首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1115篇
  免费   14篇
  国内免费   30篇
化学   632篇
晶体学   1篇
力学   32篇
数学   369篇
物理学   125篇
  2020年   7篇
  2018年   7篇
  2016年   18篇
  2015年   9篇
  2014年   17篇
  2013年   70篇
  2012年   32篇
  2011年   39篇
  2010年   28篇
  2009年   32篇
  2008年   40篇
  2007年   37篇
  2006年   42篇
  2005年   35篇
  2004年   45篇
  2003年   43篇
  2002年   36篇
  2001年   21篇
  2000年   18篇
  1999年   9篇
  1998年   7篇
  1997年   19篇
  1996年   23篇
  1995年   24篇
  1993年   20篇
  1992年   8篇
  1991年   17篇
  1990年   12篇
  1989年   14篇
  1988年   18篇
  1987年   19篇
  1986年   10篇
  1985年   25篇
  1984年   17篇
  1983年   17篇
  1982年   25篇
  1981年   26篇
  1980年   29篇
  1979年   28篇
  1978年   19篇
  1977年   18篇
  1976年   19篇
  1975年   13篇
  1974年   14篇
  1973年   16篇
  1972年   11篇
  1971年   8篇
  1970年   7篇
  1967年   7篇
  1966年   7篇
排序方式: 共有1159条查询结果,搜索用时 31 毫秒
1.
By transplanting identity elements into E. coli tRNAfMet, we have engineered an orthogonal initiator tRNA (itRNATy2) that is a substrate for Methanocaldococcus jannaschii TyrRS. We demonstrate that itRNATy2 can initiate translation in vivo with aromatic non-canonical amino acids (ncAAs) bearing diverse sidechains. Although the initial system suffered from low yields, deleting redundant copies of tRNAfMet from the genome afforded an E. coli strain in which the efficiency of non-canonical initiation equals elongation. With this improved system we produced a protein containing two distinct ncAAs at the first and second positions, an initial step towards producing completely unnatural polypeptides in vivo. This work provides a valuable tool to synthetic biology and demonstrates remarkable versatility of the E. coli translational machinery for initiation with ncAAs in vivo.  相似文献   
2.
3.
4.
5.
Hydrogen–deuterium exchange (HDX) experiments analyzed by mass spectrometry (MS) provide information about the dynamics and the solvent accessibility of protein backbone amide hydrogen atoms. Continuous improvement of MS instrumentation has contributed to the increasing popularity of this method; however, comprehensive automated data analysis is only beginning to mature. We present Hexicon 2, an automated pipeline for data analysis and visualization based on the previously published program Hexicon (Lou et al. 2010). Hexicon 2 employs the sensitive NITPICK peak detection algorithm of its predecessor in a divide-and-conquer strategy and adds new features, such as chromatogram alignment and improved peptide sequence assignment. The unique feature of deuteration distribution estimation was retained in Hexicon 2 and improved using an iterative deconvolution algorithm that is robust even to noisy data. In addition, Hexicon 2 provides a data browser that facilitates quality control and provides convenient access to common data visualization tasks. Analysis of a benchmark dataset demonstrates superior performance of Hexicon 2 compared with its predecessor in terms of deuteration centroid recovery and deuteration distribution estimation. Hexicon 2 greatly reduces data analysis time compared with manual analysis, whereas the increased number of peptides provides redundant coverage of the entire protein sequence. Hexicon 2 is a standalone application available free of charge under http://hx2.mpimf-heidelberg.mpg.de.
Figure
?  相似文献   
6.
The linear stability analysis of vertical throughflow of power law fluid for double-diffusive convection with Soret effect in a porous channel is investigated in this study. The upper and lower boundaries are assumed to be permeable, isothermal and isosolutal. The linear stability of vertical through flow is influenced by the interactions among the non-Newtonian Rayleigh number (Ra), Buoyancy ratio (N), Lewis number (Le), Péclet number (Pe), Soret parameter (Sr) and power law index (n). The results indicate that the Soret parameter has a significant influence on convective instability of power law fluid. It has also been noticed that buoyancy ratio has a dual effect on the instability of fluid flow. Further, it is noticed that the basic temperature and concentration profiles have singularities at \(Pe = 0\) and \(Le = 1\), the convective instability is looked into for the limiting case of \(Pe\rightarrow 0\) and \(Le \rightarrow 1\). For the case of pure thermal convection with no vertical throughflow, the present numerical results coincide with the solution of standard Horton–Rogers–Lapwood problem. The present results for critical Rayleigh number obtained using bvp4c and two-term Galerkin approximation are compared with those available in the literature and are tabulated.  相似文献   
7.
