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   19776篇
  免费   1084篇
  国内免费   970篇
  2024年   17篇
  2023年   820篇
  2022年   1004篇
  2021年   923篇
  2020年   1028篇
  2019年   708篇
  2018年   902篇
  2017年   1027篇
  2016年   1030篇
  2015年   1083篇
  2014年   1485篇
  2013年   1376篇
  2012年   1004篇
  2011年   1004篇
  2010年   1202篇
  2009年   1215篇
  2008年   738篇
  2007年   747篇
  2006年   624篇
  2005年   526篇
  2004年   438篇
  2003年   410篇
  2002年   401篇
  2001年   385篇
  2000年   324篇
  1999年   357篇
  1998年   150篇
  1997年   122篇
  1996年   102篇
  1995年   67篇
  1994年   51篇
  1993年   56篇
  1992年   61篇
  1991年   42篇
  1990年   32篇
  1989年   26篇
  1988年   32篇
  1987年   16篇
  1986年   20篇
  1985年   20篇
  1984年   27篇
  1983年   19篇
  1982年   11篇
  1981年   12篇
  1980年   16篇
  1979年   19篇
  1978年   11篇
  1977年   15篇
  1976年   10篇
  1973年   11篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
《Genomics》2019,111(4):579-589
ATP sulfurylase (ATPS, EC: 2.7.7.4) is a crucial enzyme for sulfate assimilation pathway in both plastids and cytosol in plants. In this study, genome-wide and comparative analyses of ATPSs in 11 higher plant species, including sequence and structural analyses have been performed. Expression of ATPS genes in sorghum under cadmium (Cd) and salinity (NaCl) stresses were also investigated to provide a model experimental data for the regulation of ATPS genes under stress conditions. Thirty-one ATPS genes from 11 plant species were found. It showed that ATPSs from different species have high sequence divergences, which cause structural differences among them. Phylogenetic analysis has shown that there are two major types of ATPSs evolved in dicots while monocots were evolved to have one type of ATPs. Finally, expression analysis of ATPS genes revealed tissue and stress dependent expression pattern, which indicates expressions of ATPS genes are tightly regulated.  相似文献   
4.
Dendrobium officinale has been considered over past centuries to be extremely valuable for use as an herbal medicine in South Asian countries. In this work, the chemical profiles of D. officinale from different habitats were systematically characterized using ultra-performance liquid chromatography time-of-flight mass spectrometry (UPLC-Q-TOF-MS) and multivariate analysis. The principal component analysis (PCA), partial least squares discriminate analysis (PLS-DA) and orthogonal partial least squares analysis (OPLS-DA) of UPLC-Q-TOF-MS data displayed an obvious separation. Several flavonoids and terpenoids derivatives contribute to the quantitative chemotypic variation within and between the samples as observed. These findings lead to a better understanding of the phytochemical variation of D. officinale which can aid in quality control of raw material.  相似文献   
5.
Ulcerative colitis is the typical progression of chronic inflammatory bowel disease. Amino acids, particularly tryptophan, have been reported to exert a protective effect against colitis induced by dextran sodium sulfate (DSS), but the precise underlying mechanisms remain incompletely clarified. Tryptophan metabolites are recognized to function as endogenous ligands for aryl hydrocarbon receptor (Ahr), which is a critical regulator of inflammation and immunity. Thus, we conducted this study to investigate whether dietary tryptophan supplementation protects against DSS-induced colitis by acting through Ahr. Female wild-type (WT) and Ahr-deficient (knockout; KO) mice (10–12 weeks old) were divided into four groups and fed either a control or 0.5% tryptophan diet. The tryptophan diet ameliorated DSS-induced colitis symptoms and severity in WT mice but not in KO mice, and the diet reduced the mRNA expression of Il-6, Tnfα, Il-1β and the chemokines Ccl2, Cxcl1 and Cxcl2 in the WT groups. Furthermore, Il-22 and Stat3 mRNA expression in the colon was elevated in WT mice fed with the tryptophan diet, which mainly protected epithelial layer integrity, and Ahr also modulated immune homeostasis by regulating Foxp3 and Il-17 mRNA expression. These data suggest that tryptophan-containing diet might ameliorate DSS-induced acute colitis and regulate epithelial homeostasis through Ahr. Thus, tryptophan could serve as a promising preventive agent in the treatment of ulcerative colitis.  相似文献   
6.
