全文获取类型
收费全文 | 6035篇 |
免费 | 599篇 |
国内免费 | 1篇 |
出版年
2022年 | 41篇 |
2021年 | 98篇 |
2020年 | 60篇 |
2019年 | 100篇 |
2018年 | 114篇 |
2017年 | 93篇 |
2016年 | 166篇 |
2015年 | 255篇 |
2014年 | 264篇 |
2013年 | 336篇 |
2012年 | 421篇 |
2011年 | 385篇 |
2010年 | 276篇 |
2009年 | 254篇 |
2008年 | 363篇 |
2007年 | 340篇 |
2006年 | 341篇 |
2005年 | 347篇 |
2004年 | 287篇 |
2003年 | 286篇 |
2002年 | 285篇 |
2001年 | 99篇 |
2000年 | 63篇 |
1999年 | 69篇 |
1998年 | 79篇 |
1997年 | 53篇 |
1996年 | 58篇 |
1995年 | 52篇 |
1994年 | 42篇 |
1993年 | 32篇 |
1992年 | 61篇 |
1991年 | 54篇 |
1990年 | 44篇 |
1989年 | 46篇 |
1988年 | 46篇 |
1987年 | 40篇 |
1986年 | 42篇 |
1985年 | 41篇 |
1984年 | 61篇 |
1983年 | 52篇 |
1982年 | 45篇 |
1981年 | 47篇 |
1980年 | 37篇 |
1979年 | 36篇 |
1977年 | 34篇 |
1976年 | 25篇 |
1975年 | 25篇 |
1974年 | 26篇 |
1973年 | 37篇 |
1972年 | 26篇 |
排序方式: 共有6635条查询结果,搜索用时 765 毫秒
71.
F. K. Ewete J. T. Arnason J. Larson B. J. R. Philogène 《Entomologia Experimentalis et Applicata》1996,80(3):531-537
Preliminary investigations with ethanolic (EtOH) extracts from five Nigerian plants show that extracts of Piper guineense Schum and Thonn (Piperaceae), Cedrela odorata L. (Meliaceae), Dennettia tripetala G. Baker (Annonaceae) and Aframomum melegueta (Rosch) K. Schum (Zingiberaceae) in artificial diets significantly reduced larval growth of European corn borer (ECB), Ostrinia nubilalis Hubner, at a concentration of 1000 ppm (0.1%). An extract of Xylopia aethiopica (Dunal) A. Rich (Annonaceae) was ineffective. When the extracts were subsequently incorporated into artificial diets at 300 ppm and offered to neaonates, larval mortality increased in the order A. melegueta (13%), D. tripetala (13%), P. guineense (27%), and C. odorata (48%). Larval and adult emergence periods increased with increasing concentration of P. guineense, C. odorata and D. tripetala indicating a toxic response. Nutritional indices for habituated third instar larvae with the two most promising plant extracts, P. guineense and C. odorata, showed that the efficiencies of conversion of digested food (ECD) was significantly reduced at 300 ppm suggesting a postdigestive toxicity of the extracts. P. guineense and C. odorata extracts show the best potential for development as botanical insecticides. 相似文献
72.
73.
The CD45 tyrosine phosphatase regulates specific pools of antigen receptor-associated p59fyn and CD4-associated p56lck tyrosine in human T-cells.
