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131.
Sutherland FC Badenhorst D de Jager AD Scanes T Hundt HK Swart KJ Hundt AF 《Journal of chromatography. A》2001,914(1-2):45-51
A sensitive method for the simultaneous determination of fluoxetine and its major active metabolite norfluoxetine in plasma was developed, using high-performance liquid chromatographic separation with tandem mass spectrometric detection. The samples were extracted from alkalised plasma with hexane-isoamyl alcohol (98:2, v/v) followed by back-extraction into formic acid (2%). Chromatography was performed on a Phenomenex Luna C18 (2) 5 microm, 150x2 mm column with a mobile phase consisting of acetonitrile-0.02% formic acid (340:660, v/v) at a flow-rate of 0.35 ml/min. Detection was achieved by a Perkin-Elmer Sciex API 2000 mass spectrometer (LC-MS-MS) set at unit resolution in the multiple reaction monitoring mode. TurbolonSpray ionisation was used for ion production. The mean recoveries for fluoxetine and norfluoxetine were 98 and 97%, respectively, with a lower limit of quantification set at 0.15 ng/ml for the analyte and its metabolite. This assay method makes use of the increased sensitivity and selectivity of mass spectrometric (MS-MS) detection to allow for a more rapid (extraction and chromatography) and sensitive method for the simultaneous determination of fluoxetine and norfluoxetine in human plasma than has previously been described. 相似文献
132.
Sílvia Osuna Marcel Swart Dr. Evert Jan Baerends Prof. Dr. F. Matthias Bickelhaupt Prof. Dr. Miquel Solà Prof. Dr. 《Chemphyschem》2009,10(17):2955-2965
Herein we report density functional calculations of homolytic and heterolytic dissociation energies of the diatomic alkalimetal halides MX (M=Li, Na, K, Rb, and Cs and X=F, Cl, Br, I, and At) and their corresponding microsolvated structures MX?(H2O)n (n=1 to 4). Our results show that the homolytic dissociation energy of the MX?(H2O)n species increases with the number of water molecules involved in the microsolvated salts. On the other hand, the heterolytic dissociation energy follows exactly the opposite trend. As a result, while for the isolated diatomic alkalimetal halides, homolytic dissociation is always favored over heterolytic dissociation, the latter is preferred for CsF and CsCl already for n=2, and for n=4 it is the preferential mode of dissociation for more than half of the species studied. 相似文献
133.
Carvalho AT Fernandes PA Swart M Van Stralen JN Bickelhaupt FM Ramos MJ 《Journal of computational chemistry》2009,30(5):710-724
The enzymes of the thioredoxin family fulfill a wide range of physiological functions. Although they possess a similar CXYC active site motif, with identical environment and stereochemical properties, the redox potential and pK(a) of the cysteine pair varies widely across the family. As a consequence, each family member promotes oxidation or reduction reactions, or even isomerization reactions. The analysis of the three-dimensional structures gives no clues to identify the molecular source for the different active site properties. Therefore, we carried out a set of quantum mechanical calculations in active site models to gain more understanding on the elusive molecular-level origin of the differentiation of the properties across the family. The obtained results, together with earlier quantum mechanical calculations performed in our laboratories, gave rise to a consistent line of evidence, which points to the fact that both active site cysteines play an important role in the differentiation. In contrary to what was assumed, differentiation is not achieved through a different stabilization of the solvent exposed cysteine but, instead, through a fine tuning of the nucleophilicity of both active site cysteines. Reductant enzymes have both cysteine thiolates poorly stabilized, oxidant proteins have both cysteine thiolates highly stabilized, and isomerases have one thiolate (solvent exposed) poorly stabilized and the other (buried) thiolate highly stabilized. The feasibility of shifting the chemical equilibrium toward oxidation, reduction, or isomerization only through subtle electrostatic effects is quite unusual, and it relies on the inherent thermoneutrality of the catalytic steps carried out by a set of chemically equivalent entities all of which are cysteine thiolates. Such pattern of stabilization/destabilization, detected in our calculations is fully consistent with the observed physiological roles of this family of enzymes. 相似文献
134.
