全文获取类型
收费全文 | 694483篇 |
免费 | 70547篇 |
国内免费 | 387篇 |
专业分类
765417篇 |
出版年
2018年 | 7219篇 |
2017年 | 7024篇 |
2016年 | 9348篇 |
2015年 | 11139篇 |
2014年 | 13662篇 |
2013年 | 19751篇 |
2012年 | 22317篇 |
2011年 | 23006篇 |
2010年 | 15721篇 |
2009年 | 14236篇 |
2008年 | 20264篇 |
2007年 | 20976篇 |
2006年 | 19744篇 |
2005年 | 18896篇 |
2004年 | 18735篇 |
2003年 | 17982篇 |
2002年 | 17449篇 |
2001年 | 34326篇 |
2000年 | 34139篇 |
1999年 | 26536篇 |
1998年 | 8277篇 |
1997年 | 8722篇 |
1996年 | 8059篇 |
1995年 | 7428篇 |
1994年 | 7153篇 |
1993年 | 7082篇 |
1992年 | 20859篇 |
1991年 | 20256篇 |
1990年 | 19621篇 |
1989年 | 19274篇 |
1988年 | 17634篇 |
1987年 | 16474篇 |
1986年 | 15287篇 |
1985年 | 15211篇 |
1984年 | 12381篇 |
1983年 | 10366篇 |
1982年 | 7541篇 |
1981年 | 6694篇 |
1980年 | 6429篇 |
1979年 | 11467篇 |
1978年 | 8777篇 |
1977年 | 8073篇 |
1976年 | 7484篇 |
1975年 | 8464篇 |
1974年 | 9185篇 |
1973年 | 8973篇 |
1972年 | 8116篇 |
1971年 | 7454篇 |
1970年 | 6457篇 |
1969年 | 6191篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
851.
852.
853.
The complete sequence of recombinant human interleukin-2 expressed in Escherichia coli has been confirmed by thermospray liquid chromatography-mass spectrometry (TS-LC-MS) of a tryptic digest derived from 100 micrograms (7 nmol) of reduced carboxymethylated interleukin-2. The preparation was shown by this method to contain predominantly unprocessed N-terminal initiator Met, with some authentic N-terminal Ala; the rest of the protein was as predicted from the DNA sequence, though some deamidated material was noted. TS-LC-MS proved to be a rapid and efficient method for surveying the protein tryptic peptide products allowing all the data to be collected in one chromatographic run; all tryptic fragments were identified by their molecular ions including those for the larger peptides (Mr 1500-3500) which, due to the presence of doubly and triply charged molecular ions, were brought within the mass range of the instrument (1800 Da). It is proposed that TS-LC-MS is a good general method for analyzing recombinant protein digests with respect to sequence confirmation, processing, and post-translational modification, and since each chromatographic peak is identified allows for subsequent monitoring of the protein by LC using uv detection. The method suffers from the disadvantages that all the sample is consumed during the experiment and that no fragment (sequence) ions are generally observed. 相似文献
854.
A number of drugs cause marked increases in the steroid hydroxylase activity of hepatic microsomes. Beginning 2 days after estrus, 117 mature ewes were each given 14 injections over a 27-day period of phenobarbital sodium, diphenylhydantoin, chlorcyclizine HCl or phenylbutazone. Blood samples for luteinizing hormone (LH) and progesterone determination by radioimmunoassay (RIA) were taken on day 10 of the first estrous cycle (day 18 if no heat was observed) and on days 5 and 10 of the second cycle. On day 10 of the second cycle, the ewes were given an intravenous injection of 1 ml of 6% solution of pentobarbitol sodium anesthetic per 4.5 kg body weight, and the length of anesthetic sleep time was measured. The ewes were then killed and corpora lutea and liver were weighed.In 33 ewes treated with either phenobarbitol sodium or phenylbutazone, sleep time was shortened (18 min 29 min in untreated controls, P<.01), indicating that enzyme induction had occurred. For 41 ewes treated with either chlorcyclizine HCl or diphenylhydantoin, sleep time was lengthened to 93 min (P<.01 controls), indicating impaired liver function. Electron micrographs of liver cells verified that enzyme induction or hepatic degeneration had occurred. 相似文献
855.
