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   1648篇
  免费   134篇
  国内免费   1篇
  1783篇
  2023年   37篇
  2022年   17篇
  2021年   30篇
  2020年   29篇
  2019年   37篇
  2018年   59篇
  2017年   56篇
  2016年   95篇
  2015年   84篇
  2014年   100篇
  2013年   235篇
  2012年   105篇
  2011年   145篇
  2010年   158篇
  2009年   85篇
  2008年   56篇
  2007年   21篇
  2006年   39篇
  2005年   16篇
  2004年   34篇
  2003年   17篇
  2002年   15篇
  2001年   19篇
  2000年   12篇
  1999年   7篇
  1998年   10篇
  1997年   6篇
  1996年   15篇
  1995年   13篇
  1994年   12篇
  1993年   22篇
  1992年   20篇
  1991年   11篇
  1990年   8篇
  1989年   5篇
  1988年   12篇
  1986年   4篇
  1985年   12篇
  1984年   9篇
  1983年   8篇
  1982年   9篇
  1981年   6篇
  1980年   4篇
  1978年   7篇
  1977年   10篇
  1976年   5篇
  1975年   8篇
  1974年   6篇
  1972年   11篇
  1971年   9篇
排序方式: 共有1783条查询结果,搜索用时 15 毫秒
171.
The effects of 24-epibrassinolide (24-epiBL) on seedling growth, antioxidative system, lipid peroxidation, proline and soluble protein content were investigated in seedlings of the salt-sensitive rice cultivar IR-28. Seedling growth of rice plants was improved by 24-epiBL treatment under salt stress conditions. When seedlings treated with 24-epiBL were subjected to 120 mM NaCl stress, the activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6) and glutathione reductase (EC 1.6.4.2) did not show significant difference, whereas the activity of ascorbate peroxidase (EC 1.11.1.11) significantly increased. Increased activity of peroxidase (EC 1.11.1.7) under NaCl stress showed remarkable decrease in the 24-epiBL+NaCl-applied group. Lipid peroxidation level significantly increased under salt stress but decreased with 24-epiBL application revealing that less oxidative damage occurred in this group (24-epiBL+NaCl). In addition, increased proline content in the NaCl-applied group was decreased by 24-epiBL application in the 24-epiBL+NaCl-applied group. Soluble protein content was increased by 24-epiBL application even under NaCl stress, being also higher than control conditions (no 24-epiBL or NaCl treatment). 24-epiBL treatment considerably alleviated oxidative damage that occurred under NaCl-stressed conditions and improved seedling growth in part under salt stress in sensitive IR-28 seedlings.  相似文献   
172.
Low density lipoprotein (LDL) and oxidized LDL are associated with collagen in the arterial intima, where the collagen is coated by the small proteoglycan decorin. When incubated in physiological ionic conditions, decorin-coated collagen bound only small amounts of native and oxidized LDL, the interaction being weak. When decorin-coated collagen was first allowed to bind lipoprotein lipase (LPL), binding of native and oxidized LDL increased dramatically (23- and 7-fold, respectively). This increase depended on strong interactions between LPL that was bound to the glycosaminoglycan chains of the collagen-bound decorin and native and oxidized LDL (kDa 12 and 5.9 nM, respectively). To distinguish between binding to monomeric (inactive) and dimeric (catalytically active) forms of LPL, affinity chromatography on heparin columns was conducted, which showed that native LDL bound to the monomeric LPL, whereas oxidized LDL, irrespective of the type of modification (Cu(2+), 2, 2'-azobis(2-amidinopropane)hydrochloride, hypochlorite, or soybean 15-lipoxygenase), bound preferably to dimeric LPL. However, catalytic activity of LPL was not required for binding to oxidized LDL. Finally, immunohistochemistry of atherosclerotic lesions of human coronary arteries revealed specific areas in which LDL, LPL, decorin, and collagen type I were present. The results suggest that LPL can retain LDL in atherosclerotic lesions along decorin-coated collagen fibers.  相似文献   
173.
In this study, new chiral thiourea and 1,3-thiazolidine-4,5-dione derivatives were synthesized, it was aimed to evaluate the various biological activities and molecular docking of these compounds. Firstly, the new thioureas ( 1 – 16 ) were obtained by reacting 1-naphthylisothiocyanate with different chiral amines. Then, the chiral thioureas were cyclized with oxalyl chloride to obtain 1,3-thiazolidine-4,5-dione derivatives ( 17 – 32 ). All compounds were evaluated with several in vitro antioxidant and enzyme inhibition activities. Compound 30 was the most active compound against AChE, with a value of IC50=8.09±0.58 μM. On the other hand, all compounds were tested in silico absorption, distribution, metabolism, and excretion (ADME) assays to better understand their bioavailability. These physicochemical properties, pharmacokinetics, and drug-likeness of all compounds were calculated using SwissADME. Furthermore, according to molecular docking analyses compound 30 exhibited significant binding affinities for all enzymes. Based on our overall observations, compound 30 could be recommended as a potential lead for the therapuetic of Alzheimer's.  相似文献   
174.
