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  • 1
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    German Medical Science; Düsseldorf, Köln
    In:  International Conference on SARS - one year after the (first) outbreak; 20040508-20040511; Lübeck; DOC04sars2.01 /20040526/
    Publication Date: 2004-05-26
    Keywords: ddc: 610
    Language: English
    Type: conferenceObject
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  • 2
    ISSN: 1432-0614
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract The operon encoding aspartokinase and aspartate semialdehyde dehydrogenase was cloned and sequenced from rifamycin-SV-producing Amycolatopsis mediterranei U32 previously. In the present work, these two genes were introduced into the auxotrophic Escherichia coli strain CGSC5074 (ask −) and E. coli X6118 (asd −), respectively. The A. mediterranei U32 aspartokinase and aspartate semialdehyde dehydrogenase genes can be functionally expressed in E. coli and the gene products are able to substitute for the E. coli enzymes. Histidine-tagged aspartokinase and aspartate semialdehyde dehydrogenase were partially purified from E. coli cellular extracts and their kinetic characteristics were studied. Both aspartokinase and aspartate semialdehyde dehydrogenase showed typical Michaelis-Menten type substrate saturation patterns. Aspartokinase has K m values of 3.4 mM for aspartate and 2.3 mM for ATP, while aspartate semialdehyde dehydrogenase has K m values of 1.25 mM for dl-aspartate semialdehyde and 0.73 mM for NADP, respectively. Aspartokinase was inhibited by l-threonine, l-lysine, and l-methionine, but not by l-isoleucine and diaminopimelate. Aspartate semialdehyde dehydrogenase was not inhibited by any of the end-product amino acids at a concentration of less than 5 mM. Hill plot analysis suggested that aspartokinase was subject to allosteric control by l-threonine. Repression of both aspartokinase and aspartate semialdehyde dehydrogenase gene transcription in A. mediterranei U32 by l-lysine, l-methionine, l-threonine, and l-isoleucine were found. The network of regulation of aspartokinase and aspartate semialdehyde dehydrogenase in rifamycin SV-producing A. mediterranei U32 is presented.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We have built a relatively simple, highly efficient, THz emission and detection system centered around a 15 fs Ti:sapphire laser. In the system, 200 mW of laser power is focused to a 120 μm diam spot between two silverpaint electrodes on the surface of a semi-insulating GaAs crystal, kept at a temperature near 300 K, biased with a 50 kHz, ±400 V square wave. Using rapid delay scanning and lock-in detection at 50 kHz, we obtain probe laser quantum-noise limited signals using a standard electro-optic detection scheme with a 1-mm-thick (110) oriented ZnTe crystal or a (110) oriented 0.1-mm-thick GaP crystal. The maximum THz-induced differential signal that we observe is ΔI/I=7×10−3, corresponding to a THz peak amplitude of 95 V/cm. The THz average power was measured to be about 40 μW, to our knowledge, the highest power reported so far generated with Ti:sapphire oscillators as a pump source. The system uses off-the-shelf electronics and requires no microfabrication techniques. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 77 (2000), S. 2195-2197 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Si delta-doped GaN has been grown by metalorganic chemical-vapor deposition. A very high peak density and narrow full width at half maximum (FWHM) of the carrier profile are obtained. It is found that the peak carrier density of Si delta doping increases with the doping time and SiH4 flow rate, while the FWHM of the carrier profile decreases with both increasing doping time and SiH4 flow rate. Some saturation in the carrier density has also been observed for relatively longer doping time. Except for a broadened carrier distribution in GaN induced by Si diffusion due to high growth temperature, the Si delta-doping properties in GaN are found to be similar to those of GaAs. © 2000 American Institute of Physics.
