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排序方式: 共有196条查询结果,搜索用时 203 毫秒
191.
192.
Although the three-dimensional structure of the bacterial photosynthetic reaction center (RC) reveals a high level of structural symmetry, with two nearly equivalent potential electron transfer pathways, the RC is functionally asymmetric: Electron transfer occurs along only one of the two possible pathways. In order to determine the origins of this symmetry breaking, the internal electric field present in the RC when charge is separated onto structurally characterized sites was probed by using absorption band shifts of the chromophores within the RC. The sensitivity of each probe chromophore to an electric field was calibrated by measuring the Stark effect spectrum, the change in absorption due to an externally applied electric field. A quantitative comparison of the observed absorption band shifts and those predicted from vacuum electrostatics gives information on the effective dielectric constant of the protein complex. These results reveal a significant asymmetry in the effective dielectric strength of the protein complex along the two potential electron transfer pathways, with a substantially higher dielectric strength along the functional pathway. This dielectric asymmetry could be a dominant factor in determining the functional asymmetry of electron transfer in the RC. 相似文献
193.
Coleman JR Papamichail D Skiena S Futcher B Wimmer E Mueller S 《Science (New York, N.Y.)》2008,320(5884):1784-1787
As a result of the redundancy of the genetic code, adjacent pairs of amino acids can be encoded by as many as 36 different pairs of synonymous codons. A species-specific "codon pair bias" provides that some synonymous codon pairs are used more or less frequently than statistically predicted. We synthesized de novo large DNA molecules using hundreds of over-or underrepresented synonymous codon pairs to encode the poliovirus capsid protein. Underrepresented codon pairs caused decreased rates of protein translation, and polioviruses containing such amino acid-independent changes were attenuated in mice. Polioviruses thus customized were used to immunize mice and provided protective immunity after challenge. This "death by a thousand cuts" strategy could be generally applicable to attenuating many kinds of viruses. 相似文献
194.
Carter JA Fabrycky DC Ragozzine D Holman MJ Quinn SN Latham DW Buchhave LA Van Cleve J Cochran WD Cote MT Endl M Ford EB Haas MR Jenkins JM Koch DG Li J Lissauer JJ MacQueen PJ Middour CK Orosz JA Rowe JF Steffen JH Welsh WF 《Science (New York, N.Y.)》2011,331(6017):562-565
The Kepler spacecraft has been monitoring the light from 150,000 stars in its primary quest to detect transiting exoplanets. Here, we report on the detection of an eclipsing stellar hierarchical triple, identified in the Kepler photometry. KOI-126 [A, (B, C)], is composed of a low-mass binary [masses M(B) = 0.2413 ± 0.0030 solar mass (M(⊙)), M(C) = 0.2127 ± 0.0026 M(⊙); radii R(B) = 0.2543 ± 0.0014 solar radius (R(⊙)), R(C) = 0.2318 ± 0.0013 R(⊙); orbital period P(1) = 1.76713 ± 0.00019 days] on an eccentric orbit about a third star (mass M(A) = 1.347 ± 0.032 M(⊙); radius R(A) = 2.0254 ± 0.0098 R(⊙); period of orbit around the low-mass binary P(2) = 33.9214 ± 0.0013 days; eccentricity of that orbit e(2) = 0.3043 ± 0.0024). The low-mass pair probe the poorly sampled fully convective stellar domain offering a crucial benchmark for theoretical stellar models. 相似文献
195.
Sleep facilitates memory consolidation. A widely held model assumes that this is because newly encoded memories undergo covert reactivation during sleep. We cued new memories in humans during sleep by presenting an odor that had been presented as context during prior learning, and so showed that reactivation indeed causes memory consolidation during sleep. Re-exposure to the odor during slow-wave sleep (SWS) improved the retention of hippocampus-dependent declarative memories but not of hippocampus-independent procedural memories. Odor re-exposure was ineffective during rapid eye movement sleep or wakefulness or when the odor had been omitted during prior learning. Concurring with these findings, functional magnetic resonance imaging revealed significant hippocampal activation in response to odor re-exposure during SWS. 相似文献
196.
Carsten Schulz Steffen Günther Manfred Wirth Bernhard Rennert 《Aquaculture International》2006,14(6):577-586
To assess the effect of dietary composition on growth performance and body composition of pike perch (Sander lucioperca), fingerlings with an initial body weight of 1.36 g (just trained to accept formulated feed) were fed three experimental diets in triplicate for 90 days. Two feeding groups were fed with formulated diets (CD, CD+7) containing varying levels of crude lipid (CL) of 14.65% and 21.94% dry matter (d.m.) with crude protein (CP) levels of 59.73% and 56.56%, and one feeding group was fed a natural diet (chironomids, CP = 65.93% d.m.; CL = 7.20% d.m.). Furthermore, pike perch of the same age caught in different natural habitats were analysed to determine their naturally fluctuations in body composition. Specific growth rate (SGR; CD = 3.36, CD+7 = 3.47) and feed conversion ratio (FCR; CD = 1.02, CD+7 = 0.93) of fish fed formulated diets did not differ significantly with rising dietary lipid content, due to high variability within the individuals of each feeding groups. In contrast, pike perch fed with chironomids showed a significantly lower SGR of 2.49 and higher FCR of 2.37 (on a dry matter basis). Body composition of pike perch fed formulated diets was affected by dietary composition and showed increased lipid contents [CD=6.25% original matter (o.m.), CD+7 = 9.00% o.m.] with rising dietary lipid levels. Pike perch of CD and CD+7 feeding groups showed significant increased hepatosomatic indices (HSIs) of 1.99 and 2.05 in contrast to fish fed chironomids with HSI of 1.11. Fish caught in the different natural habitats were characterised by low body lipid and dry matter contents of 0.64–1.88% o.m. and 21.08–23.75% o.m. Higher lipid incorporation of fish fed with formulated diets accompanied with poor benefit on growth performance at higher dietary lipid content indicated that pike perch ability to utilise lipids is low when dietary crude protein content is higher than 56.56%. 相似文献