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   56304篇
  免费   4004篇
  国内免费   165篇
  2021年   434篇
  2020年   384篇
  2019年   423篇
  2018年   959篇
  2017年   904篇
  2016年   1376篇
  2015年   2029篇
  2014年   2227篇
  2013年   2933篇
  2012年   3453篇
  2011年   3291篇
  2010年   2078篇
  2009年   1556篇
  2008年   2632篇
  2007年   2630篇
  2006年   2585篇
  2005年   2417篇
  2004年   2360篇
  2003年   2203篇
  2002年   2177篇
  2001年   1623篇
  2000年   1723篇
  1999年   1352篇
  1998年   515篇
  1997年   421篇
  1996年   401篇
  1995年   378篇
  1994年   391篇
  1993年   369篇
  1992年   835篇
  1991年   805篇
  1990年   773篇
  1989年   725篇
  1988年   678篇
  1987年   604篇
  1986年   597篇
  1985年   558篇
  1984年   493篇
  1983年   451篇
  1982年   402篇
  1981年   383篇
  1979年   498篇
  1978年   371篇
  1977年   324篇
  1975年   383篇
  1974年   370篇
  1973年   337篇
  1972年   376篇
  1971年   335篇
  1968年   317篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Different thiol-containing compounds, such as diethyldithiocarbamate (DDC), glutathione, penicillamine, and dithioerythritol have been chosen to study their effect on ascorbate/Fe-ADP-induced lipid peroxidation, detected by low-level chemiluminescence and alkane production. In the concentration range used, these thiols exerted a temporary protection against lipid peroxidation by lengthening the induction period; after overcoming this induction period, no substantial inhibition of either chemiluminescence or alkane production was observed. DDC was effective in protecting against lipid peroxidation in the nanomolar range, whereas the group of other thiol-containing molecules operated in the millimolar range.  相似文献   
82.
83.
84.
By chemoenzymatic synthesis the gene for a (Leu27) analogue of human growth hormone releasing hormone-Gly45 [(Leu27)GHRH-Gly45] was constructed, cloned and expressed in Escherichia coli as a fusion protein with beta-galactosidase under the control of the lac promoter and operator. Upon induction with isopropyl-D-thio-beta-galactopyranoside the fusion protein accumulated to a yield of 15-20% of the total cellular protein. After cyanogen bromide cleavage of the fusion protein the precursor peptide (Leu27)hGHRH-Gly45 was separated by extraction and purified by ion exchange and h.p.l.c.-RP18 chromatography. The purified peptide was analysed by sequencing, isoelectric focusing, amino acid analysis and amino acid analysis after V8 protease digestion. The carboxy-terminal glycine was subsequently amidated by PAM (peptidylglycine-alpha-amidating-monooxygenase), an enzyme which was isolated and characterized from fresh bovine pituitaries. Correct amidation of the penultimate amino acid, leucine, was verified by peptide sequencing with an authentic leucine amide reference.  相似文献   
85.
86.
M F Hynes  J Quandt  M P O'Connell  A Pühler 《Gene》1989,78(1):111-120
We have constructed derivatives of the transposon Tn5 carrying the mob site (oriT) of plasmid RP4, and an nptI-sacB-sacR cassette [Ried and Collmer, Gene 57 (1987) 239-246]. The mob site, in conjunction with the antibiotic-resistance markers carried on the transposons, allows identification of transposon inserts in cryptic plasmids by mobilisation to other strains. The sacB-sacR genes allow direct selection for the loss or curing of plasmids, because only strains which no longer contain an active sacB gene are able to grow on media containing sucrose. We have tested these transposons in four strains of Rhizobium leguminosarum and two strains of Rhizobium meliloti, and have been able to demonstrate curing of several large cryptic plasmids, and generation of large deletions in many other plasmids. This method has enabled us to show that the R. leguminosarum plasmids pRL12JI and pR1eVF39f carry auxotrophic markers, and that the plasmid pR1eVF39c carries genes which affect colony morphology.  相似文献   
87.
Summary The simultaneous use of intraarterial Cis-Platinum and Radiotherapy (CP/RT) was found to be a very effective and relatively little burdened treatment for a palliative treatment concept. This affects life quality as well as the remission - and survival times. The fast and continual remission with low CP/RT concentrations, even in extreme palliative cases, is surprising. CP/RT treatment shows additive and synergistic effects which are not explainable by the single effects of the cis-platinum dose used (60 mg/1.73 m2 in our case) or the total irradiation dose (e.g., 5 Gy TD) or the fractionation (e.g., 5 × 1 Gy), especially since the doses of each which were used are by themselves without therapeutic relevance. Only the combination of the modalities with a low dose two-day preradiation program induced the described effects.Dedicated to Prof. L.E. Feinendegen on the occasion of his 60th birthday  相似文献   
88.
89.
90.
The emission maximum of the single tryptophan residue of melittin was measured in the presence of phosphatidylethanolamine liposomes and Escherichia coli cytoplasmic membranes. In both cases, the fluorescence maximum was shifted to shorter wavelengths indicating a transfer of the indole ring to an apolar environment. E. coli membranes were labelled in position 2 of their phospholipids with [14C]oleic acid. These membranes were used for measuring the activity of an endogenous phospholipase A2. A slow hydrolysis is observed, which can be accelerated by adding melittin. The extent of the stimulation depends on the molar ratio of melittin to membrane phospholipid. Under suitable conditions, the initial rate of hydrolysis is six to seven times higher in the presence than in the absence of melittin. The action of the phospholipase A2 from bee venom is also stimulated by melittin. An identical stimulation was observed with either E. coli membranes or pure phosphatidylethanolamine liposomes as substrate.  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