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   603篇
  免费   22篇
  国内免费   4篇
  2023年   8篇
  2022年   23篇
  2021年   38篇
  2020年   16篇
  2019年   30篇
  2018年   28篇
  2017年   22篇
  2016年   18篇
  2015年   38篇
  2014年   38篇
  2013年   51篇
  2012年   35篇
  2011年   41篇
  2010年   17篇
  2009年   26篇
  2008年   33篇
  2007年   27篇
  2006年   21篇
  2005年   12篇
  2004年   15篇
  2003年   11篇
  2002年   13篇
  2001年   4篇
  2000年   6篇
  1999年   8篇
  1998年   2篇
  1997年   2篇
  1996年   3篇
  1995年   5篇
  1994年   3篇
  1993年   3篇
  1992年   3篇
  1985年   2篇
  1982年   2篇
  1981年   2篇
  1975年   1篇
  1974年   1篇
  1973年   1篇
  1972年   1篇
  1971年   2篇
  1970年   1篇
  1966年   1篇
  1965年   1篇
  1963年   1篇
  1962年   1篇
  1961年   1篇
  1959年   1篇
  1958年   1篇
  1956年   1篇
  1954年   1篇
排序方式: 共有629条查询结果,搜索用时 15 毫秒
81.
Stereoselective and efficient synthesis of hydroxymethyl-substituted rac-quercitols (1315) was achieved, starting from cis-furan (Kobayashi, 2008) with photooxygenation reaction, which is readily available by the reduction of cis-phtalic anhydride. α- and β-Glucosidase enzyme activity of the target molecules was evaluated and good inhibitor activity was seen. One- and two-dimensional NMR spectroscopy, IR spectroscopy and X-ray crystallography were utilized in the structure characterization of products.  相似文献   
82.
83.
A new series of 1,4-dihydropyridine derivatives (2a–h, 3a–e, and 4a–e) were systematically designed and synthesized via ultrasound irradiation methods with easy work-up and good yields. Compounds structures were confirmed by IR, 1H NMR, 13C NMR, and mass spectra. The synthesized compounds were screened for both antimicrobial and anticoagulant activities. Compound 2e (MIC: 0.25?μg/mL) was highly active against Escherichia coli and compound 2c (MIC: 0.5?μg/mL) was also highly active against Pseudomonas aeruginosa compared with ciprofloxacin. (MIC: 1?μg/mL) The antifungal activity of 2c (MIC: 0.5?μg/mL) against Candida albicans was high relative to that of clotrimazole (MIC: 1?μg/mL). Anticoagulant activity was determined by activated partial thromboplastin time (APTT) and prothrombin time (PT) coagulation assays. Compound 4-(4-hydroxyphenyl)-2,6-dimethyl-N3,N5-bis(5-phenyl-1,3,4-thiadiazol-2-yl)-1,4-dihydropyridine-3,5-dicarboxamide 3d (>1000?s in APTT assays) was highly active in anticoagulant screening compared with the reference of heparin.Cytotoxicity was evaluated using HepG2 (liver), HeLa (cervical), and MCF-7 (breast) cancer cell lines, with high toxicities observed for 2c (GI50?=?0.02?μm) against HeLa cell line and 2e (GI50?=?0.03?μm) equipotant against MCF-7 cell line. Therefore, the compounds 2e, 2c and 3d can serve as lead molecules for the development of new classes of antimicrobial and anticoagulant agent.  相似文献   
84.
The Cyclophragma undans nucleopolyhedrovirus(Cyun NPV), a potential pest control agent, was isolated from Cyclophragma undans(Lepidoptera: Lasiocampidae), an important forest pest. In the present study, we performed detailed genome analysis of Cyun NPV and compared its genome to those of other Group Ⅰ alphabaculoviruses. Sequencing of the Cyun NPV genome using the Roche 454 sequencing system generated 142,900 bp with a G + C content of 45%. Genome analysis predicted a total of 147 hypothetical open reading frames comprising 38 baculoviral core genes, 24 lepidopteran baculovirus conserved genes, nine Group Ⅰ Alphabaculovirus conserved genes, 71 common genes, and five genes that are unique to Cyun NPV. In addition, the genome contains 13 homologous repeated sequences(hrs). Phylogenetic analysis groups Cyun NPV under a distinct branch within clade ‘‘a' of Group Ⅰ in the genus Alphabaculovirus. Unlike other members of Group Ⅰ, Cyun NPV harbors only nine of the 11 genes previously determined to be specific to Group Ⅰ viruses.Furthermore, the Cyun NPV lacks the tyrosine phosphatase gene and the ac30 gene. The Cyun NPV F-like protein contains two insertions of continuous polar amino acids, one at the conventional fusion peptide and a second insertion at the pretransmembrane domain. The insertions are likely to affect the fusion function and suggest an evolutionary process that led to inactivation of the F-like protein. The above findings imply that Cyun NPV is a distinct species under Group Ⅰ Alphabaculovirus.  相似文献   
85.