下载免费PDF全文
![点击此处可从《The EMBO journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
M Biffen D McMichael-Phillips T Larson A Venkitaraman D Alexander 《The EMBO journal》1994,13(8):1920-1929
A newly isolated T-cell line (CB1) derived from a T-acute lymphoblastic leukaemia (T-ALL) patient contained cells (40% of total) which did not express the CD45 phosphotyrosine phosphatase. The cells were sorted into CD45- and CD45+ populations and shown to be clonal in origin. T-cell receptor (TCR) cross-linking or coligation of the TCR with its CD4/CD8 co-receptors induced tyrosine phosphorylation and calcium signals in CD45+ but not in CD45- cells. Unexpectedly, whole cell p56lck and p59fyn tyrosine kinase activities were not reduced in CD45- compared to CD45+ cells. A novel technique was therefore developed to isolated specific pools of aggregated receptors expressed at the cell surface, together with their associated tyrosine kinases. Using this technique it was shown that cell surface CD4-p56lck kinase activity was 78% lower in CD45- than in CD45+ cells. Phosphorylation of TCR zeta- and gamma-chains occurred in TCR immunocomplexes from CD45+ but not CD45- cells, despite comparable levels of p59fyn and TCR proteins. Furthermore, TCR-associated tyrosine kinase activity towards an exogenous substrate was 84% lower in CD45- than in CD45+ cells. Addition of recombinant p59fyn to TCR immunocomplexes isolated from CD45-cells restored the phosphorylation of the TCR zeta- and gamma-chains. Our results demonstrate that CD45 selectively regulates the pools of p59fyn and p56lck kinases which are associated with the TCR and CD4 at the cell surface. Activation by CD45 of these receptor-associated kinase pools correlates with the ability of the TCR and its coreceptors to couple to intracellular signalling pathways. 相似文献
74.
Characterization of the interaction of the glp repressor of Escherichia coli K-12 with single and tandem glp operator variants. 总被引:2,自引:2,他引:0
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
N Zhao W Oh D Trybul K S Thrasher T J Kingsbury T J Larson 《Journal of bacteriology》1994,176(8):2393-2397
The glp operons of Escherichia coli are negatively controlled by the glp repressor. Comparison of the repressor-binding affinities for consensus and altered consensus operators in vivo showed that all base substitutions at positions 3, 4, 5, and 8 from the center of the palindromic operator caused a striking decrease in repressor binding. Substitutions at other positions had a severe to no effect on repressor binding, depending on the base substitution. The results obtained indicate that the repressor binds with highest affinity to operators with the half-site WATKYTCGWW, where W is A or T, K is G or T, and Y is C or T. Strong cooperative binding of the repressor to tandem operators was demonstrated in vivo. Cooperativity was maximal when two 20-bp operators were directly repeated or when 2 bp separated the two operators. Cooperativity decreased with the deletion of 2 bp or the addition of 4 bp between the individual operators. Cooperativity was eliminated with a 6-bp insertion between the operators. 相似文献
75.
76.
77.
78.
John A. Lowe III Weimin Qian Pamela J. Scott Stafford McLean Dianne K. Bryce Rosemary T. Crawford Jon Bordner 《Bioorganic & medicinal chemistry letters》1994,4(24):2877-2882
A series of 5,7-diphenyl-3-ureidohexahydroazepin-2-one cholecystokinin-B (CCK-B) receptor antagonists was synthesized using Beckmann ring expansion of a suitable 2,4-diphenylcyclohexanone as a key step. SAR studies revealed the importance of the 5-aryl group for high and selective CCK-B receptor affinity, as illustrated in compound (−)-10i (CCK-B IC50 = 6.8 nM). 相似文献
79.
80.
The organisation and nucleotide sequences coding for the catabolism of benzene, toluene (and xylenes), naphthalene and biphenylvia catechol and the extradiol (meta) cleavage pathway inPseudomonas are reviewed and the various factors which may have played a part in their evolution are considered. The data suggests that the complete pathways have evolved in a modular way probably from at least three elements. The commonmeta pathway operons, downstream from the ferredoxin-like protein adjacent to the gene for catechol 2,3-dioxygenase, are highly homologous and clearly share a common ancestry. This common module may have become fused to a gene or genes the product(s) of which could convert a stable chemical (benzoate, salicylate, toluene, benzene, phenol) to catechol, thus forming the lower pathway operons found in modern strains. The upper pathway operons might then have been acquired as a third module at a later stage thus increasing the catabolic versatility of the host strains. 相似文献