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排序方式: 共有619条查询结果,搜索用时 15 毫秒
1.
Hornstein MK Bajaj VS Griffin RG Temkin RJ 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》2006,34(3):524-533
We report the regulated continuous-wave (CW) operation of a second harmonic gyrotron oscillator at output power levels of over 8 W (12.4 kV and 135 mA beam voltage and current) in the TE(0,6,1) mode near 460 GHz. The gyrotron also operates in the second harmonic TE(2,6,1) mode at 456 GHz and in the TE(2,3,1) fundamental mode at 233 GHz. CW operation was demonstrated for a one-hour period in the TE(0,6,1) mode with better than 1% power stability, where the power was regulated using feedback control. Nonlinear simulations of the gyrotron operation agree with the experimentally measured output power and radio-frequency (RF) efficiency when cavity ohmic losses are included in the analysis. The output radiation pattern was measured using a pyroelectric camera and is highly Gaussian, with an ellipticity of 4%. The 460-GHz gyrotron will serve as a millimeter-wave source for sensitivity-enhanced nuclear magnetic resonance (dynamic nuclear polarization) experiments at a magnetic field of 16.4 T. 相似文献
2.
KL Britton HF Rogers Y Asano T Dairi Y Kato TJ Stillman DW Rice 《Acta crystallographica. Section D, Biological crystallography》1998,53(4):124-126
The novel NAD+-linked opine dehydrogenase from a soil isolate Arthrobacter sp. strain 1C belongs to an enzyme superfamily whose members exhibit quite diverse substrate specificites. Crystals of this opine dehydrogenase, obtained in the presence or absence of co-factor and substrates, have been shown to diffract to beyond 1.8 ? resolution. X-ray precession photographs have established that the crystals belong to space group P21212, with cell parameters a = 104.9, b = 80.0, c = 45.5 ? and a single subunit in the asymmetric unit. The elucidation of the three-dimensional structure of this enzyme will provide a structural framework for this novel class of dehydrogenases to enable a comparison to be made with other enzyme families and also as the basis for mutagenesis experiments directed towards the production of natural and synthetic opine-type compounds containing two chiral centres. 相似文献
3.
Applied Biochemistry and Biotechnology - The US chemical industry is facing maturing markets, variability in raw materials supply, and stiff competition from emerging chemical industries in other... 相似文献
4.
Bar G Bennati M Nguyen HH Ge J Stubbe JA Griffin RG 《Journal of the American Chemical Society》2001,123(15):3569-3576
High-frequency pulsed EPR and ENDOR have been employed to characterize the tyrosyl radical (Y*)-diiron cofactor in the Y2-containing R2 subunit of ribonucleotide reductase (RNR) from yeast. The present work represents the first use of 140-GHz time domain EPR and ENDOR to examine this system and demonstrates the capabilities of the method to elucidate the electronic structure and the chemical environment of protein radicals. Low-temperature spin-echo-detected EPR spectra of yeast Y* reveal an EPR line shape typical of a tyrosyl radical; however, when compared with the EPR spectra of Y* from E. coli RNR, a substantial upfield shift of the g(1)-value is observed. The origin of the shift in g(1) was investigated by 140-GHz (1)H and (2)H pulsed ENDOR experiments of the Y2-containing subunit in protonated and D(2)O-exchanged buffer. (2)H ENDOR spectra and simulations provide unambiguous evidence for one strongly coupled (2)H arising from a bond between the radical and an exchangeable proton of an adjacent residue or a water molecule. Orientation-selective 140-GHz ENDOR spectra indicate the direction of the hydrogen bond with respect to the molecular symmetry axes and the bond length (1.81 A). Finally, we have performed saturation recovery experiments and observed enhanced spin lattice relaxation rates of the Y* above 10 K. At temperatures higher than 20 K, the relaxation rates are isotropic across the EPR line, a phenomenon that we attribute to isotropic exchange interaction between Y* and the first excited paramagnetic state of the diiron cluster adjacent to it. From the activation energy of the rates, we determine the exchange interaction between the two irons of the cluster, J(exc) = -85 cm(-)(1). The relaxation mechanism and the presence of the hydrogen bond are discussed in terms of the differences in the structure of the Y*-diiron cofactor in yeast Y2 and other class I R2s. 相似文献
5.
D.C. Lankin G.W. Griffin I. Bernal J. Korp S.F. Watkins T.J. Delord N.S. Bhacca 《Tetrahedron》1984,40(15):2829-2836
Condensation of 2,5,5-trimethylhexa-2,3-dien-6-a1 with malononitrile affords an unexpected product, C15H16N4X, (), the structure of which was partially characterized by spectral (infrared, ultraviolet, 1H, and 13C nmr) methods and fully elucidated by single crystal X-ray analysis. is l-cyano-2-amino-3-(2-propenyl)-5,5-dimethy-6- dicyanomethycycohexa-l,3-diene in a half-chair conformation. Conjugation of the -aminocyanoethenyl moiety leads to intermolecular hydrogen bonding in the crystal. A reaction sequence and its mechanistic implications are proposed. 相似文献
6.
The use of DNA to template the assembly of nanoscale wires and protein-functionalized nanogap contacts is described: Specifically, the use of DNA to template the assembly of gold nanowires between conventionally patterned gold contacts on a silicon wafer substrate. Also described is the use of DNA to template the assembly of protein-functionalized nanogap gold contacts on a silicon wafer substrate. Of particular significance is the finding that suitably modified gold nanoparticles recognize and bind selectively the protein-functionalized nanogap and are localized there. 相似文献
7.
Computer-optimized selective pulses are routinely used in solution-state NMR spectroscopy. At the same time, their utility and importance for solid-state applications has yet to be fully realized. We suggest a new computational approach that makes the design of soft selective pulses with desired properties relatively straightforward. By applying this technique to the generic selective excitation problem, we have arrived at a family of high performance selective excitation pulses, dubbed E-Family, that allows more flexibility and better performance than analogous pulses previously reported in the literature. The new pulses have been successfully tested in both solid- and solution-state NMR experiments. A theoretical treatment of the effects of chemical shift anisotropy (CSA) on the selective excitation in magic-angle spinning (MAS) experiments in solids is presented. The set of heuristics that comprise our new strategy were incorporated into a general NMR simulation program SPINEVOLUTION. 相似文献
8.
G. T. Baldwin H. C. Griffin G. F. Knoll 《Journal of Radioanalytical and Nuclear Chemistry》1984,85(5):285-291
A procedure is described for the accelerator production of232Pa and its isolation in radiochemically pure form, suitable for isotope tracing of protactinium/thorium chemical separations. 相似文献
9.
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