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   27111篇
  免费   2379篇
  国内免费   21篇
  29511篇
  2023年   81篇
  2022年   227篇
  2021年   454篇
  2020年   248篇
  2019年   310篇
  2018年   427篇
  2017年   320篇
  2016年   608篇
  2015年   1051篇
  2014年   1099篇
  2013年   1454篇
  2012年   1918篇
  2011年   1990篇
  2010年   1252篇
  2009年   1155篇
  2008年   1674篇
  2007年   1683篇
  2006年   1577篇
  2005年   1533篇
  2004年   1608篇
  2003年   1481篇
  2002年   1521篇
  2001年   298篇
  2000年   178篇
  1999年   272篇
  1998年   395篇
  1997年   287篇
  1996年   235篇
  1995年   224篇
  1994年   215篇
  1993年   236篇
  1992年   173篇
  1991年   187篇
  1990年   158篇
  1989年   129篇
  1988年   147篇
  1987年   140篇
  1986年   142篇
  1985年   168篇
  1984年   153篇
  1983年   158篇
  1982年   178篇
  1981年   178篇
  1980年   152篇
  1979年   105篇
  1978年   118篇
  1977年   111篇
  1976年   105篇
  1974年   116篇
  1973年   96篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
Abstract: Phenylacetic acid, the major metabolite of phenylethylamine, has been identified and quantitated in rat brain regions by capillary column high-resolution gas chromatography mass spectrometry. Its distribution is heterogeneous and correlates with that of phenylethylamine. The values obtained were (ng/g ± SEM): whole brain, 31.2 ± 2.7; caudate nucleus, 64.6 ± 6.5; hypothalamus, 60.1 ± 7.4; cerebellum, 31.3 ± 2.9; brainstem, 33.1 ± 3.3, and the "rest," 27.6 ± 3.0.  相似文献   
22.
The reported selective serotonin Re-uptake Inhibitor Litoxetine was used as the starting point in the design of a range of potential SSRIs with high ease of synthetic accessibility. Preparation and subsequent optimization yielded a range of potent and highly selective SSRIs.  相似文献   
23.
One of the leading challenges for contemporary anthropology is to try to contribute to an understanding of the interaction between indigenous and exogenous socio‐cultural orders, especially at the frontiers of globalisation. Here I review three recent attempts to do so: (1) a model of structural transformation as developed by Marshall Sahlins; (2) a model of articulation as developed by James Clifford; (3) a model of ‘adoption’ proposed by Joel Robbins. As a test case for these models, I consider them in relation to some recent developments in local segmentary politics and verbal art in the Ku Waru region of Highland New Guinea. I show that all three models are in certain respects inadequate for understanding those developments, and offer some proposals as to what kinds of theory might be more adequate to the task.  相似文献   
24.
When expressed in Escherichia coli, a truncated form of phytochrome (oat PHYA AP3 residues 464-1129) self associates to form a series of products ranging in size from monomers to aggregates of greater than 20 subunits. When these same phytochrome sequences are coexpressed with the chaperonins GroEL and GroES, the truncated phytochrome migrates as a native-like dimer in size exclusion chromatography and no higher-order aggregates were detected. GroEL and GroES inhibition of phytochrome aggregation in E. coli presumably occurs via the suppression of folding pathways leading to incorrectly folded phytochrome.  相似文献   
25.
The photochemistry of the [(CpR)Mo(CO)(3)](2) molecules, where CpR = eta(5)-C(5)H(4)(CH(2))(2)C(O)NCH(3)(CH(2))(n)CH(3) (n = 3, 8, 13, and 18), was examined using femtosecond pump-probe transient absorption spectroscopy. The goal of this study was to investigate the importance of radical size and mass on the dynamics and efficiency of geminate radical-radical recombination. The femtosecond results demonstrated the lack of any size/mass dependence of the recombination efficiency. This finding contrasts with results from a prior study that did find a size/mass dependence using a steady-state photochemical technique. To explain these conflicting results, it is proposed that the femtosecond pump-probe results are a measurement of the efficiency of primary geminate recombination whereas the steady-state method results are a measurement of the sum of primary and secondary geminate recombination efficiencies. The size/mass dependence is evident in the latter because secondary geminate recombination is a slower diffusive recombination process and therefore depends on the steric properties of the radicals. Although the existence of primary and secondary recombination channels is often taken for granted, experimental differentiation of primary and secondary caging has proven to be difficult because it is not possible for a single experimental technique to span the entire timescale for recombination of a radical cage pair and adequately resolve these recombination pathways.  相似文献   
26.
27.
The rotating wall vessel is optimized for suspension culture, with laminar flow and adequate nutrient delivery, but minimal shear. However, higher shears may occur in vivo. During rotating wall vessel cultivation of human renal cells, size and density of glass-coated microcarrier beads were changed to modulate initial shear. Renal-specific proteins were assayed after 2 days. Flow cytometry antibody binding analysis of vitamin D receptor demonstrated peak expression at intermediate shears, with 30% reduction outside this range. Activity of cathepsin C showed the inverse pattern, lowest at midshear, with twofold increases at either extreme. Dipeptidyl-peptidase IV had no shear dependence, suggesting that the other results are specific, not universal, changes in membrane trafficking or protein synthesis. On addition of dextran, which changes medium density and viscosity but not shear, vitamin D receptor assay showed no differences from controls. Neither cell cycle, apoptosis/necrosis indexes, nor lactate dehydrogenase release varied between experiments, confirming that the changes are primary, not secondary to cell cycling or membrane damage. This study provides direct evidence that mechanical culture conditions modulate protein expression in suspension culture.  相似文献   
28.
29.
The particulate enzyme fraction from pig aorta was treated with Triton X-100 or Nonidet P-40 to yield a soluble enzyme preparation. This solubilized enzyme catalyzed the transfer of mannose from GDP-[14C]mannose, but not from [14C]mannosyl-phosphoryl-polyprenol, to G1cNAc-G1cNAc-pyrophosphoryl-polyprenol to form the trisaccharide-lipid, Man-β-GlcNAc-GlcNAc-pyrophosphoryl-polyprenol. The trisaccharide-lipid formed in these reactions was isolated by solvent fractionation and was subjected to mild acid hydrolysis to release the [14C]trisaccharide. Essentially all of the radioactivity was released from this trisaccharide as mannose upon treatment with β-mannosidase while α-mannosidase had no effect.  相似文献   
30.
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