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991.
James P Davidson Olga Lubman Thierry Rose Gabriel Waksman Stephen F Martin 《Journal of the American Chemical Society》2002,124(2):205-215
Isothermal titration calorimetry and X-ray crystallography have been used to determine the structural and thermodynamic consequences associated with constraining the pTyr residue of the pYEEI ligand for the Src Homology 2 domain of the Src kinase (Src SH2 domain). The conformationally constrained peptide mimics that were used are cyclopropane-derived isosteres whereby a cyclopropane ring substitutes to the N-Calpha-Cbeta atoms of the phosphotyrosine. Comparison of the thermodynamic data for the binding of the conformationally constrained peptide mimics relative to their equivalent flexible analogues as well as a native tetrapeptide revealed an entropic advantage of 5-9 cal mol(-1) K(-1) for the binding of the conformationally constrained ligands. However, an unexpected drop in enthalpy for the binding of the conformationally constrained ligands relative to their flexible analogues was also observed. To evaluate whether these differences reflected conformational variations in peptide binding modes, we have determined the crystal structure of a complex of the Src SH2 domain bound to one of the conformationally constrained peptide mimics. Comparison of this new structure with that of the Src SH2 domain bound to a natural 11-mer peptide (Waksman et al. Cell 1993, 72, 779-790) revealed only very small differences. Hence, cyclopropane-derived peptides are excellent mimics of the bound state of their flexible analogues. However, a rigorous analysis of the structures and of the surface areas at the binding interface, and subsequent computational derivation of the energetic binding parameters, failed to predict the observed differences between the binding thermodynamics of the rigidified and flexible ligands, suggesting that the drop in enthalpy observed with the conformationally constrained peptide mimic arises from sources other than changes in buried surface areas, though the exact origin of the differences remains unclear. 相似文献
992.
The 13C-NMR spectra of 19 different, singly, doubly, and triple 13C-labelled α-sulfur- and α-selenium-substituted 6Li-derivatives generated from methyl and phenyl thioethers, thioacetals, trithio-orthoesters and from their selenium analogues have been recorded in ethereal solutions (tetrahydrofuran (THF), 2-methyltetrahydrofuran (MTHF) at temperatures between ?30° and ?110°. The effects of H/Li-exchange upon chemical shifts and coupling constants, as well as the values and multiplicities of Li, 13C-coupling are interpreted in vie wof crystal structures of some of the same compounds. In two thirds of the cases studied, the H-decoupled 13C-NMR signals observed below ?80° were triplets, proving that the C-atoms are bonded to single 6Li-atoms. This is compatible either with monomeric or with dimeric, heteroatom-bridged structures. The direct 1H, 13C- and 13C, 13C-coupling constants (1J) decrease, the 13C, 77Se-coupling constants increase upon lithiation. More striking is that the geminal coupling 13C-S-13C (2J) is too small to be observed in the non-metalated species, while it ranges from 3.7 to 7.5 Hz in the lithiated derivatives. These observations may be interpreted as resulting from delocalization of electron density from the carbonionic center towards the adjacent heteroatom. 相似文献
993.
Quantification of the ion transport mechanism in protective polymer coatings on lithium metal anodes
Hongyao Zhou Haodong Liu Xing Xing Zijun Wang Sicen Yu Gabriel M. Veith Ping Liu 《Chemical science》2021,12(20):7023
Protective Polymer Coatings (PPCs) have been proposed to protect lithium metal anodes in rechargeable batteries to stabilize the Li/electrolyte interface and to extend the cycle life by reducing parasitic reactions and improving the lithium deposition morphology. However, the ion transport mechanism in PPCs remains unclear. Specifically, the degree of polymer swelling in the electrolyte and the influence of polymer/solvent/ion interactions are never quantified. Here we use poly(acrylonitrile-co-butadiene) (PAN–PBD) with controlled cross-link densities to quantify how the swelling ratio of the PPC affects conductivity, Li+ ion selectivity, activation energy, and rheological properties. The large difference in polarities between PAN (polar) and PBD (non-polar) segments allows the comparison of PPC properties when swollen in carbonate (high polarity) and ether (low polarity) electrolytes, which are the two most common classes of electrolytes. We find that a low swelling ratio of the PPC increases the transference number of Li+ ions while decreasing the conductivity. The activation energy only increases when the PPC is swollen in the carbonate electrolyte because of the strong ion–dipole interaction in the PAN phase, which is absent in the non-polar PBD phase. Theoretical models using Hansen solubility parameters and a percolation model have been shown to be effective in predicting the swelling behavior of PPCs in organic solvents and to estimate the conductivity. The trade-off between conductivity and the transference number is the primary challenge for PPCs. Our study provides general guidelines for PPC design, which favors the use of non-polar polymers with low polarity organic electrolytes.Protective Polymer Coatings (PPCs) protect lithium metal anodes in rechargeable batteries to stabilize the Li/electrolyte interface and to extend the cycle life by reducing parasitic reactions and improving the lithium deposition morphology. 相似文献
994.
