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排序方式: 共有993条查询结果,搜索用时 203 毫秒
61.
Dr. Michael Bodensteiner Prof. Dr. Alexey Y. Timoshkin Dr. Eugenia V. Peresypkina Dr. Ulf Vogel Prof. Dr. Manfred Scheer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(3):957-963
The reaction of [{(CO)5W}PRH2] (R=H, Ph) with H3Al ? NR3 (R=Et, Me) leads to the formation of four‐membered heterocyclic compounds [({(CO)5W}P(H)AlH ? NEt3)2] and [({(CO)5W}PhPAlH ? NMe3)2]. Upon dissolving the solid compounds, fast equilibria between the isomers are observed on the NMR timescale. Further insight into the stability and reactivity of the isomers was gained by applying theoretical methods. DFT calculations predict that hydrogen elimination in the case of [({(CO)5W}PhPAlH ? NMe3)2] may be reversible. 相似文献
62.
Alessandro Marchetti Dr. Andrea Pizzi Dr. Greta Bergamaschi Dr. Nicola Demitri Dr. Ulrike Stollberg Prof. Ulf Diederichsen Dr. Claudia Pigliacelli Prof. Pierangelo Metrangolo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(14):e202104089
Iodination has long been employed as a successful labelling strategy to gain structural insights into proteins and other biomolecules via several techniques, including Small Angle X-ray Scattering, Inductively Coupled Plasma Mass Spectrometer (ICP-MS), and single-crystal crystallography. However, when dealing with smaller biomolecular systems, interactions driven by iodine may significantly alter their self-assembly behaviour. The engineering of amyloidogenic peptides for the development of ordered nanomaterials has greatly benefitted from this possibility. Still, to date, iodination has exclusively been applied to aromatic residues. In this work, an aliphatic bis-iodinated amino acid was synthesized and included into a custom pentapeptide, which showed enhanced fibrillogenic behaviour. Peptide single crystal X-ray structure and powder X-ray diffraction on its dried water solution demonstrated the key role of iodine atoms in promoting intermolecular interactions that drive the peptide self-assembly into amyloid fibrils. These findings enlarge the library of halogenated moieties available for directing and engineering the self-assembly of amyloidogenic peptides. 相似文献
63.
We have used combined quantum mechanical and molecular mechanical (QM/MM) calculations to study the reaction mechanism of nitrogenase, assuming that none of the sulfide ligands dissociates. To avoid the problem that there is no consensus regarding the structure and protonation of the E4 state, we start from a state where N2 is bound to the cluster and is protonated to N2H2, after dissociation of H2. We show that the reaction follows an alternating mechanism with HNNH (possibly protonated to HNNH2) and H2NNH2 as intermediates and the two NH3 products dissociate at the E7 and E8 levels. For all intermediates, coordination to Fe6 is preferred, but for the E4 and E8 intermediates, binding to Fe2 is competitive. For the E4, E5 and E7 intermediates we find that the substrate may abstract a proton from the hydroxy group of the homocitrate ligand of the FeMo cluster, thereby forming HNNH2, H2NNH2 and NH3 intermediates. This may explain why homocitrate is a mandatory component of nitrogenase. All steps in the suggested reaction mechanism are thermodynamically favourable compared to protonation of the nearby His-195 group and in all cases, protonation of the NE2 atom of the latter group is preferred. 相似文献
64.
Gabriella Kervefors Kumar Bhaskar Pal Gergely L. Tolnai Mukul Mahanti Hakon Leffler Ulf J. Nilsson Berit Olofsson 《Helvetica chimica acta》2021,104(2):e2000220
β-Galactose derivatives have recently been reported to selectively inhibit galectin-3, and a library of O3-arylated galactosides with varying substitution patterns was designed to study such inhibitions further. The O3-arylated galactosides were synthesized using diaryliodonium salts under mild and transition metal free conditions, providing the target products in moderate to good yields. An O3-trifluoroethylated galactoside was also synthesized using iodonium salt chemistry. Azido-substituted products were subsequently transformed into the corresponding triazoles. After deprotection, a selection of galactoside derivatives were evaluated for inhibitory potencies against galectins-1, 3, 4 N (N-terminal domain), 4 C (C-terminal domain), 7, 8 N, 8 C, 9 N, and 9 C and one compound with promising affinity and selectivity for both the N- and C-terminal domain of galectin-9 was discovered. 相似文献
65.