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  • 5
    ISSN: 1750-3841
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition , Process Engineering, Biotechnology, Nutrition Technology
    Notes: : At the same protein concentration, the protein extracts from Se-enriched Ganoderma lucidum(Se-GLPr) exhibited strong DNA protective effects from oxidative damage, which increased with the increase of Se content as suggested by chemiluminescence analysis, indicating indirectly that Se plays an important role in increasing the antioxidant activities of protein extracts. This was confirmed by spin-trapping experiments showing that Se-GLPr exhibited higher activities of scavenging superoxide and hydroxyl radicals than its analog, common Ganoderma lucidumextract. All Se-GLPr samples showed stronger activities of attenuating the production of superoxide radical than that of hydroxyl radical.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 2186-2191 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The phenomenon of magnetization reversal in a hard magnet is studied analytically within a framework of a simplified energy minimization model for a two grain system. The transition region (a domain wall-like moment structure) is found to play a crucial role in nucleating a reversed domain and in the subsequent formation of a domain wall near the grain boundary. This is shown to result in a considerable reduction in coercivity. Temperature effects are taken into account by considering thermally activated jumps over energy barriers that hinder the motion of the transition region. We found that thermal fluctuations at room temperature can substantially reduce coercivity provided the grains are sufficiently small. © 2002 American Institute of Physics.
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  • 7
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: As a representative natural porphyrin derivative, it is interesting to know the chemical form of lanthanide in chlorophyll a. We select the natural fern dicranopteris dichotoma, the most lanthanide-concentrated plant known, to isolate the chlorophyll a for our study. It is found by EXAFS that lanthanum substitutes in the magnesium position in chlorophyll and coordinates with the porphyrin ring. The lanthanum is seen to have a similar coordination structure to a sandwich-type lanthanide complex, with the La surrounded by eight nitrogen atoms from two porphyrin rings with average La-N bond length of 2.65 Å.
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  • 8
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract This paper presents evidence for long-term optical variability and colour behaviour of the blazar 3C 345 (1641+399, z=0.595). Our results show that the amplitude of the optical variations of 3C 345 has been only about 3 magnitudes from its photometric history; existent significant correlation between brightness and colours is found for 3C 345. Our recent CCD photometry of 3C 345 in May 1996 and May 1997 showed that they are in good agreement with our prediction of the optical variability period of about 10 years (see Zhang et al., 1998; Webb et al., 1988).
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 79 (2001), S. 2567-2569 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: GaN films were grown on sapphire and SiC substrates. The crystal qualities of GaN films were investigated by photoluminescence, atomic force microscopy, and electron-beam-induced current measurements, etc. It was found that the crystal quality of GaN on SiC is better than the one on sapphire. Ni/Au Schottky contacts were formed on the both samples. The electronic characteristics were obtained by current–voltage and capacitance–voltage measurements. Schottky diodes on sapphire substrate show breakdown voltage of −80 V. While for SiC substrate, the strong breakdown was not observed even at −100 V. The reverse leakage current of diodes based on SiC is over three orders of magnitude lower than that of sapphire substrate when the reverse voltage is above 50 V, which is due to the presence of low dislocation density and high thermal conductivity. © 2001 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 5532-5534 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The underlying mechanism of coercivity in permanent magnets has been a topic of intense interest for many years. It is motivated by the fact that the measured coercivity approaches only 20%–40% of the theoretical nucleation fields as derived from micromagnetic theory. We address this problem by proposing an analytical model, within the framework of the micromagnetic approximation, to examine the mechanism of magnetization reversal in hard magnetic materials. The exchange interaction between neighboring grains with different easy axes orientations can result in the formation of a domain wall-like magnetization structure (transition region) in the grain boundary. We propose that the transition region can propagate between neighboring grains provided that it is energetically favorable. The subsequent nucleation of a domain wall is shown to reduce the critical field considerably. Applying our model to a thin film, whose magnetic grains have a randomly oriented in-plane easy axis distribution, we have calculated the coercivity for the film to be 0.14HK, where HK is the anisotropy field. It is found that the coercivity decreases with increasing film thickness. For a material with a three-dimensional random easy axis distribution, we obtain the coercivity as 0.16HK. These results are substantially lower than that given by the Stoner-Wohlfarth model and are consistent with available experimental results. © 2000 American Institute of Physics.
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