A differential screening study using high-resolution (HR)-hydrophilic interaction chromatography (HILIC)-electrospray ionization (ESI)–quadrupole time-of-flight mass spectrometry (Q-TOF MS) was conducted to identify saxitoxin (STX) analogues in the marine dinoflagellate toxic sub-clone Alexandrium tamarense Axat-2 and the non-toxic sub-clone UAT-014-009 derived from the same Japanese isolate. One unknown compound was identified only in the toxic sub-clone and was found to have the molecular formula C9H16N6O2. This structure differed from that of decarbamoyl STX (dcSTX; C9H16N6O3) by the loss of a single oxygen. A 12-deoxy-dcSTX standard (a mixture of 12α- and β-deoxy-dcSTX) was chemically prepared from dcSTX by reduction with sodium borohydride. The unknown compound in the toxic strain of A. tamarense was identified as 12β-deoxy-dcSTX by comparison of its HR-HILIC-LC–MS retention time and HR–MS/MS spectrum with those of the chemically prepared standard, and the identification was confirmed by high-sensitivity HPLC analysis with post-column fluorescent derivatization. Moreover, two Japanese isolates of A. catenella showing toxin profiles different from that of A. tamarense were also found to contain 12β-deoxy-dcSTX. Previously, 12β-deoxy-dcSTX was isolated from the freshwater cyanobacterium Lyngbya wollei, which produces a unique set of STX analogues. This study is the first evidence of the presence of 12β-deoxy-dcSTX in marine dinoflagellates.  相似文献   
7.
This study evaluated whether showing a video of the different ways of raising goats for milk affected consumer acceptability. Four combinations, 2 Videos (intensive [INT] and semiextensive [SEM] system) × 2 Milk Types (semiskimmed [S] and whole [W] milk), were evaluated by 70 habitual consumers of goat milk, who scored their liking and purchase intention during blind (B), expected (E), and informed (I) acceptability sessions. In the B session, consumers tasted both milk types without information. S samples were preferred over W samples. In the E session, SEM video created high expectations in terms of milk liking and purchase intent, whereas the opposite happened when showing INT video. In the I session, consumers showed a clear preference for combinations created using SEM video, regardless of milk type. W-SEM and S-INT were worse (negative disconfirmation) and better (positive disconfirmation) than expected, respectively. A complete assimilation toward expectations occurred only for S-INT. INT video adversely affected the acceptability of S samples. Concerning purchase intent, W-SEM and S-SEM were worse than expected, but the assimilation was complete only for S-SEM: SEM video increased purchase intent for S samples.  相似文献   
8.
9.
A synthetic flavone derivative 2-(benzo[d][1,3]dioxol-5-yl)-4H-chromen-4-one (BDC) was synthesized by the one pot reaction method and assessed for α-glucosidase inhibitory activity. The BDC demonstrated dose dependent inhibition of α-glucosidase activity. A maximum inhibition (99.3 ± 0.26%) of α-glucosidase was observed at 27.6 µM. The maximum α-glucosidase inhibitory activity depicted by BDC 27.6 µM concentration was 22.4 fold over the maximum inhibition observed with acarbose (97.72 ± 0.59% at 669.57 µM), a standard commercial anti-diabetic drug. In contrast to acarbose that depicted competitive type inhibition, kinetic studies of α-glucosidase inhibition by BDC demonstrated non-competitive inhibition with Km of 0.71 mM−1 and a Vmax of 0.028 mmol/min. In silico studies suggest allosteric interaction of BDC with α-glucosidase at a minimum binding energy (ΔG) of −8.64 kcal/mol and Ki of 465.3 nM, whereas, acarbose interacted at the active site of α-glucosidase with ΔG of −9.23 kcal/mol and Ki of 172 nM. Thus BDC significantly inhibited α-glucosidase in comparison to acarbose. Moreover, BDC has been endorsed for drug likeness by evaluating it as per Lipinski rule of five. Thus, BDC can be a lead compound for the management of type-2 diabetes mellitus.  相似文献   
10.
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