Pia Hellström Anette Heijnesson-Hultén Magnus Paulsson Helena Håkansson Ulf Germgård 《Cellulose (London, England)》2014,21(3):1489-1503
A fully bleached birch kraft pulp was treated with acidic hydrogen peroxide in the presence of ferrous ions (Fenton’s reagent) and thereafter treated mechanically in a colloid mill to produce a product containing microfibrillated cellulose (MFC). The produced MFC products were chemically and morphologically characterized and compared with MFC products produced without pretreatment as well as with enzymatic hydrolysis. Fenton treatment resulted in an increase in total charge and number of carbonyl groups while the intrinsic viscosity decreased. The Fenton treated pulps were easier to process mechanically i.e. they reached a higher specific surface area at a given mechanical treatment time and the MFC produced had a stable water-fibre suspension for at least 8 weeks compared to enzymatic pretreated pulps and pulps not subjected to any pretreatment. 相似文献
66.
67.
Octav Caldararu Martin A. Olsson Christoph Riplinger Frank Neese Ulf Ryde 《Journal of computer-aided molecular design》2017,31(1):87-106
We have tried to calculate the free energy for the binding of six small ligands to two variants of the octa-acid deep cavitand host in the SAMPL5 blind challenge. We employed structures minimised with dispersion-corrected density-functional theory with small basis sets and energies were calculated using large basis sets. Solvation energies were calculated with continuum methods and thermostatistical corrections were obtained from frequencies calculated at the HF-3c level. Care was taken to minimise the effects of the flexibility of the host by keeping the complexes as symmetric and similar as possible. In some calculations, the large net charge of the host was reduced by removing the propionate and benzoate groups. In addition, the effect of a restricted molecular dynamics sampling of structures was tested. Finally, we tried to improve the energies by using the DLPNO–CCSD(T) approach. Unfortunately, results of quite poor quality were obtained, with no correlation to the experimental data, systematically too positive affinities (by ~50 kJ/mol) and a mean absolute error (after removal of the systematic error) of 11–16 kJ/mol. DLPNO–CCSD(T) did not improve the results, so the accuracy is not limited by the energy function. Instead, four likely sources of errors were identified: first, the minimised structures were often incorrect, owing to the omission of explicit solvent. They could be partly improved by performing the minimisations in a continuum solvent with four water molecules around the charged groups of the ligands. Second, some ligands could bind in several different conformations, requiring sampling of reasonable structures. Third, there is an indication the continuum-solvation model has problems to accurately describe the binding of both the negatively and positively charged guest molecules. Fourth, different methods to calculate the thermostatistical corrections gave results that differed by up to 30 kJ/mol and there is an indication that HF-3c overestimates the entropy term. In conclusion, it is a challenge to calculate binding affinities for this octa-acid system with quantum–mechanical methods. 相似文献
68.
Langlet A Latypov NV Wellmar U Bemm U Goede P Bergman J Romero I 《The Journal of organic chemistry》2002,67(22):7833-7838
Nitration of some 2-substituted pyrimidine-4,6-diones in sulfuric acid was studied, which afforded previously unknown 5,5-gem-dinitropyrimidine-4,6-diones in high yields. The gem-dinitro products were easily attacked by nucleophiles with concomitant formation of gem-dinitroacetyl derivatives, which in turn could be further hydrolyzed to salts of dinitromethane and triureas. 相似文献
69.
Ulf Ryde 《Journal of computer-aided molecular design》1996,10(2):153-164
Summary The coordination number of the catalytic zinc ion in alcohol dehydrogenase has been studied by integrated ab initio quantum-chemical and molecular mechanics geometry optimisations involving the whole enzyme. A four-coordinate active-site zinc ion is 100–200 kJ/mol more stable than a five-coordinate one, depending on the ligands. The only stable binding site for a fifth ligand at the zinc ion is opposite to the normal substrate site, in a small cavity buried behind the zinc ion. The zinc coordination sphere has to be strongly distorted to accommodate a ligand in this site, and the ligand makes awkward contacts with surrounding atoms. Thus, the results do not support proposals attributing an important role to five-coordinate zinc complexes in the catalytic mechanism of alcohol dehydrogenase. The present approach makes it possible also to quantify the strain induced by the enzyme onto the zinc ion and its ligands; it amounts to 42–87 kJ/mol for four-coordinate active-site zinc ion complexes and 131–172 kJ/mol for five-coordinate ones. The four-coordinate structure with a water molecule bound to the zinc ion is about 20 kJ/mol less strained than the corresponding structure with a hydroxide ion, indicating that the enzyme does not speed up the reaction by forcing the zinc coordination sphere into a structure similar to the reaction intermediates. 相似文献
70.