Toluene dioxygenase-mediated oxidation of dibromobenzenes. Absolute stereochemistry of new metabolites and synthesis of (−)-conduritol E

Dibromobenzenes (o-, m-, and p-isomers) were converted to the corresponding cis-cyclohexadiene diols by whole-cell fermentation with Escherichia coli JM 109 (pDTG601A), an organism over-expressing the enzyme toluene dioxygenase (TDO). Absolute stereochemistry of new metabolites was determined, and (−)-conduritol was synthesized.     

Catalytic vapor-phase oxidation of 2,2,2-trifluoroethanol

The synthesis of trifluoroacetaldehyde by vapor-phase oxidation of 2,2,2-trifluoroethanol using supported vanadium catalysts was studied. Significant differences were observed in the reaction outcomes resulting from different types of catalysts. The ZrO₂- and Al₂O₃-supported catalyst demonstrated both high catalytic activity and selectivity. The addition of co-catalysts such as MoO₃ or SnO₂ improved catalytic performance (Selectivity: up to 91%; S.T.Y.: >200 g L⁻¹ h⁻¹). The experimental results on catalyst lifetime showed a marked decrease in the activity of the Al₂O₃-supported catalyst within tens of hours, while the ZrO₂-supported catalyst showed little, if any, performance alterations for 2000 h.     

Multiresidue method for the determination of residues of 251 pesticides in fruits and vegetables by gas chromatography/mass spectrometry and liquid chromatography with fluorescence detection

A method is described for the determination of 251 pesticide and degradation product residues in fruit and vegetable samples. Extraction of the sample with acetonitrile is followed by a salting-out step. Co-extractives are removed by passing a portion of the acetonitrile extract through an octadecyl (C18) solid-phase extraction cleanup cartridge and then, in a second cleanup, through a carbon cartridge coupled to an amino propyl cartridge. Determination is by gas chromatography with mass-selective detection in the selected-ion monitoring mode, and by liquid chromatography with post-column reaction and fluorescence detection for N-methyl carbamates. The method has been used for analysis of various fruits and vegetables, such as apple, banana, cabbage, carrot, cucumber, lettuce, orange, pear, pepper, and pineapple. Limits of detection range between 0.02 and 1.0 mg/kg for most compounds. Over 80% of the compounds have a limit of detection of < or = 0.04 mg/kg.     

Interpretation of contact angle measurements on two different fluoropolymers for the determination of solid surface tension

Contact angle measurements with a large number of liquids on the semi-fluorinated acryl polymer EGC-1700 films are reported. The surface tension was determined to be gammasv=13.84 mJ/m2 from contact angles of octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS). Inertness of these two liquids makes them ideal for determination of surface tension of low-energy fluoropolymers. On the other hand, contact angles of many other liquids deviated somewhat from a smooth contact angle pattern that represents the EGC-1700 surface tension. It is argued that noninertness of the molecules of these liquids gives rise to specific interactions with the polymer film, causing the deviations. Furthermore, contact angles of a series of n-alkanes (n-hexane to n-hexadecane) showed systematic deviations from this curve, similar to the trend observed for n-alkanes/Teflon AF 1600 systems studied earlier. Adsorption of vapor of short-chain liquids onto the polymer film caused their contact angles to fall above the gammasv=13.84 mJ/m2 curve, and a parallel alignment of molecules of the long-chain n-alkanes in the vicinity of the solid was the explanation for the deviation of their contact angles below it. It is found that vapor adsorption effect is more significant in the case of Teflon AF 1600, while the alignment of liquid molecules close to the surface is more pronounced for EGC-1700.     

Generic implementation of semi-analytical CI gradients for NDDO-type methods

Generic semi-analytical energy gradients are derived and implemented for NDDO-type methods, by using numerical integral and Fock matrix derivatives in the context of an otherwise analytical approach for configuration interaction (CI) and other non-variational treatments. The correctness, numerical precision, and performance of this hybrid approach are established through comparisons with fully numerical and fully analytical calculations. The semi-analytical evaluation of the CI gradient is generally much faster than the fully numerical computation, but somewhat slower than a fully analytical calculation, which however shows the same scaling behavior. It is the method of choice whenever a fully analytical CI gradient is not available due to the lack of analytical integral derivatives. The implementation is generic in the sense that it can easily be extended to any new NDDO-type Hamiltonian. The present development of a semi-analytical CI gradient will facilitate studies of electronically excited states with recently proposed NDDO methods that include orthogonalization corrections. Dedicated to Professor Karl Jug on the occasion of his 65th birthday     

A criterion to demarcate the dual Diels-Alder and σ-complex behaviour of aromatic and heteroaromatic superelectrophiles

A frontier demarcating superelectrophilic compounds that undergo both Diels-Alder reactivity and σ-complexation reactions, from traditional electrophiles, is afforded through the thermodynamic tendency for σ-complexation as defined by the pK_a values for H₂O addition to electron-deficient aromatic and heteroaromatic substrates.     

