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   1574215篇
  免费   144690篇
  国内免费   1696篇
  2021年   18128篇
  2019年   16312篇
  2018年   19403篇
  2017年   18184篇
  2016年   29442篇
  2015年   43212篇
  2014年   51331篇
  2013年   77702篇
  2012年   44220篇
  2011年   34985篇
  2010年   46979篇
  2009年   47365篇
  2008年   31971篇
  2007年   30649篇
  2006年   33948篇
  2005年   35222篇
  2004年   34003篇
  2003年   31323篇
  2002年   29229篇
  2001年   50085篇
  2000年   47743篇
  1999年   43383篇
  1998年   27114篇
  1997年   27052篇
  1996年   26129篇
  1995年   24354篇
  1994年   23939篇
  1993年   23218篇
  1992年   36093篇
  1991年   34215篇
  1990年   32855篇
  1989年   33209篇
  1988年   30403篇
  1987年   28676篇
  1986年   27033篇
  1985年   28768篇
  1984年   27045篇
  1983年   23673篇
  1982年   22458篇
  1981年   21492篇
  1980年   20137篇
  1979年   22968篇
  1978年   20743篇
  1977年   19770篇
  1976年   18626篇
  1975年   18602篇
  1974年   19325篇
  1973年   19649篇
  1972年   16874篇
  1971年   15352篇
排序方式: 共有10000条查询结果,搜索用时 828 毫秒
61.
62.
63.
64.
As part of a program towards the development of novel antibiotics, a convenient method for solid-phase synthesis of the cyclic cationic peptide polymyxin B1 and analogues thereof is described. The methodology, based on cleavage-by-cyclization using Kenner's safety-catch linker, yields crude products with purities ranging from 37-67%. Antibacterial assays revealed that analogues 23-26, in which the (S)-6-methyloctanoic acid moiety is replaced with shorter acyl chains, exhibit distinct antimicrobial activity. The results suggest that the length of the acyl chain is rather critical for antimicrobial activity. On the other hand, substitution of the hydrophobic ring-segment D-Phe-6/Leu-7 in polymyxin B1 with dipeptide mimics (i.e. analogues 27-33) resulted in almost complete loss of antimicrobial activity.  相似文献   
65.
66.
S Yokota  H Tsuji  K Kato 《Histochemistry》1985,82(2):141-148
Light and electron microscopic localization of cathepsin D in rat liver was investigated by post-embedding immunoenzyme and protein A-gold techniques. By light microscopy, cytoplasmic granules of parenchymal cells and Kupffer cells were stained for cathepsin D. Weak staining was also noted in sinusoidal endothelial cells. In the parenchymal cells many of positive granules located around bile canaliculi. In the Kupffer cells and the endothelial cells, diffuse staining was noted in the cytoplasm in addition to granular staining. By electron microscopy, gold particles representing the antigenic sites for cathepsin D were seen in typical secondary lysosomes and some multivesicular bodies of the parenchymal cells and Kupffer cells. The lysosomes of the endothelial cells and fat-storing cells were weakly labeled. Quantitative analysis of the labeling density in the lysosomes of these three types of cells demonstrated that the lysosomes of parenchymal cells and Kupffer cells are main containers of cathepsin D in rat liver. The results suggest that cathepsin D functions in the intracellular digestive system of parenchymal cells and Kupffer cells but not so much in that of the endothelial cells.  相似文献   
67.
Experiments on 330 rats were made to study the influence of benzodiazepines (diazepam, dormicum and phenazepam) on 5'-nucleotidase activity in brain homogenates. It was discovered that diazepam and dormicum in doses of 3 and 4 mg, phenazepam in doses of 3.75 and 5 mg per 200 g bw provoked a 16-20% reduction in 5'-nucleotidase activity. The maximal effect of diazepam (3 and 4 mg doses) was attained 1 h after intraperitoneal injection, that of dormicum (3 mg) 30 min and of phenazepam (5 mg) 1 h after intraperitoneal injection. It is assumed that benzodiazepines are involved in AMP metabolism.  相似文献   
68.
1. Examination of the cerebrospinal fluid (CSF) of head-injured patients reveals that the concentration of intraventricular xanthine is elevated and that of uridine is decreased relative to those of adult lumbar CSF. 2. No correlations were observed between CSF lactate and CSF hypoxanthine, xanthine, or uridine, suggesting that changes in purine metabolites and the pyrimidine nucleoside do not index similar cellular events as does lactic acid production. 3. Ventricular CSF from hydrocephalic infants had uridine and hypoxanthine concentrations not significantly different from those of normal adult lumbar CSF, but xanthine was significantly elevated. 4. Since uridine has anticonvulsant properties and is a crucial substrate for cerebral metabolism, it may be useful to evaluate this pyrimidine for use in the management of patients with head injury.  相似文献   
69.
70.
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