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81.
Solid-state 31P NMR spectroscopy was applied to measure the isotropic chemical shifts, chemical shift anisotropies and asymmetry parameters of three phosphorylated amino acids, O-phospho-L-serine, O-phospho-L-threonine and O-phospho-L-tyrosine. The cross-polarization buildup rates and longitudinal relaxation times of 31P and 1H were-determined and compared with the values measured for a triphosphate (GppCH2p) bound to a crystalline protein (Ras). It is shown that the phosphorylated amino acids are well-suited model compounds, e.g. for the optimization of experiments on crystalline proteins. Two-dimensional exchange experiments on O-phospho-L-tyrosine indicate the existence of an exchange between the two different conformations of the molecule. 相似文献
82.
The scope of substrate-controlled diastereoselective hydroborations can be considerably enhanced by a boron-zinc exchange reaction, providing organozinc derivatives that react with a broad range of electrophiles. Even normally unreactive boronic esters, obtained by Rh-catalyzed hydroboration with catecholborane, react readily with iPr2Zn providing the corresponding zinc reagents in high diastereoselectivity. 相似文献
83.
Eike Kirchner Frauke Thiem Rudolt Dernick Jochen Heukeshoven Joachim Thieina 《Journal of carbohydrate chemistry》2013,32(2):453-486
Abstract Glycal derivatives of N-acetylneuraminic acid were prepared and their N-iodosuccinimide-mediated glycosylation shown to proceed only with most reactive alcohols, Their reduced enol ether reactivity is attributed to the α unsaturated ester reature. Thus several reduced glycal derivatives were synthesized. These couid be glycosyiatec with simple alcohols as well as otner saccnarides as aglycones in averaae to modest yields with the trans-diaxiai additions compounas prevailing. A number or selective and specific prepparation led to both the anomeric phenylthioglycosides or N dcety Ineuraminic acid. These could be used in phenylmercurl triflate-promoted glycosylations to afford several derivatives. 相似文献
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86.
Transition-Metal-Induced Rearrangement of [(PhSn)4S6] Towards Ternary CuI/Sn/S or CuII/Sn/S Clusters
Eike Dornsiepen Dr. Florian Weigend Prof. Dr. Stefanie Dehnen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(10):2486-2490
Direct access of ternary copper-tin sulfide clusters by reactions of a binary organotin sulfide cluster, [(PhSn)4S6] ( A ), with transition metal complexes was achieved for the first time without extra addition of further chalcogenide sources. This indicates that an in situ rearrangement of the inorganic core takes place even without initial formation of anionic fragments. The use of [Cu(PPh3)3Cl] or [Cu(PPh3)2Cl2] as reactants yielded the ternary clusters [(CuPPh3)4(PhSn)18Cu6S31Cl2] ( 1 ) and [{Cu(PPh3)2}2(PhSn)3(SnCl)S8] ( 2 ), respectively. Whereas 1 represents the largest neutral Cu/Sn/S cluster known to date, compound 2 , which is the first example of a ternary Cu/Sn/E (E=S, Se) cluster containing copper in the +II oxidation state, may be viewed as a very early stage of cluster formation. Apparently, the presence of CuII inhibits effective cluster growth, which rationalizes the lack of such species so far. The two ternary clusters exhibit very similar optical absorption energies despite their markedly different cluster sizes. According to time-dependent DFT calculations, this is due to different characters of the electronic excitation in the triplet compound 2 , as compared to the excitation of the closed shell cluster 1 , which serve to compensate for the different extensions of the clusters. 相似文献
87.
88.
Solving chemical master equations numerically on a large state space is known to be a difficult problem because the huge number of unknowns is far beyond the capacity of traditional methods. We present an adaptive method which compresses the problem very efficiently by representing the solution in a sparse wavelet basis that is updated in each step. The step-size is chosen adaptively according to estimates of the temporal and spatial approximation errors. Numerical examples demonstrate the reliability of the error estimation and show that the method can solve large problems with bimodal solution profiles. 相似文献
89.
Matthias A. Grasser Tobias Pietsch Jan Blasius Dr. Oldamur Hollóczki Prof. Dr. Eike Brunner Prof. Dr. Thomas Doert Prof. Dr. Michael Ruck 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(7):e202103770
Elemental tellurium readily dissolves in ionic liquids (ILs) based on tetraalkylphosphonium cations even at temperatures below 100 °C. In the case of ILs with acetate, decanoate, or dicyanamide anions, dark red to purple colored solutions form. A study combining NMR, UV-Vis and Raman spectroscopy revealed the formation of tellurium anions (Ten)2− with chain lengths up to at least n=5, which are in dynamic equilibrium with each other. Since external influences could be excluded and no evidence of an ionic liquid reaction was found, disproportionation of the tellurium is the only possible dissolution mechanism. Although the spectroscopic detection of tellurium cations in these solutions is difficult, the coexistence of tellurium cations, such as (Te4)2+ and (Te6)4+, and tellurium anions could be proven by cyclic voltammetry and electrodeposition experiments. DFT calculations indicate that electrostatic interactions with the ions of the ILs are sufficient to stabilize both types of tellurium ions in solution. 相似文献
90.
The partitioning of non-polar analytes into the silicone polydimethylsiloxane (PDMS) is the basis for many analytical approaches such as solid phase microextraction (SPME), stir bar sorptive extraction (SBSE) and environmental passive sampling. Recently, the methods have been applied to increasingly complex sample matrices. The present work investigated the possible effect of complex matrices on the sorptive properties of PDMS. First, SPME fibers with a 30 μm PDMS coating were immersed in 15 different matrices, including sediment, suspensions of soil and humic substances, mayonnaise, meat, fish, olive oil and fish oil. Second, the surface of the fibers was wiped clean, and together with matrix-free control fibers, they were exposed via headspace to 7 non-polar halogenated organic chemicals in spiked olive oil. The fibers were then solvent-extracted, analyzed, and the ratios of the mean concentrations in the matrix-immersed fibers to the control fibers were determined for all matrices. These ratios ranged from 92% to 112% for the four analytes with the highest analytical precision (i.e. polychlorinated biphenyls (PCBs) 3, 28, 52 and brominated diphenyl ether (BDE) 3), and they ranged from 74% to 133% for the other three compounds (i.e. PCBs 101, 105 and γ-hexachlorocyclohexane (HCH)). We conclude that, for non-polar, hydrophobic chemicals, the sorptive properties of the PDMS were not modified by the diverse investigated media and consequently that PDMS is suited for sampling of these analytes even in highly complex matrices. 相似文献