den Burger JC Wilhelm AJ Chahbouni A Vos RM Sinjewel A Swart EL 《Analytical and bioanalytical chemistry》2012,404(6-7):1803-1811
In the last few years, an increasing number of dried blood spot (DBS) sampling assays have been developed. With this increase, more insight is gained in the factors that possibly influence the performance of DBS assays. We have developed an assay for four commonly used immunosuppressants; some of them are possibly concomitantly prescribed: cyclosporin A (CsA), tacrolimus (TcR), sirolimus (SiR), and everolimus (EvE). Chromatographic separation from possible ion suppression was obtained within the total runtime of 4.2?min. Trifluoroacetic acid and ammonium acetate were used as mobile phase additives. The linearity ranged from 23.6 to 787, 1.14 to 30.3, 1.34 to 35.8, and 1.26 to 33.7?μg/L, for CsA, TcR, SiR, and EvE, respectively. Between- and within-run accuracy and precision were all within 15?% and extensive validation for DBS samples, such as hematocrit, blood spot volume, and spot punch location was performed. None of these factors were found to be of influence on the performance of the DBS assay. 相似文献
135.
K.G. Tshabalala S.-H. Cho J.-K. Park Shreyas S. Pitale I.M. Nagpure R.E. Kroon H.C. Swart O.M. Ntwaeaborwa 《Physica B: Condensed Matter》2012,407(10):1489-1492
In this study, a solution combustion method was used to prepare green emitting Ce3+–Tb3+ co-activated ZnAl2O4 phosphor. The samples were annealed at 700 °C in air or hydrogen atmosphere to improve their crystallinity and optical properties. X-ray diffraction study confirmed that both as-prepared and post-preparation annealed samples crystallized in the well known cubic spinel structure of ZnAl2O4. An agglomeration of irregular platelet-like particles whose surfaces were encrusted with smaller spheroidal particles was confirmed by scanning electron microscopy (SEM). The fluorescence data collected from the annealed samples with different concentrations of Ce3+ and Tb3+ show the enhanced green emission at 543 nm associated with 5D4→7F5 transitions of Tb3+. The enhancement was attributed to energy transfer from Ce3+ to Tb3+. Possible mechanism of energy transfer via a down conversion process is discussed. Furthermore, cathodoluminescence (CL) intensity degradation of this phosphor was also investigated and the degradation data suggest that the material was chemically stable and the CL intensity was also stable after 10 h of irradiation by a beam of high energy electrons. 相似文献
136.
Abdub G. Ali Francis B. Dejene Hendrik C. Swart 《Central European Journal of Physics》2012,10(2):478-484
Un-doped and Mn-doped ZnO nanoparticles were successfully synthesized in an ethanolic solution by using a sol-gel method.
Material properties of the samples dependence on preparation conditions and Mn concentrations were investigated while other
parameters were controlled to ensure reproducibility. It was observed that the structural properties, particle size, band
gap, photoluminescence intensity and wavelength of maximum intensity were influenced by the amount of Mn ions present in the
precursor. The XRD spectra for ZnO nanoparticles show the entire peaks corresponding to the various planes of wurtzite ZnO,
indicating a single phase. The diffraction peaks of doped samples are slightly shifted to lower angles with an increase in
the Mn ion concentration, signifying the expansion of the lattice constants and increase in the band gap of ZnO. All the samples
show the absorption in the visible region. The absorbance spectra show that the excitonic absorption peak shifts towards the
lower wavelength side with the Mn-doped ZnO nanoparticles. The PL spectra of undoped ZnO consist of UV emission at 388 nm
and broad visible emission at 560 nm with varying relative peak intensities. The doping of ZnO with Mn quenches significantly
the green emission while UV luminescence is slightly affected. 相似文献
137.
