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排序方式: 共有245条查询结果,搜索用时 15 毫秒
61.
Jennifer Solarczek Dr. Felix Kaspar Pia Bauer Prof. Anett Schallmey 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(72):e202202343
Halohydrin dehalogenases are promiscuous biocatalysts, which enable asymmetric ring opening reactions of epoxides with various anionic nucleophiles. However, despite the increasing interest in such asymmetric transformations, the substrate scope of G-type halohydrin dehalogenases toward cyclic epoxides has remained largely unexplored, even though this subfamily is the only one known to display activity with these sterically demanding substrates. Herein, we report on the exploration of the substrate scope of the two G-type halohydrin dehalogenases HheG and HheG2 and a newly identified, more thermostable member of the family, HheG3, with a variety of sterically demanding cyclic epoxides and anionic nucleophiles. This work shows that, in addition to azide and cyanide, these enzymes facilitate ring-opening reactions with cyanate, thiocyanate, formate, and nitrite, significantly expanding the known repertoire of accessible transformations. 相似文献
62.
Alongi KS Dibble TS Shields GC Kirschner KN 《The journal of physical chemistry. A》2006,110(10):3686-3691
The hydroperoxy radical (HO2) plays a critical role in Earth's atmospheric chemistry as a component of many important reactions. The self-reaction of hydroperoxy radicals in the gas phase is strongly affected by the presence of water vapor. In this work, we explore the potential energy surfaces of hydroperoxy radicals hydrogen bonded to one or two water molecules, and predict atmospheric concentrations and vibrational spectra of these complexes. We predict that when the HO2 concentration is on the order of 10(8) molecules x cm(-3) at 298 K, that the number of HO2...H2O complexes is on the order of 10(7) molecules x cm(-3) and the number of HO2...(H2O)2 complexes is on the order of 10(6) molecules x cm(-3). Using the computed abundance of HO2...H2O, we predict that, at 298 K, the bimolecular rate constant for HO2...H2O + HO2 is about 10 times that for HO2 + HO2. 相似文献
63.
E. Kuzmann Z. Homonnay A. Vértes I. Halász J. Bánkuti I. Kirschner 《Journal of Radioanalytical and Nuclear Chemistry》1989,136(2):121-125
57Fe Mössbauer spectroscopy was used to study57Fe doped high Tc TlBaCaCuO4.5+y superconductor between 70 and 295 K. Significant anomalous changes in the shape of the Mössbauer spectrum were observed around the Tc /105–120 K/. This experimental fact could be reproduced. Our finding can be associated with structural changes taking place around the Tc in TlBaCaCuO4.5+y superconductor. 相似文献
64.
B. Ermolaev R. Kirschner L. Szymanowski 《The European Physical Journal C - Particles and Fields》1999,7(1):65-72
We study the small-x behaviour of the polarized photon structure function F3gF_3^{\gamma}, measuring the gluon transversity distribution, in the leading logarithmic approximation of perturbative QCD. There are two contributions, both arising from two-gluon exchange. The leading contribution to small-x is related to the BFKL pomeron and behaves like x-1-w2x^{-1-\omega_2}, w2 = O(aS)\omega_2 ={\cal O}(\alpha_S). The other contribution includes in particular the ones summed by the DGLAP equation and behaves like x1-w0(+)x^{1-\omega_0^{(+)}}, w0(+) = O(?{aS})\omega_0^{(+)} = {\cal O}(\sqrt{\alpha_S}). 相似文献
65.
N. A. Willoughby T. Kirschner M. P. Smith R. Hjorth N. J. Titchener-Hooker 《Journal of chromatography. A》1999,840(2):375-204
Alcohol dehydrogenase (ADH) from solutions of homogenised packed bakers’ yeast has been successfully purified using immobilised metal-ion affinity chromatography in an expanded bed. Method scouting carried out using pure ADH solutions loaded onto 5-ml HiTrap columns charged with Zn2+, Ni2+ and Cu2+ and eluted using 0–50 mM EDTA gradient found that charging with Zn2+ gave the highest recovery and the lowest EDTA concentration required for elution. These results were used to develop a protocol for the expanded bed system and further tested using clarified yeast homogenate loaded onto XK16/20 packed beds (approximately 30 ml) packed with Chelating Sepharose FastFlow matrix in order to determine the optimum elution conditions using EDTA. The ADH was found to elute at 5 mM EDTA and the dynamic and total binding capacities of Streamline chelating for ADH were found to be 235 U/ml and 1075 U/ml matrix, respectively. Expanded bed work based on a step EDTA elution protocol demonstrated that ADH could be successfully eluted from unclarified homogenised bakers’ yeast diluted to 10 mg/ml total protein content with a recovery of 80–100% that was maintained over five consecutive runs with a vigorous clean-in-place procedure between each run. 相似文献
66.
