄₃P₂)Cl₂H]·0.5C₆H₁₄ or [(^tBuPCP)IrH(μ₂‐Cl)₂Ir(COD)][^tBuPCP is κ³‐2,6‐(^tBu₂PCH₂)₂C₆H₃ and COD is η⁴‐2,5‐cyclooctadiene], is an Ir^III/Ir^I dimer bridged by two chloride ions. The Ir₂Cl₂ framework is nearly planar, with a dihedral angle of 13.04 (4)° between the two Ir centers. The compound was isolated as a hexane hemisolvate. A list of distances found in Ir(PCP) compounds is given.     

Experimental and theoretical studies of charge transfer and deuterium ion transfer between D2O+ and C2H4

The charge transfer and deuterium ion transfer reactions between D(2)O(+) and C(2)H(4) have been studied using the crossed beam technique at relative collision energies below one electron volt and by density functional theory (DFT) calculations. Both direct and rearrangement charge transfer processes are observed, forming C(2)H(4) (+) and C(2)H(3)D(+), respectively. Independent of collision energy, deuterium ion transfer accounts for approximately 20% of the reactive collisions. Between 22 and 36 % of charge transfer collisions occur with rearrangement. In both charge transfer processes, comparison of the internal energy distributions of products with the photoelectron spectrum of C(2)H(4) shows that Franck-Condon factors determine energy disposal in these channels. DFT calculations provide evidence for transient intermediates that undergo H/D migration with rearrangement, but with minimal modification of the product energy distributions determined by long range electron transfer. The cross section for charge transfer with rearrangement is approximately 10(3) larger than predicted from the Rice-Ramsperger-Kassel-Marcus isomerization rate in transient complexes, suggesting a nonstatistical mechanism for H/D exchange. DFT calculations suggest that reactive trajectories for deuterium ion transfer follow a pathway in which a deuterium atom from D(2)O(+) approaches the pi-cloud of ethylene along the perpendicular bisector of the C-C bond. The product kinetic energy distributions exhibit structure consistent with vibrational motion of the D-atom in the bridged C(2)H(4)D(+) product perpendicular to the C-C bond. The reaction quantitatively transforms the reaction exothermicity into internal excitation of the products, consistent with mixed energy release in which the deuterium ion is transferred in a configuration in which both the breaking and the forming bonds are extended.     

Parallel separations of oligonucleotides with optically gated sample introduction on multichannel microchips

With the release of the human genome sequence, there has been increasing attention given to other genetic analyses, including the detection of genetic variations and fast sequencing of multiple samples for pharmacogenomics studies. Rapid injections of samples in multiplexed separation channels by optically gated sample introduction are shown here for DNA separation. Serial separations of four amino acids are shown in less than four seconds on a microchip with four multiplexed channels. Five short oligonucleotides have also been rapidly separated in 2% LPA with four channels using this technique. In addition, multiple unique samples have been simultaneously separated and five-base resolution has been demonstrated.     

A novel effective electrophile: β-trifluoroacetylketene diphenyldithioacetal S,S-tetroxide

A synthesis of a novel electrophilic reagent--β-trifluoroacetylketene diphenyldithioacetal S,S-tetroxide 3 is described. The reaction of 3 with various electron-rich aromatics such as 1,3-dimethoxybenzene, 2-methylthiophene, N-methylpyrrole, and 2-methylindole was investigated. In the course of these reactions an unusual migration of a phenylsulfonyl group takes place. An easy and ready for scale-up procedure for the synthesis of previously unknown β-aryl, α-phenylsulphonyl substituted α,β-unsaturated trifluoromethyl ketones is reported.