We have carried out an extensive exploration of the gas-phase basicity of archetypal neutral bases across the periodic system using the generalized gradient approximation (GGA) of the density functional theory (DFT) at BP86/QZ4P//BP86/TZ2P. First, we validate DFT as a reliable tool for computing proton affinities and related thermochemical quantities: BP86/QZ4P//BP86/TZ2P is shown to yield a mean absolute deviation of 2.0 kcal/mol for the proton affinity at 298 K with respect to experiment, and 1.2 kcal/mol with high-level ab initio benchmark data. The main purpose of this work is to provide the proton affinities (and corresponding entropies) at 298 K of the neutral bases constituted by all maingroup-element hydrides of groups 15-17 and the noble gases, that is, group 18, and periods 1-6. We have also studied the effect of step-wise methylation of the protophilic center of the second- and third-period bases. 相似文献
138.
van Bochove MA Swart M Bickelhaupt FM 《Journal of the American Chemical Society》2006,128(33):10738-10744
Pentacoordinate phosphorus species play a key role in organic and biological processes. Yet, their nature is still not fully understood, in particular, whether they are stable, intermediate transition complexes (TC) or labile transition states (TS). Through systematic, theoretical analyses of elementary S(N)2@C, S(N)2@Si, and S(N)2@P reactions, we show how increasing the coordination number of the central atom as well as the substituents' steric demand shifts the S(N)2@P mechanism stepwise from a single-well potential (with a stable central TC) that is common for substitution at third-period atoms, via a triple-well potential (featuring a pre- and post-TS before and after the central TC), back to the double-well potential (in which pre- and postbarrier merge into one central TS) that is well-known for substitution reactions at carbon. Our results highlight the steric nature of the S(N)2 barrier, but they also show how electronic effects modulate the barrier height. 相似文献
139.
Van Der Poorten Alfred J.; Swart Christine S. 《Bulletin London Mathematical Society》2006,38(4):546-554
In his celebrated memoir, Morgan Ward's definition of ellipticdivisibility sequences has the remarkable feature that it doesnot become at all clear until deep into the paper that thereexist nontrivial examples of such sequences. Even then, Ward'sproof of the coherence of his definition relies on displayinga sequence of values of quotients of Weierstraß -functions.We give a direct proof of coherence and show, rather more generally,that a sequence defined by a so-called Somos relation of width4 is always also given by three-term Somos relations of alllarger widths 5, 6, 7, .... 2000 Mathematics Subject Classification11B83, 11G05 (primary), 11A55, 14H05, 14H52 (secondary). 相似文献
140.
James Charles Truscott Jeanet Conradie Hendrik C. Swart Mart-Marie Duvenhage Hendrik Gideon Visser 《Acta Crystallographica. Section C, Structural Chemistry》2019,75(8):1045-1052
Single crystals of the aluminium and gallium complexes of 6,6′‐{(1E,1′E)‐[1,2‐phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2‐methoxyphenol), namely diaqua(6,6′‐{(1E,1′E)‐[1,2‐phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2‐methoxyphenolato)‐κ4O1,N,N′,O1′)aluminium(III) nitrate ethanol monosolvate, [Al(C22H18N2O4)(H2O)2]NO3·C2H5OH, 1 , and diaqua(6,6′‐{(1E,1′E)‐[1,2‐phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2‐methoxyphenolato)‐κ4O1,N,N′,O1′)gallium(III) nitrate ethanol monosolvate, [Ga(C22H18N2O4)(H2O)2]NO3·C2H5OH, 2 , were obtained after successful synthesis in ethanol. Both complexes crystallized in the triclinic space group P, with two molecules in the asymmetric unit. In both structures, in one of the independent molecules the tetradentate ligand is almost planar while in the other independent molecule the ligand shows significant distortions from planarity, as illustrated by the largest distance from the plane constructed through the central metal atom and the O,N,N′,O′‐coordinating atoms of the ligand in 1 of 1.155 (3) Å and a distance of 1.1707 (3) Å in 2 . The possible reason for this is that there are various strong π‐interactions in the structures. This was confirmed by density functional theory (DFT) calculations, as were the other crystallographic data. DFT was also used to predict the outcome of cyclic voltammetry experiments. Ligand oxidation is more stabilized in the gallium complex. Solid‐state photoluminescence gave an 80 nm red‐shifted spectrum for the gallium complex, whereas the aluminium complex maintains the ligand curve with a smaller red shift of 40 nm. 相似文献