Purple blotch (Alternaria porri), having symptom similar to stemphylium blight (Stemphylium vesicarium), is one of the important diseases that significantly lowered shallot yield in Indonesia. However, stemphylium blight has never been reported. Purple blotch pathogens from farmers’ crops in Bantul and experimental crop in Centre of Innovative Agricultural Technology Universitas Gadjah Mada (CIAT-UGM) Sleman Regencies were identified to observe whether Stemphylium sp. was found with A. porri. Thirteen isolates obtained showed variability in colony texture, colour and pigmentation. DNA fingerprinting through Polymerase Chain Reaction of BOX and Enterobacterial Repetitive Intergenic Consensus region dispersed those isolates into two main groups. Identification using Alternaria-specific primers, Dir1ITSAlt/Inv1ITSAlt revealed that 10 isolates were Stemphylium sp. and 3 isolates were Alternaria sp. A. porri was only identified from CIAT-UGM. This finding emphasise that the Alternaria-specific primer was able to amplify Stemphylium sp. Besides, stemphylium blight disease may have been occurred in farmers’ crops in Bantul.  相似文献   
86.
From the bark of Zizyphus sativa, in addition to already described cyclopeptide alkaloids, two new compounds of this class, sativanine-A(1) and sativanine-B(2), were isolated. Both alkaloids contain 14-membered ring systems. 1 belongs to the integerrine type, while 2 is similar to nummularine-G, with an additional ring in the side chain.  相似文献   
87.
88.
89.
Various concentrations of salt (NaCI) were shown to have an influence on the differentiation of tissues in the root and stem ofBacopa monniera (L) Wettst. Higher concentrations induced drastic changes in roots grown on salt-supplemented media; epidermal and cortical cells experienced changes in shape, size, and orientation and/or were got disintegrated. A low concentration of salt induced a profuse development of root hairs which gradually disappeared at higher concentrations. Air spaces in the stem cortex were enlarged and xylem cell walls in the vascular ring were thickened.  相似文献   
90.
More than fifteen years ago, it was noted that the failure rate of antidepressant clinical trials was high, and such negative outcomes were thought to be related to the increasing magnitude of placebo response. However, there is considerable debate regarding this phenomenon and its relationship to outcomes in more recent antidepressant clinical trials. To investigate this, we accessed the US Food and Drug Administration (FDA) reviews for sixteen antidepressants (85 trials, 115 trial arms, 23,109 patients) approved between 1987 and 2013. We calculated the magnitude of placebo and antidepressant responses, antidepressant‐placebo differences, as well as the effect sizes and success rates, and compared these measures over time. Exploratory analysis investigated potential changes in trial design and conduct over time. As expected, the magnitude of placebo response has steadily grown in the past 30 years, increasing since 2000 by 6.4% (r=0.46, p<0.001). Contrary to expectations, a similar increase has occurred in the magnitude of antidepressant response (6.0%, r=0.37, p<0.001). Thus, the effect sizes (0.30 vs. 0.29, p=0.42) and the magnitude of antidepressant‐placebo differences (10.5% vs. 10.3%, p=0.37) have remained statistically equivalent. Furthermore, the frequency of positive trial arms has gone up in the past 15 years (from 47.8% to 63.8%), but this difference in frequency has not reached statistical significance. Trial design features that were previously associated with a possible lower magnitude of placebo response were not implemented, and their relationship to the magnitude of placebo response could not be replicated. Of the 34 recent trials, two implemented enhanced interview techniques, but both of them were unsuccessful. The results of this study suggest that the relationship between the magnitude of placebo response and the outcome of antidepressant clinical trials is weak at best. These data further indicate that antidepressant‐placebo differences are about the same for all of the sixteen antidepressants approved by the FDA in the past thirty years.  相似文献   
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