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991.
Andreas Thomas Simon Beuck Jens Christian Eickhoff Sven Guddat Oliver Krug Matthias Kamber Wilhelm Schänzer Mario Thevis 《Analytical and bioanalytical chemistry》2010,396(8):2899-2908
Influencing the endurance in elite sports is one of the key points in modern sports science. Recently, a new class of prohibited
substances reached in the focus of doping control laboratories and their misuse was classified as gene doping. The adenosine
monophosphate activated protein kinase activator 5-amino-4-imidazolecarboxyamide ribonucleoside (AICAR) was found to significantly
enhance the endurance even in sedentary mice after treatment. Due to endogenous production of AICAR in healthy humans, considerable
amounts were present in the circulation and, thus, were excreted into urine. Considering these facts, the present study was
initiated to fix reference values of renally cleared AICAR in elite athletes. Therefore a quantitative analytical method by
means of isotope-dilution liquid chromatography (analytical column: C6-phenyl) coupled to tandem mass spectrometry, after
a sample preparation consisting of a gentle dilution of native urine, was developed. Doping control samples of 499 athletes
were analysed, and AICAR concentrations in urine were determined. The mean AICAR value for all samples was 2,186 ng/mL with
a standard deviation of 1,655 ng/mL. Concentrations were found to differ depending on gender, type of sport and type of sample
collection (in competition/out of competition). The method was fully validated for quantitative purposes considering the parameters
linearity, inter- (12%, 7% and 10%) and intraday precision (14%, 9% and 12%) at low, mid and high concentration, robustness,
accuracy (approx. 100%), limit of quantification (100 ng/mL), stability and ion suppression effects, employing an in-house
synthesised 13C5-labelled AICAR as internal standard. 相似文献
992.
Two-dimensional ultrasound Doppler velocimeter for velocity field measurements of liquid metal flows
Sven Franke Andreas Fischer Lars Büttner Jürgen Czarske Dirk Räbiger Sven Eckert 《PAMM》2011,11(1):649-650
We present a novel fluid flow measurement system based on the pulsed-wave ultrasound Doppler velocimetry being able to determine two-dimensional velocity fields. It applies for the measurement of unsteady liquid metal flows driven by electromagnetic forces concerning the research field of magnetohydrodynamics. The application of advanced processing techniques enable high data acquisition rates and concurrently a high spatial resolution facilitating to resolve transient liquid metal flow structures which could not been acquired so far. An experimental setup utilizing liquid metal in a cubic vessel exposed to a stationary rotating magnetic field was used to validate the reliability of the measurement system. The swirling fluid motion in its horizontal section could be resolved into a velocity field grid of 24 × 24 vectors while achieving frame rates of about 30 fps. Results from a further study driving liquid metal in a cylindrical vessel by a pulsed rotating magnetic field are presented. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
993.
Sven Tobisch Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(11):3441-3458
The present computational mechanistic study explores comprehensively the organoactinide‐mediated intramolecular hydroamination/cyclisation (IHC) of aminodienes by employing a reliable DFT method. All the steps of a plausible catalytic reaction course have been scrutinised for the IHC of (4E,6)‐heptadienylamine 1 t by [(CGC)Th(NMe2)2] precatalyst 2 (CGC=[Me2Si(η5‐Me4C5)(tBuN)]2?). For each of the relevant elementary steps the most accessible pathway has been identified from a multitude of mechanistic possibilities. The operative mechanism involves rapid substrate association/dissociation equilibria for the 3 t ‐S resting state and also for azacyclic intermediates 4 a , 4 s , easily accessible and reversible exocyclic ring closure, supposedly facile isomerisation of the azacycle’s butenyl tether prior to turnover‐limiting protonolysis. The following aspects are in support of this scenario: 1) the derived rate law is consistent with the experimentally obtained empirical rate law; 2) the accessed barrier for turnover‐limiting protonolysis does agree remarkably well with observed performance data; 3) the ring‐tether double‐bond selectivity is consistently elucidated, which led to predict the product distribution correctly. This study provides a computationally substantiated rationale for observed activity and selectivity data. Steric demands at the CGC framework appear to be an efficient means for modulating both performance and ring‐tether double‐bond selectivity. The careful comparison of (CGC)4f‐element and (CGC)5f‐element catalysts revealed that aminodiene IHC mediated by organoactinides and organolanthanides proceeds through a similar mechanistic scenario. However, cyclisation and protonolysis steps, in particular, feature a markedly different reactivity pattern for the two catalyst classes, owing to enhanced bond covalency of early actinides when compared to lanthanides. 相似文献
994.
995.
