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排序方式: 共有239条查询结果,搜索用时 15 毫秒
61.
The distillation and volatility of ionic liquids 总被引:2,自引:0,他引:2
Earle MJ Esperança JM Gilea MA Lopes JN Rebelo LP Magee JW Seddon KR Widegren JA 《Nature》2006,439(7078):831-834
It is widely believed that a defining characteristic of ionic liquids (or low-temperature molten salts) is that they exert no measurable vapour pressure, and hence cannot be distilled. Here we demonstrate that this is unfounded, and that many ionic liquids can be distilled at low pressure without decomposition. Ionic liquids represent matter solely composed of ions, and so are perceived as non-volatile substances. During the last decade, interest in the field of ionic liquids has burgeoned, producing a wealth of intellectual and technological challenges and opportunities for the production of new chemical and extractive processes, fuel cells and batteries, and new composite materials. Much of this potential is underpinned by their presumed involatility. This characteristic, however, can severely restrict the attainability of high purity levels for ionic liquids (when they contain poorly volatile components) in recycling schemes, as well as excluding their use in gas-phase processes. We anticipate that our demonstration that some selected families of commonly used aprotic ionic liquids can be distilled at 200-300 degrees C and low pressure, with concomitant recovery of significant amounts of pure substance, will permit these currently excluded applications to be realized. 相似文献
62.
针对有限单元法计算精度受网格密度制约的问题,提出一种求任意复杂杆系静、动力学问题精确解的方法.该方法基于节点分析法,逐个节点列位移相容方程和内力平衡方程,以谱系数作为未知数,杆件变形和截面内力则采用相应问题的精确解来描述.用代数符号化的方式,对方程系数矩阵的组建过程进行了详细描述,为通用程序编制提供依据.对建模过程中的分布式载荷和铰接接头的处理方法进行了探讨并给出处理方法.通过平面钢架屈曲分析和空间管路谐响应分析,证实所提方法具有计算精度高、划分单元数少的特点,特别适合于需要精确解的场合. 相似文献
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Unique astrocyte ribbon in adult human brain contains neural stem cells but lacks chain migration 总被引:1,自引:0,他引:1
Sanai N Tramontin AD Quiñones-Hinojosa A Barbaro NM Gupta N Kunwar S Lawton MT McDermott MW Parsa AT Manuel-García Verdugo J Berger MS Alvarez-Buylla A 《Nature》2004,427(6976):740-744
The subventricular zone (SVZ) is a principal source of adult neural stem cells in the rodent brain, generating thousands of olfactory bulb neurons every day. If the adult human brain contains a comparable germinal region, this could have considerable implications for future neuroregenerative therapy. Stem cells have been isolated from the human brain, but the identity, organization and function of adult neural stem cells in the human SVZ are unknown. Here we describe a ribbon of SVZ astrocytes lining the lateral ventricles of the adult human brain that proliferate in vivo and behave as multipotent progenitor cells in vitro. This astrocytic ribbon has not been observed in other vertebrates studied. Unexpectedly, we find no evidence of chains of migrating neuroblasts in the SVZ or in the pathway to the olfactory bulb. Our work identifies SVZ astrocytes as neural stem cells in a niche of unique organization in the adult human brain. 相似文献
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Martínez-Salgado C Rodríguez-Peña AB López-Novoa JM 《Cellular and molecular life sciences : CMLS》2008,65(3):477-492
The mechanisms involved in the development of renal fibrosis are poorly understood. Small Ras GTPases control cell proliferation,
differentiation, cellular growth and apoptosis, with cell-specific expression in the kidney. Cytokines, high glucose medium
or advanced glycation end-products activate Ras in different renal cells. Increased Ras activation has been found in experimental
tubulointerstitial fibrosis. Transforming growth factor-β1 (TGF-β1) and Ras signalling pathways are close related: TGF-β1
overcomes Ras mitogenic effects, and Ras counteracts TGF-β signalling. However, Ras activation is also an intracellular signal
transduction point for several molecules (e.g. TGF-β1) involved in kidney damage. Ras isoforms play different roles in regulating extracellular matrix synthesis in fibroblasts
and mesangial cells. These data give evidence for a role for Ras in renal fibrosis, but no reviews are available on the role
of p21 Ras in this process. Thus, our goal is to review the role of Ras activation and signalling in renal fibrosis.
Received 7 June 2007; received after revision 17 September 2007; accepted 1 October 2007 相似文献
68.
Franco-Zorrilla JM Valli A Todesco M Mateos I Puga MI Rubio-Somoza I Leyva A Weigel D García JA Paz-Ares J 《Nature genetics》2007,39(8):1033-1037
MicroRNAs (miRNA) regulate key aspects of development and physiology in animals and plants. These regulatory RNAs act as guides of effector complexes to recognize specific mRNA sequences based on sequence complementarity, resulting in translational repression or site-specific cleavage. In plants, most miRNA targets are cleaved and show almost perfect complementarity with the miRNAs around the cleavage site. Here, we examined the non-protein coding gene IPS1 (INDUCED BY PHOSPHATE STARVATION 1) from Arabidopsis thaliana. IPS1 contains a motif with sequence complementarity to the phosphate (Pi) starvation-induced miRNA miR-399, but the pairing is interrupted by a mismatched loop at the expected miRNA cleavage site. We show that IPS1 RNA is not cleaved but instead sequesters miR-399. Thus, IPS1 overexpression results in increased accumulation of the miR-399 target PHO2 mRNA and, concomitantly, in reduced shoot Pi content. Engineering of IPS1 to be cleavable abolishes its inhibitory activity on miR-399. We coin the term 'target mimicry' to define this mechanism of inhibition of miRNA activity. Target mimicry can be generalized beyond the control of Pi homeostasis, as demonstrated using artificial target mimics. 相似文献
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Anisotropy and Corotation of Galactic Cosmic Rays 总被引:1,自引:0,他引:1
Based on some billion cosmic ray events collected from to by the Tibet Air Shower Array experiment 《中国科学基金(英文版)》2007,15(2):18-20
Based on some 40 billion cosmic ray events collected from 1997 to 2005 by the Tibet Air Shower Array experiment (a major scientific collaboration between China and Japan) operating at the YangBaJing Cosmic Ray Observatory (90.522 E, 30. 102 N; 4300 m above sea level) near Lhasa in Tibet, a two-dimensiondl cosmic-ray intensity map in the sky was obtained with very high directional granularity and unprecedented precision in intensity at a level of 10-4. 相似文献