The Gaussian-3 (G3) model chemistry method has been used to calculate the relative deltaG(o) values for all possible conformers of neutral clusters of water, (H2O)n, where n = 3-5. A complete 12-fold conformational search around each hydrogen bond produced 144, 1728, and 20,736 initial starting structures of the water trimer, tetramer, and pentamer. These structures were optimized with PM3, followed by HF/6-31G* optimization, and then with the G3 model chemistry. Only two trimers are present on the G3 potential energy hypersurface. We identified 5 tetramers and 10 pentamers on the potential energy and free-energy hypersurfaces at 298 K. None of these 17 structures were linear; all linear starting models folded into cyclic or three-dimensional structures. The cyclic pentamer is the most stable isomer at 298 K. On the basis of this and previous studies, we expect the cyclic tetramers and pentamers to be the most significant cyclic water clusters in the atmosphere. 相似文献
67.
We present experimental evidence for a three-dimensional noncollinear antiferromagnetic spin structure in ultrathin single-crystalline fcc Fe50Mn50 layers using magnetic circular dichroism photoelectron emission microscopy and x-ray magnetic linear dichroism. Layer-resolved as-grown domain images of epitaxial trilayers grown on Cu(001) in which FeMn is sandwiched between ferromagnetic layers with different easy axes reveal the presence of antiferromagnetic spin components in the film plane and normal to the film plane. An FeMn spin structure with no collinear order in the film plane is consistent with the absence of x-ray magnetic linear dichroism in Fe L3 absorption in FeMn/Co bilayers. 相似文献
68.
A.?IvanovEmail author R.?Kirschner 《The European Physical Journal C - Particles and Fields》2004,36(1):43-48
The large t behavior of the helicity amplitudes of diffractive photoproduction is estimated relying on models of the photon and meson light-cone wave functions and on the double-logarithmic approximation to the exchanged gluon interaction. The role of large-size color dipole contributions to the photon-meson transition impact factor is discussed.Received: 17 November 2003, Revised: 10 March 2004, Published online: 23 June 2004A. Ivanov: Corresponding author. 相似文献
69.
The stable magnetization configurations of a ferromagnet on a quasiperiodic tiling have been derived theoretically. The magnetization configuration is investigated as a function of the ratio of the exchange to the dipolar energy. The exchange coupling is assumed to decrease exponentially with the distance between magnetic moments. It is demonstrated that for a weak exchange interaction the new structure, the quasiferromagnetic decagonal configuration, corresponds to the minimum of the free energy. The decagonal state represents a new class of frustrated systems where the degenerated ground state is aperiodic and consists of two parts: ordered decagon rings and disordered spin-glass-like phase inside the decagons. 相似文献
70.
The theory of spin-polarized low-energy electron diffraction (SPLEED) is outlined with an emphasis on symmetry-induced properties and models for the effective single-sphere potential. Details are given of the experimental set-up involving a new type of spin polarization detector based on SPLEED. Numerical calculations for the W(001)(1× 1) surface proceeded in close coordination with measurements of spin polarization (normal to the scattering plane) and intensity. A variety of calculated spin polarization and intensity profiles exhibits features, which are very sensitive to changes in the top interlayer spacing, as well as features sensitive to various ingredients of the potential model. The overall agreement between theory and experiment is very good not only for intensities, but also for spin polarization. It implies a contraction of the topmost interlayer spacing of W(001) with respect to the bulk spacing by 7% ± 1.5%, and supports the use of an energy-dependent exchange part and a nonuniform absorption part in the effective ion-core potential. 相似文献