Exploiting the Synthetic Potential of Sesquiterpene Cyclases for Generating Unnatural Terpenoids 下载免费PDF全文
Clara Oberhauser Vanessa Harms Dr. Katja Seidel Dr. Benjamin Schröder Kimia Ekramzadeh Dr. Sascha Beutel Sven Winkler Lukas Lauterbach Prof. Dr. Jeroen S. Dickschat Prof. Dr. Andreas Kirschning 《Angewandte Chemie (International ed. in English)》2018,57(36):11802-11806
The substrate flexibility of eight purified sesquiterpene cyclases was evaluated using six new heteroatom‐modified farnesyl pyrophosphates, and the formation of six new heteroatom‐modified macrocyclic and tricyclic sesquiterpenoids is described. GC‐O analysis revealed that tricyclic tetrahydrofuran exhibits an ethereal, peppery, and camphor‐like olfactoric scent. 相似文献
996.
Bolanca T Cerjan-Stefanović S Regelja M Regelja H Loncarić S 《Journal of chromatography. A》2005,1085(1):74-85
This paper describes development of artificial neural network (ANN) retention model, which can be used for method development in variety of ion chromatographic applications. By using developed retention model it is possible both to improve performance characteristic of developed method and to speed up new method development by reducing unnecessary experimentation. Multilayered feed forward neural network has been used to model retention behaviour of void peak, lithium, sodium, ammonium, potassium, magnesium, calcium, strontium and barium in relation with the eluent flow rate and concentration of methasulphonic acid (MSA) in eluent. The probability of finding the global minimum and fast convergence at the same time were enhanced by applying a two-phase training procedure. The developed two-phase training procedure consists of both first and second order training. Several training algorithms were applied and compared, namely: back propagation (BP), delta-bar-delta, quick propagation, conjugate gradient, quasi Newton and Levenberg-Marquardt. It is shown that the optimized two-phase training procedure enables fast convergence and avoids problems arisen from the fact that every new weight initialization can be regarded as a new starting position and yield irreproducible neural network if only second order training is applied. Activation function, number of hidden layer neurons and number of experimental data points used for training set were optimized in order to insure good predictive ability with respect to speeding up retention modelling procedure by reducing unnecessary experimental work. The predictive ability of optimized neural networks retention model was tested by using several statistical tests. This study shows that developed artificial neural network are very accurate and fast retention modelling tool applied to model varied inherent non-linear relationship of retention behaviour with respect to mobile phase parameters. 相似文献
997.
998.
999.
Monty Kessler Dr. Sven Hansen Christian Godemann Dr. Anke Spannenberg Dr. Torsten Beweries 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(20):6350-6357
The synthesis of a series of ansa‐titanocene dichlorides [Cp′2TiCl2] (Cp′=bridged η5‐tetramethylcyclopentadienyl) and the corresponding titanocene bis(trimethylsilyl)acetylene complexes [Cp′2Ti(η2‐Me3SiC2SiMe3)] is described. The ethanediyl‐bridged complexes [C2H4(C5Me4)2TiCl2] ( 2 ‐Cl2) and [C2H4(C5Me4)2Ti(η2‐Me3SiC2SiMe3)] ( 2‐ btmsa; btmsa=η2‐Me3SiC2SiMe3) can be obtained from the hitherto unknown calcocenophane complex [C2H4(C5Me4)2Ca(THF)2] ( 1 ). Furthermore, a heterodiatomic bridging unit containing both, a dimethylsilyl and a methylene group was introduced to yield the ansa‐titanocene dichloride [Me2SiCH2(C5Me4)2TiCl2] ( 3 ‐Cl2) and the bis(trimethylsilyl)acetylene complex [Me2SiCH2(C5Me4)2Ti(η2‐Me3SiC2SiMe3)] ( 3 ‐btmsa). Besides, tetramethyldisilyl‐ and dimethylsilyl‐bridged metallocene complexes (structural motif 4 and 5 , respectively) were prepared. All ansa‐titanocene alkyne complexes were reacted with stoichiometric amounts of water; the hydrolysis products were isolated as model complexes for the investigation of the elemental steps of overall water splitting. Compounds 1 , 2 ‐btmsa, 2 ‐(OH)2, 3 ‐Cl2, 3 ‐btmsa, 4 ‐(OH)2, 3 ‐alkenyl and 5 ‐alkenyl were characterised by X‐ray diffraction analysis. 相似文献
1000.
Within the last decades we witnessed the discovery of a number of mechanisms that enable the use of nucleic acids for therapeutic purposes. Small RNA and DNA molecules can be used to specifically suppress the expression of individual genes. Aptamers provide an alternative to monoclonal antibodies. A prerequisite for the pharmacological use of nucleic acids is an enhanced stability towards the body's degrading enzymes. This can be achieved for instance by employing non‐natural mirror‐image nucleic acids. The article describes the basic principles of stereochemistry underlying this approach and shows how these translate into the discovery of mirror‐image aptamers. Furthermore, it explains why the stereospecificity of Watson‐Crick base pairing has precluded mirror‐image nucleic acids from gene silencing methods and introduces a new approach that may help to overcome this. 相似文献