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991.
[reaction: see text] L-(+)-carbafuranomycin is a novel analogue of L-(+)-furanomycin, an unusual antibiotic alpha-amino acid that attracted great interest due to its activity as an isoleucine antagonist. We present here a concise and efficient asymmetric synthesis of this carba-analogue starting with the 1,3-dipolar cycloaddition of a chiral nitrile oxide with cyclopentadiene. Notably, the methyl group was introduced by an S(N)2' cuprate substitution with high stereo- and regioselectivity. 相似文献
992.
High-dimensional, multispectral data on complex physical systems are increasingly common. As the amount of information in data sets increases, the difficulty of effectively utilizing it also increases. For such data, summary information is required for understanding and modeling the underlying dynamics. It is here proposed to use an extension of computational mechanics [C. R. Shalizi and J. P. Crutchfield, J. Stat. Phys. 104, 817 (2001)] to arbitrary spatiotemporal and spectral dimension, for providing such summary information. An example of the use of these tools to identify state evolution in the brain, an archetypal, complex biophysical system, serves as an illustration. 相似文献
993.
Although time-frequency analysis is effective for characterizing dispersive wave signals, the time-frequency tilings of most conventional analysis methods do not take into account dispersion phenomena. An adaptive time-frequency analysis method is introduced whose time-frequency tiling is determined with respect to the wave dispersion characteristics. In the dispersion-based time-frequency tiling, each time-frequency atom is adaptively rotated in the time-frequency plane, depending on the local wave dispersion. Although this idea can be useful in various problems, its application to the analysis of dispersive wave signals has not been made. In this work, the adaptive time-frequency method was applied to the analysis of dispersive elastic waves measured in waveguide experiments and a theoretical investigation on its time-frequency resolution was presented. The time-frequency resolution of the proposed transform was then compared with that of the standard short-time Fourier transform to show its effectiveness in dealing with dispersive wave signals. In addition, to facilitate the adaptive time-frequency analysis of experimentally measured signals whose dispersion relations are not known, an iterative scheme for determining the relationships was developed. The validity of the present approach in dealing with dispersive waves was verified experimentally. 相似文献
994.
Kim YY Park CI Cho SH Han SW 《The Journal of the Acoustical Society of America》2005,117(6):3459-3468
For the efficient long-range nondestructive structural health inspection of pipes, guided waves have become widely used. Among the various guided wave modes, the torsional wave is most preferred since its first branch is nondispersive. Our objective in this work is to develop a new magnetostrictive transducer configuration to transmit and receive torsional waves in cylindrical waveguides. The conventional magnetostrictive transducer for the generation and measurement of torsional waves consists of solenoid coils and a nickel strip bonded circumferentially to test pipes. The strip must be premagnetized by a permanent magnet before actual measurements. Because of the premagnetization, the transducer is not suitable for the long-term on-line monitoring of pipes buried underground. To avoid the cumbersome premagnetization and to improve the transduction efficiency, we propose a new transducer configuration using several pieces of nickel strips installed at 45 degrees with respect to the pipe axis. If a static bias magnetic field is also applied, the transducer output can be substantially increased. Several experiments were conducted to study the performance of the proposed transducer configuration. The proposed transducer configuration was also applied for damage detection in an aluminum pipe. 相似文献
995.
Cai YQ Mao HK Chow PC Tse JS Ma Y Patchkovskii S Shu JF Struzhkin V Hemley RJ Ishii H Chen CC Jarrige I Chen CT Shieh SR Huang EP Kao CC 《Physical review letters》2005,94(2):025502
The near K-edge structure of oxygen in liquid water and ices III, II, and IX at 0.25 GPa and several low temperatures down to 4 K has been studied using inelastic x-ray scattering at 9884.7 eV with a total energy resolution of 305 and 175 meV. A marked decrease of the preedge intensity from the liquid phase and ice III to ices II and IX is attributed to ordering of the hydrogen bonds in the proton-ordered lattice of the latter phases. Density functional theory calculations including the influence of the Madelung potential of the ice IX crystal correctly account for the remaining preedge feature. Furthermore, we obtain spectroscopic evidence suggesting a possible new phase of ice at temperatures between 4 and 50 K. 相似文献
996.
