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81.
Origin of summating potential   总被引:2,自引:0,他引:2  
  相似文献   
82.
83.
The rapid, stereoselective, and specific transfer reduction of alkynes to alkenes using specially-modified, but easily prepared, palladium catalysts is described. Additional use of a phase transfer catalyst in these systems affords a simple method for phase transfer catalysed catalytic transfer reduction which can be carried out in the presence of other potentially reducible functional groups.  相似文献   
84.
The thermal lens apparatus as usually designed for energy transfer and thermal conduction studies, has been used to measure the thermal conductivity coefficients of ethylene and ethyl chloride (2.2 × 10?2 and 3.1 × 10?2 J m?1 s?1 K?1). Contrary to previous analyses, a relaxation model based on the IR laser beam diameter must be used in order to get order of magnitude agreement between these data and literature values. The importance of Fresnel diffraction in greatly limiting the accuracy of the above data is determined and it is shown that the traditional design of apparatus is thus wholly unsuitable for a simple qualitative model. A new apparatus design is suggested which is amenable to simple quantitative as well as qualitative interpretation.  相似文献   
85.
The structure and absolute configuration of the marine antithrombotic product dysinosin A was confirmed by total synthesis. The strategy involved disconnections to three subunits, of which two were synthesized from the readily available l-glutamic acid, d-leucine, and d-mannitol. The Grubbs olefin metathesis carbocyclization reaction was utilized to prepare two intermediates.  相似文献   
86.
The high nutrient concentrations that would exist near the nutrient injection well during the application of cometabolicin situ bioremediation may lead to the development of significant quantities of biomass at this point in the subsurface. This biomass can decrease the porosity of the soil to such an extent that nutrient injection is no longer possible. In this work, experiments were conducted using a porous media biofilm reactor, operated under constant substrate loading conditions, such that the pressure drop across the reactor was allowed to increase to maintain a constant volumetric flow rate through the reactor. Results suggest that biomass production, and hence biofilm thickness, near the injection feed port is highly sensitive to substrate loading. In addition, these variations in biofilm thickness produce dramatic differences in the pressure drop that is attained across the reactor. Use of the Kozeny-Carman equation can be used to predict that once a critical depth has been exceeded, the pressure drop across the bed will increase exponentially within biofilm depth. This result means that pressure is not a reliable indicator of the onset of pore plugging.  相似文献   
87.
88.
We present a concept of openness for geometric morphisms between toposes, which extends the usual notion of open map of spaces, and investigate some of its properties. Our two main results are that open maps are precisely those whose inverse image functors preserve full first-order logic, and that open maps and open surjections are stable under bounded pullback in Top. These two results extend earlier work of C. J. Mikkelsen [11] and M. Coste [3] respectively; they are also closely related to recent work of A. Joyal and M. Tierney [9].  相似文献   
89.
A trapping reagent for formaldehyde, based on the pararosaniline reaction, was evaluated as a method of determination of formaldehyde in the aqueous or vapour phase. Collection of formaldehyde vapour relied upon passive diffusion of formaldehyde into the trapping media and quantitative results were obtained without the need for liquid impingers, bubblers or active sampling pumps. Moreover, a novel, hand-held absorption spectrophotometric measurement device was designed to provide on-site, quantitative measurements. It is proposed that the full measurement system devised would be ideally suited to specific sampling applications such as those found in museum enclosures.  相似文献   
90.
Ferric complexes of triscatechol siderophores may assume one of two enantiomeric configurations at the iron site. Chirality is known to be important in the iron uptake process, however an understanding of the molecular features directing stereospecific coordination remains ambiguous. Synthesis of the full suite of (DHBL/DLysL/DSer)3 macrolactone diastereomers, which includes the siderophore cyclic trichrysobactin (CTC), enables the effects that the chirality of Lys and Ser residues exert on the configuration of the Fe(iii) complex to be defined. Computationally optimized geometries indicate that the Λ/Δ configurational preferences are set by steric interactions between the Lys sidechains and the peptide backbone. The ability of each (DHBL/DLysL/DSer)3 diastereomer to form a stable Fe(iii) complex prompted a genomic search for biosynthetic gene clusters (BGCs) encoding the synthesis of these diastereomers in microbes. The genome of the plant pathogen Dickeya chrysanthemi EC16 was sequenced and the genes responsible for the biosynthesis of CTC were identified. A related but distinct BGC was identified in the genome of the opportunistic pathogen Yersinia frederiksenii ATCC 33641; isolation of the siderophore from Y. frederiksenii ATCC 33641, named frederiksenibactin (FSB), revealed the triscatechol oligoester, linear-(DHBLLysLSer)3. Circular dichroism (CD) spectroscopy establishes that Fe(iii)–CTC and Fe(iii)–FSB are formed in opposite enantiomeric configuration, consistent with the results of the ferric complexes of the cyclic (DHBL/DLysL/DSer)3 diastereomers.

Synthesis and genome mining leverage access to diastereomers of the siderophore cyclic trichrysobactin. Computational modeling and CD spectroscopy address effects that ligand-based stereochemistry exerts on the configuration at Fe(iii).  相似文献   
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