J. Ciciolil Hilario-Martínez Fernando Murillo Jair García-Mndez Eugenia Dzib Jesús Sandoval-Ramírez Miguel ngel Muoz-Hernndez Sylvain Berns Lszl Kürti Fernanda Duarte Gabriel Merino María A. Fernndez-Herrera 《Chemical science》2020,11(47):12764
Herein, we report for the first time a “trans-hydroboration–oxidation product” isolated and characterized under traditional hydroboration–oxidation conditions using cholesterol and diosgenin as substrates. These substrates are excellent starting materials because of the rigidity and different structural environments around the double bond. Further investigations based on experimental evidence, in conjunction with theoretical studies, indicate that the formation of this trans-species occurs via a retro-hydroboration of the major product to generate the corresponding Δ6-structure and the subsequent hydroboration by the β-face. Besides, the corresponding Markovnikov type products have been isolated in synthetically useful yields. The behavior of the reaction under a range of temperatures is also investigated.A trans-product is isolated and characterized under traditional hydroboration–oxidation conditions using Δ5-steroids as substrates. Experimental and theoretical studies indicate that the trans-species occurs via a retro-hydroboration mechanism. 相似文献
995.
Cancès E Castella F Chartier P Faou E Le Bris C Legoll F Turinici G 《The Journal of chemical physics》2004,121(21):10346-10355
We introduce high-order formulas for the computation of statistical averages based on the long-time simulation of molecular dynamics trajectories. In some cases, this allows us to significantly improve the convergence rate of time averages toward ensemble averages. We provide some numerical examples that show the efficiency of our scheme. When trajectories are approximated using symplectic integration schemes (such as velocity Verlet), we give some error bounds that allow one to fix the parameters of the computation in order to reach a given desired accuracy in the most efficient manner. 相似文献
996.
We study the properties of electrical double layers separated by a charged plate of finite thickness. The nonlinear Poisson-Boltzmann equation is analytically solved for this system. It is shown that, for an unsymmetrically charged, narrow plate, the charged fluids at both sides of the plate are strongly correlated, and the local electroneutrality condition (LEC) is not satisfied. The LEC is satisfied only for an infinitely thick plate. Analytical expressions for the induced charge and mean electrostatic potential are given and analyzed. These findings could be relevant for the understanding of protein adsorption on membranes. 相似文献
997.
[reaction: see text] Methods for synthesizing N-aryl beta-amino alcohols and O-aryl beta-amino alcohols are described. The presence of a neighboring hydroxyl or amino group, respectively, is thought to activate beta-amino alcohols toward these transformations. These protocols significantly increase access to a variety of arylated beta-amino alcohols. 相似文献
998.
Carré V Aubriet F Scheepers PT Krier G Muller JF 《Rapid communications in mass spectrometry : RCM》2005,19(7):871-880
Stainless steel factories are known to release particles into the atmosphere. Such particulate matter contains significant amounts of heavy metals or toxic inorganic compounds and organic pollutants such as, for example, Cr(VI) and polycyclic aromatic hydrocarbons (PAHs). The investigation of Cr(VI) and PAHs is often complicated by the associated matrix. Organic and inorganic pollutants present in stainless steel dust particles have been investigated with the same laser microprobe mass spectrometer according to two original methodologies. These analytical methods do not require time-consuming pretreatment (extraction, solubilization) or preconcentration steps. More specifically, experiments are conducted with a Fourier transform ion cyclotron resonance mass spectrometer coupled to an ArF (193 nm) or a tripled frequency Nd-YAG (355 nm) laser. Experiments at 355 nm allow the nature of the most frequently occurring Cr(III)/Cr(VI) compounds in dust particles to be identified. Examination of PAHs at 193 nm is assisted by the formation of pi-complexes with 7,7',8,8'-tetracyanoquinodimethane to prevent their evaporation in the mass spectrometer during analysis and to ensure an increase in sensitivity. 相似文献
999.
Gabriel Plancque Valérie Moulin Pierre Toulhoat Christophe Moulin 《Analytica chimica acta》2003,478(1):11-22
Time-resolved laser-induced fluorescence has been used to investigate Eu complexes formed with a few main ligands encountered in natural waters: hydroxide, carbonate and humic substances. By varying pH and concentrations of ligands at fixed europium concentration and ionic strength, it was possible, together with free europium Eu3+, to identify spectrally and temporally carbonate complexes, namely Eu(CO3)+, Eu(CO3)2− and Eu(CO3)33− and humate complexes EuHA. For hydroxide complexes, no differences were found in terms of fluorescence spectra and lifetimes. A spectral interpretation is described by using deconvolution for all systems. 相似文献
1000.
Local time control methods are used in the simulation of quantum control phenomena because they conveniently ensure an increase of a predefined performance index and also avoid singularities associated with tracking procedures. However, the drawback of the existing implementations is that they only take into account one-photon, direct transitions and may stop at nonoptimal values of the index. We propose in this paper a modification of the currently used algorithms that addresses this issue and explain how the convergence is improved. Furthermore, when iterations are required, we show that this approach can be inserted into a monotonically convergent algorithm. 相似文献