Amination and [2,3]-sigmatropic rearrangement of propargylic sulfides using a ketomalonate-derived oxaziridine: synthesis of N-allenylsulfenimides

Amination of propargylic sulfides with a ketomalonate-derived oxaziridine under metal free conditions gives N-Boc-N-allenylsulfenimides via [2,3]-sigmatropic rearrangement.     

Model systems for flavoenzyme activity: a tuneable intramolecularly hydrogen bonded flavin-diamidopyridine complex

We report the electrochemically tuneable intramolecular hydrogen bonding interactions between a covalently linked flavin-diamidopyridine unit.     

Ferrocene incorporating host-guest dyads with electrochemically controlled three-pole hydrogen bonding properties

We describe the electrochemically controlled hydrogen bonding interactions between the isobutyl flavin/2,6-diferrocenylamidopyridine (2·5) and 9,10-phenanthrenequinone/1-ferrocenyl-3-hexylurea (4·6) dyads. Cyclic and square wave voltammetry studies have shown that the binding efficiencies between these moieties can be electrochemically actuated in non-polar (CH₂Cl₂ for 2·5) or polar (DMF for 4·6) organic solvents between three distinct states.     

Hyperfine field calculations: Search for muon stopping sites in Fe3O4

Muon Spin Rotation (SR) results for magnetite (Fe₃O₄) are analyzed and discussed. At room temperature, a SR signal is observed due to the presence of an internal magnetic field (B_int) at the muon site. External transverse field measurements show that B_int is parallel to the magnetic spin direction, the <111> direction in zero applied field. Calculations of the hyperfine field to pinpoint muon stopping sites in magnetite show that the local field contains supertransfer (covalent) and dipolar field contributions. The implanted muons appear to stop at sites structurally similar to those reported for hematite (-Fe₃O₄), where muon-oxygen bond formation was strongly indicated.Research partially supported by US Department of Energy     

The bound nucleotide of actin.

The extent of actin polymerization has been studied for samples in which the bound nucleotide of the actin was ATP, ADP, or an analog of ATP that was not split (AMPPNP). The equilibrium constants for the addition of a monomer to a polymer end were determined from the concentration of monomer coexisting with the polymer. An analysis of these results concludes that the bound ATP on G-actin provides little energy to promote the polymerization of the actin. AMPPNP was incorporated into F-actin and the interaction of F-actin - AMPPNP with myosin was studied. F-actin - AMPPNP activated the ATPase of myosin to the same extent as did F-actin - ADP. However, the rate of superprecipitation was slower in the case of F-actin - AMPPNP than in the control.     

Hydrogen bonding in redox-modulated molecular recognition. An experimental and theoretical investigation

Two receptors, a diaminotriazine derivative (DAT) and diamidopyridine (DAP), are complementary to the electroactive naphthalimide (N) through three-point hydrogen bonding. The association constants of the two receptors were evaluated for both the fully oxidized and the radical anion forms of N. In the oxidized state, the two receptors displayed identical binding constants. Diamidopyridine, however, lowers the reduction potential of naphthalimide to a far greater extent than does diaminotriazine, indicating a greater affinity for diamidopyridine by naphthalimide in the radical anion form. This behavior was mirrored by EPR experiments that showed small deviations from the hyperfine coupling pattern of N(red) in the presence of DAT, with greater effects seen for the N(red).DAP complex. Computational simulations using the UB3LYP/6-311+G(d,p)//UHF/6-31G(d) hybrid gave theoretical hyperfine constants in good quantitative agreement with the experimental results. Using this correlation, we determined that electrostatics and hydrogen bond polarizability play key roles in controlling redox-modulated molecular recognition.     