By transplanting identity elements into E. coli tRNAfMet, we have engineered an orthogonal initiator tRNA (itRNATy2) that is a substrate for Methanocaldococcus jannaschii TyrRS. We demonstrate that itRNATy2 can initiate translation in vivo with aromatic non‐canonical amino acids (ncAAs) bearing diverse sidechains. Although the initial system suffered from low yields, deleting redundant copies of tRNAfMet from the genome afforded an E. coli strain in which the efficiency of non‐canonical initiation equals elongation. With this improved system we produced a protein containing two distinct ncAAs at the first and second positions, an initial step towards producing completely unnatural polypeptides in vivo. This work provides a valuable tool to synthetic biology and demonstrates remarkable versatility of the E. coli translational machinery for initiation with ncAAs in vivo.  相似文献   
8.
Among various N‐heterocyclic carbenes (NHCs) tested, only 1,3‐bis(tert‐butyl)imidazol‐2‐ylidene (NHCtBu) proved to selectively promote the catalytic conjugate addition of alcohols onto (meth)acrylate substrates. This rather rare example of NHC‐catalyzed 1,4‐addition of alcohols was investigated as a simple means to trigger the polymerization of both methyl methacrylate and methyl acrylate (MMA and MA, respectively). Well‐defined α‐alkoxy poly(methyl (meth)acrylate) (PM(M)A) chains, the molar masses of which could be controlled by the initial [(meth)acrylate]0/[ROH]0 molar ratio, were ultimately obtained in N,N‐dimethylformamide at 25 °C. A hydroxyl‐terminated poly(ethylene oxide) (PEO‐OH) macro‐initiator was also employed to directly access PEO‐b‐PMMA amphiphilic block copolymers. Investigations into the reaction mechanism by DFT calculations revealed the occurrence of two competitive concerted pathways, involving either the activation of the alcohol or that of the monomer by NHCtBu.  相似文献   
9.
The quadratic multiple knapsack problem (QMKP) consists in assigning a set of objects, which interact through paired profit values, exclusively to different capacity-constrained knapsacks with the aim of maximising total profit. Its many applications include the assignment of workmen to different tasks when their ability to cooperate may affect the results. Strategic oscillation (SO) is a search strategy that operates in relation to a critical boundary associated with important solution features (such as feasibility). Originally proposed in the context of tabu search, it has become widely applied as an efficient memory-based methodology. We apply strategic oscillation to the quadratic multiple knapsack problem, disclosing that SO effectively exploits domain-specific knowledge, and obtains solutions of particularly high quality compared to those obtained by current state-of-the-art algorithms.  相似文献   
10.
The activation behavior of two N‐heterocyclic carbenes (NHCs), namely, 1,3‐bis(isopropyl)imidazol‐2‐ylidene(NHCiPr) and 1,3‐bis(tert‐butyl) imidazol‐2‐ylidene (NHCtBu), as organic nucleophiles in the reaction with methyl methacrylate (MMA) is described. NHCtBu allows the polymerization of MMA in DMF at room temperature and in toluene at 50 °C, whereas NHCiPr reacts with two molecules of MMA, forming an unprecedented imidazolium–enolate cyclodimer (NHCiPr/MMA=1:2). It is proposed that the reaction mechanism occurs by initial 1,4‐nucleophilic addition of NHCiPr to MMA, generating a zwitterionic enolate 2 , followed by addition of 2 to a second MMA molecule, forming a linear imidazolium–enolate 3 (NHCiPr/MMA=1:2). Proton transfer, generating intermediate 5 , followed by cyclization and release of methanol yielded the aforementioned zwitterionic cyclodimer 1:2 adduct 7 , the molecular structure of which has been established by NMR spectroscopy, X‐ray diffraction, and mass spectrometry. This unexpected difference between NHCtBu and NHCiPr in the reaction with MMA (polymerization and cyclodimerization, respectively) can be rationalized by using DFT calculations. In particular, the nature of the NHC strongly influences the cyclodimerization pathway, the cyclization of 5 and the release of methanol are the discriminating step and limiting step, respectively. In the case of NHCtBu, both steps are strongly disfavoured compared with that of NHCiPr (energetic difference of around 14 and 9 kcal mol?1, respectively), preventing the cyclization mechanism from a kinetic viewpoint. Moreover, addition of a third molecule of MMA in the polymerization pathway results in a lower activation barrier than that of the limiting step in the cyclodimerization pathway (difference of around 14 kcal mol?1), in agreement with the formation of polymethyl methacrylate (PMMA) by using NHCtBu as nucleophile.  相似文献   
1 [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] 下一页 » 末  页»
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号