Tethered 2 + 2 + 2 cycloaddition reactions of cobalt-cycloheptyne complexes

Cycloheptyne-dicobalt hexacarbonyl complexes, substituted by propargylic ether functions, undergo 2 + 2 + 2 cycloaddition reactions with alkynes to give tricyclic benzocycloheptanes; an all-intramolecular version of this transformation is also possible.     

The microstructure and optical transmittance thermal analysis of sodium borosilicate bio-glasses

The conditions to fabricate the bulk porous specimens have been studied on account of sodium borosilicate (NBS) glasses. Glass composition, heat treatment at phase separation and TiO₂ addition have been considered in this study. Original glass samples of composition in mol%: sample A: 9.19 Na₂O - 23.58 B₂O₃ - 67.23 SiO₂, sample B: 9.29 Na₂O - 3.17 TiO₂ - 23.82 B₂O₃ - 63.72 SiO₂ were prepared by melting reagent grade chemicals (Na₂CO₃, HBO₃, SiO₂ and AgNO₃) in platinum crucibles at 1480°C for 1 h in air. The melts were poured onto stainless steel plates and were annealed at 500°C for 0.5 h after cooling. Thus, obtained samples were phase separated at 700°C for 2, 15, 25 and 50 h to study their microstructure by scanning electron microscope (SEM). Besides the direct study of the microstructure by SEM, information on glass structural changes of samples are provided by measuring in situ changes by the optical transmittance thermal analysis. The isothermal measurements were carried out at 700, 720 and 740°C. The temperature of phase separation, the leaching and nucleator addition (TiO₂), significantly influence the microstructure of the resulting leached product. TiO₂ additive seems to suppress crystallization of cristobalite: especially at the extended above heat treatment phase separation runs. The phase-separated domains of glasses containing above 80 moles of SiO₂ are so small that it is very hard to observe them by SEM. The glass composition in our case was selected in a way to have relatively large phase separated areas easily observed by SEM at magnification 20 000·. The influence of TiO₂ is not too pronounced. It seems to suppress the cristobalite crystallization, especially of longer heating runs. The image analysis of leached glasses shows the prevailing content of the skeletal phase in a comparison to pores. The TiO₂ content diminishes the content of the skeletal phase. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemistry of phosphine-borane adducts at platinum centers: synthesis and reactivity of PtII complexes with phosphinoborane ligands

Reaction of [Pt(PEt(3))(3)] with the primary and secondary phosphine-borane adducts PhRPH x BH(3) (R=H, Ph) resulted in oxidative addition of a P-H bond at the Pt(0) center to afford the complexes trans-[PtH(PPhR x BH(3))(PEt(3))(2)] (1: R=H; 2: R=Ph). The products 1 and 2 were characterized by (1)H, (11)B, (13)C, (31)P, and (195)Pt NMR spectroscopy, and the molecular structures were verified by X-ray crystallography. In both cases, a trans arrangement of the hydride ligand with respect to the phosphidoborane ligand was observed. When 2 was treated with PhPH(2) x BH(3), a novel phosphidoborane ligand-exchange reaction occurred which yielded 1 and Ph(2)PH x BH(3). Treatment of 2 with one equivalent of depe (depe=1,2-bis(diethylphosphino)ethane) resulted in the formation of the complex cis-[PtH(PPh(2) x BH(3))(depe)] (3), in which the hydride ligand and the phosphidoborane ligand are in a cis arrangement. Treatment of 3 with PhPH(2) x BH(3) was found to result in an exchange of the phosphidoborane ligands to give the complex cis-[PtH(PPhH x BH(3))(depe)] (4) and Ph(2)PH x BH(3). Complex 4 was found to undergo further reaction in the presence of PhPH(2) x BH(3) to give meso-cis-[Pt(PPhH x BH(3))(2)(depe)] (5) and rac-cis-[Pt(PPhH x BH(3))(2)(depe)] (6).     

Observation of Na(+)-bound oligomers of quercetin in the gas phase

While developing a liquid chromatography/tandem mass spectrometry method for the analysis of the flavonoid quercitin, it was observed that quercetin (3,3',4',5,7-pentahydroxyflavone) exhibited clustering in both the positive and negative ion mode. Two series of positive ion clusters were observed; the first series corresponds to singly charged [2M + Na](+) at m/z 627.2 to [13M + Na](+) at m/z 3947.5, while the second series corresponds to doubly charged [7M + 2Na](2+) at m/z 1080.4 to [25M + 2Na](2+) at m/z 3798.5. In the negative ion mode, the behavior of quercetin parallels that of apigenin (4',5,7-trihydroxyflavone) in that [M + NO(3)](-), [2M + NO(3)](-), and [3M + NO(3)](-) were observed at m/z 364.1, 666.0, and 968.9, respectively; in addition, quercitin clusters with chloride ions ([2M + Cl](-) at m/z 638.9 and [3M + Cl](-) at m/z 940. 9) were observed. The results of tandem mass spectrometric examination of several cluster ions are reported.