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31.
The application of the fundamental equation for isotopic dilution mass spectrometry to real samples often requires using approximations which can lead to significant errors. A new approach relating the composition of isotopic mixtures to the average mass of the mass spectral pattern is presented. The fundamental equations are completely general; they should be applicable to a wide range of problems and may provide improved accuracy and precision in some cases.  相似文献   
32.
The l-azabicyclo[2.2.1]heptan-3-exo-ol ( 2 ) was resolved by fractional crystallisation of its hydrogen tartrate salts. The enantiomers (+)- and (?)- 2 were oxidised to the ketones (?)- 4 and (+)- 4 , respectively (Scheme). CD spectroscopy suggested that (?)- 4 possesses the (1R,4S)-configuration. This absolute configuration was confirmed by single-crystal X-ray diffraction of the derivative (+)-(1R,4R)-3-(1,3-dithian-2-ylidene)-1-azabicyclo [2.2.1]-heptane ((+)- 5 ).  相似文献   
33.
Factor F430 from Methanogenic Bacteria: Structure of the Protein-free Factor Factor F430, the porphinoid nickel-containing coenzyme of the methylcoenzyme-M reductase of metanogenic bacteria is shown to be the 33,83,122,133,182-pentaacid derivative of the pentamethylester F430M, the structure of which had been determined previously (see structural formulae 1 and 2 ). The structure assignment rests on chromatographic, UV/VIS-, CD-, IR-, and 13C-NMR-spectroscopic as well as FAB-mass spectral comparision of F430 with F430M and the pentaacid prepared by acid-catalyzed hydrolysis of F430M. In the cells of Methanobacterium thermoautotrophicum, factor F430 is present in a ‘bound’ and also, depending on the growth conditions, in ‘free’ form, the latter being defined as the part of total F430 that can be extracted from the cells under extremely mild conditions (80% EtOH at 0–4°). From the (protein)-‘bound’ form, F430 is extracted by subsequently treating the cells at 0–4° with 80% EtOH containing (e.g.), 2m LiCi. From both sources, the extracted factor is the same pentaacid, and there is no indication for the existence of a protein-free F430 species that would contain additional (covalently bound) structural elements.  相似文献   
34.
The selectivity-determining step in enantioselective copper-catalyzed cyclopropanation with diazo compounds has been studied by experimental and computational methods. The addition of the very reactive metallacarbene intermediate in an early transition state to the substrate alkene is concerted but strongly asynchronous, with substantial cationic character on one alkene carbon in the neighborhood of the transition state. Evidence from isotope effects and Hammett studies supports the nature of the transition state. Formation of a metallacyclobutane intermediate by a [2+2] addition is kinetically disfavored. Ligand-substrate interactions influencing the enantio- and diastereoselectivity have been identified, and the preferred orientation of the alkene substrate during the addition is suggested.  相似文献   
35.
Huinink KD  Venema K  Roelofsen H  Korf J 《The Analyst》2005,130(8):1168-1174
Frequent in vivo sampling of blood proteins is often stressful, making it difficult to obtain more than a few samples. As a result, only limited time-profiles can be made. We have developed an ultrafiltration collection device (UCD) for continuous sampling. The UCD consists of a hollow fiber, a coil and a flow creator. Hollow fiber membranes are often hydrophobic and this can result in adsorption of protein and/or peptides, leading to clogged membranes. Adsorption was tested with a hydrophobic and hydrophilic peptide and two biocompatible hollow fibers made from different materials. The hollow fiber made from poly(ethylene) coated with ethylenevinyl alcohol gave near 100% recovery for both peptides. This was in contrast to the poly(sulfone) hollow fiber when sampling the hydrophobic peptide. Filling the coil with various peptide concentrations gave good recovery and insignificant diffusion even after storage for 6 d at 37 degrees C. Continuous pulse-free sampling was tested by vacuum. An average flow rate of 423 +/- 50 nl min(-1) over a period of 4 d was created using S-Monovette. The flow rate gradually declined during this period by <5% every consecutive day. In addition, we also examined a complex sample-serum in the poly(ethylene) hollow fiber. Serum and ultrafiltrate were spotted onto a protein chip and analyzed by surface-enhanced laser desorption/ionization time-of-flight mass spectrometry (SELDI-TOF-MS). Six proteins out of 64 were found to be significantly different between serum and the ultrafiltrate (p < 0.05). The UCD has the potential to be used for in vivo real-time monitoring.  相似文献   
36.
