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71.
Stéphanie Miquet Dr. Nicolas Vanthuyne Dr. Paul Brémond Prof. Gérard Audran 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(32):10632-10642
The first total syntheses of the proposed structures of kopeolin ( 1 ) and kopeolone ( 3 ) have been achieved from a common enantiopure chiral building block obtained by a chemoenzymatic enantioconvergent methodology. The syntheses feature two key steps: a one‐pot reduction/diastereoselective protonation followed by a highly diastereoselective addition of an organocerate. The synthetic structures were fully characterized and all stereocenters were confirmed. The results show that the two previously reported structures were not assigned correctly, and suggest an initial structural misassignment during the isolation of the natural products. Thus, two revised structures, 1′ for kopeolin and 3′ for kopeolone, are proposed. 相似文献
72.
Dr. Noam Shemesh Dr. Jean‐Nicolas Dumez Prof. Lucio Frydman 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(39):13002-13008
Nuclear magnetic resonance spectroscopy is governed by longitudinal (T1) relaxation. For protein and nucleic acid experiments in solutions, it is well established that apparent T1 values can be enhanced by selective excitation of targeted resonances. The present study explores such longitudinal relaxation enhancement (LRE) effects for molecules residing in biological tissues. The longitudinal relaxation recovery of tissue resonances positioned both down‐ and upfield of the water peak were measured by spectrally selective excitation/refocusing pulses, and compared with conventional water‐suppressed, broadband‐excited counterparts at 9.4 T. Marked LRE effects with up to threefold reductions in apparent T1 values were observed as expected for resonances in the 6–9 ppm region; remarkably, statistically significant LRE effects were also found for several non‐exchanging metabolite resonances in the 1–4 ppm region, encompassing 30–50 % decreases in apparent T1 values. These LRE effects suggest a novel means of increasing the sensitivity of tissue‐oriented experiments, and open new vistas to investigate the nature of interactions among metabolites, water and macromolecules at a molecular level. 相似文献
73.
Cellulose was acylated by N-phthaloyl amino acid chlorides in the presence of pyridine to prepare an original library of cellulose N-phthaloyl-amino acid esters as chiral solid supports for enantioselective adsorption of racemates. Cellulose esters derived from N-phthaloyl glycine, alanine, valine, leucine, phenylalanine, phenylglycine and isoleucine were isolated in high yields and with degrees of substitution approaching 3. Interestingly, the use of an optically pure (d or l) or a rac-amino acid led to the same cellulose ester according to (1) the measured optical rotation, (2) the enantiomeric excesses of the amino acids resulting from a non-racemising and total hydrolysis and (3) the enantioselective adsorptions of rac-benzoin, rac-Pirkle’s alcohol and rac-Tröger’s base. This suggests that either the formation of a prochiral ketene intermediate which was diastereoselectively attacked by the cellulose alcohols or, alternatively (or concomitantly), the occurrence of a chiral induction in the formed triesters during prolonged contact with the pyridine base in the reaction medium. Enantiomeric excesses in favour of the (S) form ranged from 8 % to 50 % depending on the N-phthaloyl amino acid. The memory of the chirality of the starting material was lost and new chirality was imprinted by the cellulose backbone on the amino acid residues. 相似文献
74.
Nicolas Zigon Manabu Hoshino Shota Yoshioka Yasuhide Inokuma Makoto Fujita 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(31):9161-9165
Crystal structures of α‐humulene, a cyclic sesquiterpene, and its oxidized subproducts, were analyzed by the crystalline sponge method. Regio‐ and stereochemistry, including absolute configuration when a chiral oxidant was applied, and the stable conformations of all the scaffold‐related compounds were successfully determined for samples on a 5–50 μg scale. 相似文献
75.
Peggy Cénac Brigitte Chauvin Frédéric Paccaut Nicolas Pouyanne 《Random Structures and Algorithms》2015,46(1):117-141
Common assumptions on the source producing the words inserted in a suffix trie with n leaves lead to a height and saturation level. We provide an example of a suffix trie whose height increases faster than a power of n and another one whose saturation level is negligible with respect to . Both are built from VLMC (Variable Length Markov Chain) probabilistic sources and are easily extended to families of tries having the same properties. The first example corresponds to a “logarithmic infinite comb” and enjoys a non uniform polynomial mixing. The second one corresponds to a “factorial infinite comb” for which mixing is uniform and exponential. © 2013 Wiley Periodicals, Inc. Random Struct. Alg., 46, 117–141, 2015 相似文献
76.