Abstract: Problem Animal Control Registers, where farmers report livestock losses due to predators as a prerequisite for financial compensation, allow quantifying the human-predator conflict. We analyzed such registers from the Kweneng District of Botswana to assess the impact of native predators on livestock over 3 years. Leopards (Panthera pardus), lions (Panthera leo), wild dogs (Lycaon pictus), brown hyenas (Hyaena brunnea), and cheetahs (Acinonyx jubatus) altogether claimed 2,272 head of livestock. During 2002, the year with the highest impact, the number of animals reported depredated (954) represented 0.34% of the livestock in the district. Leopards and lions caused 64% of the losses. Leopard livestock predation mainly affected calves and was consistent over the district and over time. In contrast, lion predation concentrated on adult cattle, was characterized by local hot spots close to reserve borders, decreased with increasing distance to a reserve, and increased during 2002, an unusually dry year. Interviews with 60 farmers and herders within 30 km of Khutse and Central Kalahari Game Reserves revealed an annual loss of 2.2% of their livestock to predators. Here, small farms (max. 100 domestic animals) suffered relatively higher losses than large, commercial farms, not only due to predation (small farms: 11.7%; large farms: 1.0%) but also from other causes (small: 12.6%; large: 2.8%), even though herders on large farms guarded 5 times more livestock per person than those on small farms. To reduce livestock predation in most of the district where lions are absent, we recommend maternity corrals for pregnant females and calves to better protect vulnerable calves during day and night. In areas close to a reserve where lions roam, herders' incentives to keep all livestock protected in a corral at night have to be enhanced because, according to the registers, only 3 predation cases were reported to have happened inside a corral.  相似文献   
175.
In this study, as a continuation of our research for new (arylalkyl)imidazole anticonvulsant compounds, the design, synthesis and anticonvulsant/antimicrobial activity evaluation of a series of 2-acetylnaphthalene derivatives have been described. Molecular design of the compounds has been based on the modification of nafimidone [1-(2-naphthyl)-2-(imidazol-1-yl)ethanone], which is a representative of the (arylalkyl)imidazole anticonvulsant compounds as well as its active metabolite, nafimidone alcohol (3, 4). In general, these compounds were variously substituted at the alkyl chain between naphthalene and imidazole rings and subjected to some other modifications to evaluate additional structure–activity relationships. The anticonvulsant activity profile of those compounds was determined by maximal electroshock seizure (MES) and subcutaneous metrazol (scM) seizure tests, whereas their neurotoxicity was examined using rotarod test. All the ester derivatives of nafimidone alcohol (5ah), which were designed as prodrugs, showed anticonvulsant activity against MES-induced seizure model. Four of the most active compounds were chosen for further anticonvulsant evaluations. Quantification of anticonvulsant protection was calculated via the ip route (ED50 and TD50) for the most active candidate (5d). Observed protection in the MES model was 38.46 mg kg?1 and 123.83 mg kg?1 in mice and 20.44 mg kg?1, 56.36 mg kg?1 in rats, respectively. Most of the compounds with imidazole ring also showed antibacterial and/or antifungal activities to a certain extent in addition to their anticonvulsant activity.  相似文献   
176.
Therapeutic effect of interferon-β (IFN-β) treatment has been associated with modulation of the balance between Th1, Th17, Th2 and regulatory T (Treg) cells, whereas the impact of disease modifying drugs on Th9-immunity in multiple sclerosis (MS) has not been studied. To investigate the short-term effects of IFN-β treatment on cytokines in MS, we determined serum levels of IL-17, IL-23, IL-10, IL-4, IFN-γ, IL-9 and TGF-β in relapsing remitting MS patients before and 2 months after IFN-β treatment by ELISA. MS patients showed increased IL-17, IL-23 and IL-4 levels and decreased IL-9 levels as compared to healthy controls. IFN-β treatment only reduced IL-17 and IL-23 levels, whereas the levels of other cytokines remained unchanged. IFN-β treatment appears to exert its earliest therapeutic effect on Th17-immunity. The influence of IL-9 on MS pathogenesis needs to be further studied.  相似文献   
177.
In this study, a new phthalonitrile derivative namely 4-[(2,4-difluorophenyl)ethynyl]phthalonitrile ( 1 ) and its metal phthalocyanines ( 2 and 3 ) were synthesized. The resultant compounds were conjugated to silver nanoparticles and characterized using transmission electron microscopy (TEM) images. The biological properties of compounds ( 1 – 3 ), their nanoconjugates ( 4 – 6 ), and silver nanoparticles ( 7 ) were examined for the first time in this study. The antioxidant activities of biological candidates ( 1 – 7 ) were studied by applying the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay. The highest antioxidant activity was obtained 97.47 % for 200 mg/L manganese phthalocyanine-silver nanoconjugates ( 6 ). The antimicrobial and antimicrobial photodynamic therapy (APDT) activities of biological candidates ( 1 – 7 ) were examined using a micro-dilution assay. The highest MIC value was obtained 8 mg/L for nanoconjugate 6 against E. hirae. The studied compounds and their silver nanoconjugates exhibited high APDT activities against all the studied microorganisms. The most effective APDT activities were obtained 4 mg/L for nanoconjugates ( 5 and 6 ) against L. pneumophila and E. hirae, respectively. All the studied biological candidates displayed high cell viability inhibition activities against E. coli cell growth. The biofilm inhibition activities of the tested biological candidates were also investigated against S. aureus and P. Aeruginosa. Biological candidates ( 1 – 6 ) can be considered efficient metal nanoparticle-based materials for multi-disciplinary biological applications.  相似文献   
178.
179.
180.
The antioxidant role of novel N-substituted indole-2-carboxamides (I2CDs) was investigated for their inhibitory effects on superoxide anion (O2? ) and lipid peroxidation (LP). Among the synthesized I2CDs, 3, 4, 6, 8 and 9 significantly inhibited O2· ? with an inhibition range at 70–98%. Examination of substituent effects on activity showed that both the ortho- and para-positions of the benzamide residue needs to be dichlorinated in order to get a maximum inhibitory effect on superoxide anion. In general, halogenated derivatives were found more active then the non-halogenated ones. However, none of the I2CDs had a significant inhibitory effects on the level of lipid peroxidation; only compounds 7 and 10 moderately decreased LP levels by over 50% at 10? 3 M concentrations.  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