Microwave spectra and structures of KrAuF, KrAgF, and KrAgBr; 83Kr nuclear quadrupole coupling and the nature of noble gas-noble metal halide bonding

Microwave spectra of the complexes KrAuF and KrAgBr have been measured for the first time using a cavity pulsed jet Fourier transform microwave spectrometer. The samples were prepared by laser ablation of the metal from its solid and allowing the resulting plasma to react with an appropriate precursor (Kr, plus SF6 or Br2) contained in the backing gas of the jet (usually Ar). Rotational constants; geometries; centrifugal distortion constants; vibration frequencies; and 197Au, 79Br, and 81Br nuclear quadrupole coupling constants have all been evaluated. The complexes are unusually rigid and have short Kr-Au and Kr-Ag bonds. The 197Au nuclear quadrupole coupling constant differs radically from its value in an AuF monomer. In addition 83Kr hyperfine structure has been measured for KrAuF and the previously reported complex KrAgF. The geometry of the latter has been reevaluated. Large values for the 83Kr nuclear quadrupole coupling constants have been found for both complexes. Both the 197Au and 83Kr hyperfine constants indicate a large reorganization of the electron distribution on complex formation. A thorough assessment of the nature of the noble gas-noble metal bonding in these and related complexes (NgMX; Ng is a noble gas, M is a noble metal, and X is a halogen) has been carried out. The bond lengths are compared with sums of standard atomic and ionic radii. Ab initio calculations have produced dissociation energies along with Mulliken populations and other data on the electron distributions in the complexes. The origins of the rigidity, dissociation energies, and nuclear quadrupole coupling constants are considered. It is concluded that there is strong evidence for weak noble gas-noble metal chemical bonding in the complexes.     

Evidence for selectivity of absorption of volatile organic compounds by a polydimethylsiloxane solid-phase microextraction fibre

Solid-phase microextraction using a 30 microns polydimethylsiloxane fibre has been used to sample the volatile organic compounds from standard mixtures and from mixtures produced by the decomposition of organic compounds. This method of sampling has been compared with the direct injection of an aliquot of headspace gas and shows an enrichment factor of approximately 100 over a 1 ml gas injection for organosulphur gases such as dimethyldisulphide. The performance of the fibre has been evaluated with respect to accuracy and precision at several concentrations in representing the composition of multicomponent mixtures. It was found that the presence of a second component in a gas sample reduced the capacity of the fibre to absorb the primary component. The selectivity of the fibre for various volatile compounds with differing functionality was also studied. It was found that the non-polar polydimethylsiloxane fibre preferentially absorbed the non-polar components of a mixture, e.g. nonane and, correspondingly, under reported the more polar components, e.g. ethanol. Hence, the fibre discriminates in favour of non-polar and against polar components in a mixture in comparison with direct analysis of a headspace sample. Thus, quantitation of a component in a multi-component mixture is liable to error from competitive interference from other components. A major advantage of the technique, however, is that it does not absorb, and therefore introduce, water into the analytical system.     

Element-selective detection using capillary gas chromatography with atomic emission detection

Capillary GC coupled to an atomic emission detector (AED) provides a powerful new hyphenated technique for the separation and characterization of complex mixtures and compounds. The AED provides simultaneous and truly specific multi-element detection. The specificity of detection reduces the need for the complex sample pretreatment procedures which are necessary to reduce the interference from co-eluted substances which is experienced with detectors such as the FID and the ECD. A range of environmentally significant problems has been studied, including PCB analysis, the characterization of the reaction products of a novel waste treatment process, and the profiling of sulfur-containing species formed by the pyrolysis of various types of coal.     

Kinetics and mechanisms of the reactions of OH with some oxygenated compounds of importance in tropospheric chemistry

The kinetics of the reactions have been studied in a discharge flow system under pseudo-first-order conditions. The OH concentration was monitored by laser induced fluorescence and helium was used as the carrier gas. Values of k₁ = (8.1 ± 1.7) × 10^?13, k₂ = (1.31 ± 0.26) × 10^?11, k₃ = (2.6 ± 0.5) × 10^?11, and k₄ = (2.5 ± 0.4) × 10^?11 cm³ molecule^?1 s^?1, at 298 K and 1 torr total pressure, were obtained. To validate the newly constructed system the rate constant for the reaction was determined in a similar manner. The value of k₅ = (6.7 ± 0.9) × 10^?12 cm³ molecule^?1 s^?1 at 298 K and 1 torr total pressure is in very good agreement with other literature values. The mechanisms for the atmospheric degradation of these compounds have been constructed to allow their incorporation in a photochemical trajectory computer model, to assess their impact on photochemical ozone creation in the troposphere. © 1994 John Wiley & Sons, Inc.