856.
Reconstitution of the Na+K+ pump of Ehrlich ascites tumor and enhancement of efficiency by quercetin 总被引:1,自引:0,他引:1
Plasma membranes from Ehrlich ascites tumor cells were solubilized by octylglucoside in the presence of phospholipids. The Na+K+-ATPase was purified from this extract by adsorption and elution from thio-Seph-arose 4B. The enzyme (specific activity, 7 mumoles of ATP hydrolyzed min-1 mg of protein -1) was reconstituted into liposomes by the octyglucoside dilution procedure. An ATP-dependent Na+ influx with low efficiency was observed. On addition of appropriate amounts of quercetin, the Na+ flux/ATP hydrolysis ratio was increased from 0.4 to 1.4. 相似文献
857.
858.
Inhibition of leukocyte adhesion by the in vivo and in vitro administration of cannabinoids 总被引:1,自引:0,他引:1
Cannabinoids have been shown to affect various aspects of arachidonic acid metabolism both in vivo and in vitro. Eicosanoid metabolites of arachidonate and related octadecanoate are believed to be involved in cell adhesion processes as agonists in some instances and as antagonists in other cases. This report shows data in which cannabinoids exhibit marked inhibitory effects on the adhesion of mouse peritoneal cells to polystyrene culture dishes. The effects could be seen by in vivo administration of the drugs as well as by direct exposure of the cells in vitro. The data suggest that this inhibition of adhesion is mediated by one or more products generated by stimulation of a lipoxygenase pathway. 相似文献
859.
A survey of cultivated heirloom tomato varieties identifies four new mutant alleles at the green-flesh locus 总被引:1,自引:0,他引:1
Cornelius S. Barry Priyanka Pandey 《Molecular breeding : new strategies in plant improvement》2009,24(3):269-276
The process of crop domestication occurs through the selection and subsequent propagation of novel alleles that improve traits
of interest. Cultivated tomato (Solanum lycopersicum), particularly heirloom varieties, exhibit a wide range of variation in fruit size, shape and color. The green-flesh mutant of tomato possesses a stay-green phenotype resulting in fruits that ripen to a red-brown color, due to the retention
of chlorophyll and the simultaneous accumulation of lycopene. The recent identification of the GREEN-FLESH gene provides a molecular tool with which to investigate the origin of a subset of cultivated tomato varieties that resemble
the green-flesh mutant. Sequence analysis of the GF locus from 26 varieties revealed the existence of four previously unidentified null alleles. This study illustrates the potential
of cultivated tomato varieties, including heritage cultivars, heirlooms, and land races, for uncovering new alleles in genes
of interest. 相似文献
860.
Determination of products of acetohydroxy acid synthase by the colorimetric method, revisited 总被引:2,自引:0,他引:2
The enzyme acetohydroxy acid synthase (AHAS, EC 4.1.3.18) catalyzes two competing reactions of physiological importance: condensation of two molecules of pyruvate to form acetolactate (AL) or condensation of pyruvate and 2-ketobutyrate to form acetohydroxybutyrate (AHB). The activity of AHAS is most frequently analyzed using the Westerfeld method, in which the acetoin formed upon decarboxylation of AL is determined by colorimetric reaction with creatine and alpha-naphthol. However, there has been confusion as to the interpretation of the results of this assay in the presence of both substrates, conditions which lead to formation of both AL and AHB. By applying this assay to enzymatically prepared samples of AL and AHB which have also been analyzed by two other independent methods, we show here that the color yield for AHB in the commonly used assay is 35-40% that for equivalent amounts of acetoin or AL. The relative color yield is not significantly affected by varying the time or temperature of various steps in the color-forming reaction. This information could in principle be used, together with an independent specific assay for AHB, to determine the composition of an AHAS product mixture; it would, however, be less accurate than a simultaneous chromatographic method. 相似文献