The pertinence of expression of heat shock proteins (HSPs) to the efficacy of cryopreservation in HELAs 总被引:1,自引:0,他引:1
HELAs (Hela cells, passed cells of human cervical carcinoma) were heat or cold treated (named heat or cold shock) and then resumed normal culture for 2, 4 or 8 hours respectively. The expressions of heat shock protein 70 (HSP70) and 90 (HSP90) of the HELAs were measured by Northern and Western blotting. HELAs after 4-hour culture were exposed to or cryopreserved with different concentration of dimethyl sulfoxide (Me2SO, 2.5%, 5%, 10%, 15% and 20% respectively, V/V). Meanwhile, the HELAs after different culture time (2, 4 and 6 hours of culture) were cryopreserved with 5% Me2SO. After exposure or cryopreservation, the number of live HELAs was counted and the survival rate was calculated. The results showed that heat shock increased the expression of HSP70 and HSP90 of HELAs, while cold shock decreased the expression of the two proteins. When the concentrations of Me2SO were 10%, 15% and 20%, the survival rates of HELAs after exposure to Me2SO or cryopreservation were much lower than those when the concentrations were small. The survival rates of the heat shocked HELAs were significantly higher than those of the cold shocked and control HELAs. After cryopreservation with 5% Me2SO, the survival rate of heat shocked HELAs group with 2 hours culture time was the lowest among all the groups of HELAs with different cultural time. From the results of this study, we conclude that the expressions of HSP70 and HSP90 in HELAs increased significantly after heat shock, while cold shock decreased the expressions of these two proteins. The over-expressions of HSPs in the heat shocked HELAs could protect the cells from both injury caused by potential toxicity of high concentrations of Me2SO and cryoinjury caused by the freeze-thawing/cryopreservation procedure. 相似文献
997.
We propose and demonstrate a photonic phase shifter based a on vector-sum technique that uses polarization-maintaining fibers (PMFs). We achieved a continuous and full phase shift up to 2pi at 30.48 GHz by controlling the polarization state of the light injected into two pieces of PMF of different lengths and applying two different modulator bias voltages. 相似文献
998.
We propose and demonstrate a novel optically tunable dynamic dispersion-compensation device with coupled-cavity etalon structure. Experimentally, it was achieved with an all-fiber coupled-cavity etalon made in an Er/Yb codoped fiber pumped by a 980-nm laser diode. The dispersion was tuned from -300 to +400 ps/nm in the experiment. The potential advantages of such optically tunable dispersion compensators include fast response time and remotely controllable operation. 相似文献
999.
Dewald EL Suter LJ Landen OL Holder JP Schein J Lee FD Campbell KM Weber FA Pellinen DG Schneider MB Celeste JR McDonald JW Foster JM Niemann C Mackinnon AJ Glenzer SH Young BK Haynam CA Shaw MJ Turner RE Froula D Kauffman RL Thomas BR Atherton LJ Bonanno RE Dixit SN Eder DC Holtmeier G Kalantar DH Koniges AE Macgowan BJ Manes KR Munro DH Murray JR Parham TG Piston K Van Wonterghem BM Wallace RJ Wegner PJ Whitman PK Hammel BA Moses EI 《Physical review letters》2005,95(21):215004
The first hohlraum experiments on the National Ignition Facility (NIF) using the initial four laser beams tested radiation temperature limits imposed by plasma filling. For a variety of hohlraum sizes and pulse lengths, the measured x-ray flux shows signatures of filling that coincide with hard x-ray emission from plasma streaming out of the hohlraum. These observations agree with hydrodynamic simulations and with an analytical model that includes hydrodynamic and coronal radiative losses. The modeling predicts radiation temperature limits with full NIF (1.8 MJ), greater, and of longer duration than required for ignition hohlraums. 相似文献
1000.
Two spatially separate Bose-Einstein condensates were prepared in an optical double-well potential. A bidirectional coupling between the two condensates was established by two pairs of Bragg beams which continuously outcoupled atoms in opposite directions. The atomic currents induced by the optical coupling depend on the relative phase of the two condensates and on an additional controllable coupling phase. This was observed through symmetric and antisymmetric correlations between the two outcoupled atom fluxes. A Josephson optical coupling of two condensates in a ring geometry is proposed. The continuous outcoupling method was used to monitor slow relative motions of two elongated condensates and characterize the trapping potential. 相似文献