Carbon‐Coated LiTi2(PO4)3: An Ideal Insertion Host for Lithium‐Ion and Sodium‐Ion Batteries 下载免费PDF全文
Dr. Vanchiappan Aravindan Dr. Wong Chui Ling Steffen Hartung Nicolas Bucher Prof. Srinivasan Madhavi 《化学:亚洲杂志》2014,9(3):878-882
We report the extraordinary performance of carbon‐coated sodium super ion conductor (NASICON)‐type LiTi2(PO4)3 as an ideal host matrix for reversible insertion of both Li and Na ions. The NASICON‐type compound was prepared by means of a Pechini‐type polymerizable complex method and was subsequently carbon coated. Several characterization techniques such as XRD, thermogravimetric analysis (TGA), field‐emission (FE) SEM, TEM, and Raman analysis were used to study the physicochemical properties. Both guest species underwent a two‐phase insertion mechanism during the charge/discharge process that was clearly evidenced from galvanostatic and cyclic voltammetric studies. Unlike that of Li (≈1.5 moles of Li), Na insertion exhibits better reversibility (≈1.59 moles of Na) while experiencing a slightly higher capacity fade (≈8 % higher than Li) and polarization (780 mV) than Li. However, excellent rate capability profiles were noted for Na insertion relative to its counterpart Li. Overall, the Na insertion properties were found to be superior relative to Li insertion, which makes carbon‐coated NASICON‐type LiTi2(PO4)3 hosts attractive for the development of next‐generation batteries. 相似文献
77.
Pierre Gras Nicolas Ratel‐Ramond Sbastien Teychn Christian Rey Erik Elkaim Batrice Biscans Stphanie Sarda Christle Combes 《Acta Crystallographica. Section C, Structural Chemistry》2014,70(9):862-866
Calcium pyrophosphate hydrate (CPP, Ca2P2O7·nH2O) and calcium orthophosphate compounds (including apatite, octacalcium phosphate etc.) are among the most prevalent pathological calcifications in joints. Even though only two dihydrated forms of CPP (CPPD) have been detected in vivo (monoclinic and triclinic CPPD), investigations of other hydrated forms such as tetrahydrated or amorphous CPP are relevant to a further understanding of the physicochemistry of those phases of biological interest. The synthesis of single crystals of calcium pyrophosphate monohydrate (CPPM; Ca2P2O7·H2O) by diffusion in silica gel at ambient temperature and the structural analysis of this phase are reported in this paper. Complementarily, data from synchrotron X‐ray diffraction on a CPPM powder sample have been fitted to the crystal parameters. Finally, the relationship between the resolved structure for the CPPM phase and the structure of the tetrahydrated calcium pyrophosphate β phase (CPPT‐β) is discussed. 相似文献
78.
Karla Vega-Granados Yadira Gochi-Ponce Nicolas Alonso-Vante 《Current Opinion in Electrochemistry》2022
The use of single-atom metals (SAM) as catalysts of energy conversion reactions is a recent topic, which has gained popularity in the last two decades. Transition metal dichalcogenides emerged as important electrocatalysts since it was discovered that their chalcogenide edge sites are active towards the electrocatalytic hydrogen evolution reaction (HER) and could also serve as supports for other metals within the same applications. Currently, several groups have reported a novel metal?chalcogenide arrangement, with the possibility of isolating metals at specific sites on chalcogenides to enhance their properties resulting in a synergistic effect in which both chalcogenide and single-atom metal features are exploited, either as promoters or active sites. Theoretical studies have been the basis of these reports. 相似文献
79.
Giraud N Blackledge M Böckmann A Emsley L 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2007,184(1):51-61
The effect of nitrogen-15 proton-driven spin diffusion on quantitative (15)N T(1) measurements in solid proteins is investigated, and the impact on the measurement of dynamic parameters is assessed. A simple model of exchange between neighboring nitrogens is used to reproduce the evolution of (15)N spin systems whose longitudinal relaxation rates and exchange rates are compatible with experimental measurements. We show that the induced error in the measured T(1) and its effect on the determination of dynamics parameters is likely to be less than the current experimental error. The use of deuterated protein samples is shown to have a small but sometimes visible effect, and may also considerably slow down or even suppress the exchange of magnetization due to spin diffusion. 相似文献
80.
Melkonian JM Forget N Bretenaker F Drag C Lefebvre M Rosencher E 《Optics letters》2007,32(12):1701-1703
We report on what we believe to be the first active mode locking of near-degenerate, doubly and singly resonant cw-pumped optical parametric oscillators (OPOs). We show experimentally that a steady-state regime of short pulses is reached in a few tens of microseconds under cw pumping. The oscillation buildup dynamics of both OPOs is also explored, evidencing an unusual transient behavior in the mode-locking process. 相似文献