Operational Research (OR) is the science of decision-making. From its military origins on the eve of World War II, OR has over the past seven decades matured to become a discipline that is recognised worldwide for its contributions to managerial planning and complex operations on all levels within both private companies and public institutions. Besides being an indispensable tool as a means for decision support, OR is today a well-established academic discipline and a field with its own institutions. Thus, OR-professionals are joined in national societies worldwide, assembled since 1959 in the global organization International Federation of OR Societies (IFORS) which again is subdivided into four Regional Groupings. Among those is the Association of European OR Societies (EURO) having as members the national societies of 31 countries notably in Europe. Two questions will be addressed: what is OR all about? How do National OR societies fit within the frameworks of IFORS and EURO? Partial answers are provided to both.  相似文献   
37.
38.
Optical properties of nanomaterials such as semiconductor and metal quantum dots are important for sensors and photovoltaic applications. We report on optical, microscopic, and AFM investigations on bulk and single nanoobjects such as metal and semiconducting nanoparticles. Firstly, of special interest is the investigation of Ag metal nanoaggregates formed in zeolites. Here, the defined structure of the zeolite serves both as size directing and a stabilizing agent. The size selected Ag aggregates fluoresce in the zeolite cages even after storage under ambient conditions for almost one year. In addition, single Ag particles escape the cages and can be investigated by fluorescence microscopy also with respect to sensor applications. Secondly, with respect to photovoltaic applications, energy transfer among organic dye molecules and semiconductor quantum dots is of great importance. We report on the extension of the optical absorption of ZnSe quantum dots into the UV regime and investigate excitation energy transfer within self-assembled nanoaggregates of surface functionalized QDs and fluorescent styrylpyridine dyes.  相似文献   
39.
NIFTy , “Numerical Information Field Theory,” is a software framework designed to ease the development and implementation of field inference algorithms. Field equations are formulated independently of the underlying spatial geometry allowing the user to focus on the algorithmic design. Under the hood, NIFTy ensures that the discretization of the implemented equations is consistent. This enables the user to prototype an algorithm rapidly in 1D and then apply it to high‐dimensional real‐world problems. This paper introduces NIFTy  3, a major upgrade to the original NIFTy  framework. NIFTy  3 allows the user to run inference algorithms on massively parallel high performance computing clusters without changing the implementation of the field equations. It supports n‐dimensional Cartesian spaces, spherical spaces, power spaces, and product spaces as well as transforms to their harmonic counterparts. Furthermore, NIFTy  3 is able to handle non‐scalar fields, such as vector or tensor fields. The functionality and performance of the software package is demonstrated with example code, which implements a mock inference inspired by a real‐world algorithm from the realm of information field theory. NIFTy  3 is open‐source software available under the GNU General Public License v3 (GPL‐3) at https://gitlab.mpcdf.mpg.de/ift/NIFTy/tree/NIFTy_3 .  相似文献   
40.
When a high-voltage direct-current is applied to two beakers filled with polar liquid dielectrica like water or methanol, a horizontal bridge forms between the two beakers. By repeating a version of Pellat’s experiment, it is shown that a horizontal bridge is stable by the action of electrohydrodynamic pressure. Thus, the static and dynamic properties of the phenomenon called a ‘floating water bridge’ can be explained by the gradient of Maxwell pressure, replenishing the liquid within the bridge against any drainage mechanism. It is also shown that a number of liquids can form stable and long horizontal bridges. The stability of such a connection, and the asymmetry in mass flow through such bridges caused by the formation of ion clouds in the vicinity of the electrodes, is also discussed by two further experiments.  相似文献   
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