全文获取类型
收费全文 | 123928篇 |
免费 | 10035篇 |
国内免费 | 11423篇 |
专业分类
145386篇 |
出版年
2024年 | 302篇 |
2023年 | 1645篇 |
2022年 | 3725篇 |
2021年 | 6201篇 |
2020年 | 4338篇 |
2019年 | 5329篇 |
2018年 | 5122篇 |
2017年 | 3794篇 |
2016年 | 5239篇 |
2015年 | 7671篇 |
2014年 | 9034篇 |
2013年 | 9509篇 |
2012年 | 11427篇 |
2011年 | 10300篇 |
2010年 | 6455篇 |
2009年 | 5780篇 |
2008年 | 6687篇 |
2007年 | 5971篇 |
2006年 | 5293篇 |
2005年 | 4130篇 |
2004年 | 3562篇 |
2003年 | 3273篇 |
2002年 | 2786篇 |
2001年 | 2184篇 |
2000年 | 1939篇 |
1999年 | 1904篇 |
1998年 | 1204篇 |
1997年 | 1140篇 |
1996年 | 1079篇 |
1995年 | 942篇 |
1994年 | 902篇 |
1993年 | 702篇 |
1992年 | 918篇 |
1991年 | 700篇 |
1990年 | 551篇 |
1989年 | 525篇 |
1988年 | 408篇 |
1987年 | 392篇 |
1986年 | 310篇 |
1985年 | 327篇 |
1984年 | 213篇 |
1983年 | 201篇 |
1982年 | 145篇 |
1981年 | 138篇 |
1980年 | 89篇 |
1979年 | 108篇 |
1978年 | 72篇 |
1977年 | 86篇 |
1975年 | 81篇 |
1973年 | 77篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
A new adsorption chromatography procedure for the purification of calmodulin from bovine brain was developed using polymeric adsorbent 3520. Calmodulin was first isolated by DEAE-Cellulose column chromatography and further purified to apparent homogeneity following elution with 50% ethanol from the adsorbent column. Polyacrylamide gel electrophoresis showed one band either in the presence of Ca2+ or EGTA. The polymeric adsorbent 3520 is a non-polar polymer lacking exchangeable groups. The selective adsorption of calmodulin is based on hydrophobic interaction within the matrix, and is Ca2+ independent. Neither high salt (0.5 M NaC1) nor EGTA (5 mM) was able to elute the CaM from the adsorption column whereas ethanol (50%) eluted it completely. This method is simple to use and it provides highly purified calmodulin with high yield. 相似文献
102.
Extractive bioconversion in aqueous two-phase systems 总被引:1,自引:0,他引:1
Summary The transformation of hydrocortisone to prednisolone was studied in aqueous two-phase systems, as a model for the extractive bioconversion of fine chemicals. The bacterium, Arthrobacter simplex, was able to grow in the two-phase system and the cells could be revitalized after a period of use. Use of aqueous two-phase systems made it possible to operate the reaction at higher substrate concentrations than in pure buffer solution. An adsorptive method to remove the product from the top phase was tested and shown to be both efficient and compatible with the overall process. In order to reduce the costs of operation in aqueous two-phase systems, a cheaper starch-based polymer, Reppal-PES, was successfully used as a substitute for dextran.Dedicated to Professor Dr. Georg Manecke on the occasion of his 70th birthday 相似文献
103.
104.
105.
The UDS induced in cultured FL cells by exposure to chemicals was measured as hydroxyurea-resistant incorporation of 3H-TdR in the acid-insoluble fraction of the 14C-TdR-prelabelled cells synchronized by the combination of arginine starvation and pretreatment with hydroxyurea. The level of UDS is represented by the ratios of 3H/14C radioactivities which are measures of specific activities of 3H. Two direct-acting alkylating agents, MMS and MNNG, a cross-linking agent, mitomycin C, and 3 procarcinogens, B(a)P, AFB1 and cyclophosphamide elicited UDS in the absence or presence of the liver-metabolizing system. Three chemicals of unknown carcinogenicity were also able to induce UDS in this assay system, i.e., bis-(O,O-diethylphosphinothioyl)-disulphide, 4-chlorophenoxy acetic acid (sodium salt) and caramelized malt sugar. With the exception of 4-chlorophenoxy acetic acid, they were also active in the Ames test. 相似文献
106.
Preparation of [B23-D-alanine]des-(B25-B30)-hexapeptide-insulin by a combination of enzymic and non-enzymic synthesis. 下载免费PDF全文
Des-(B25-B30)-hexapeptide-insulin with B23-glycine replaced by D-alanine was prepared by a combination of enzymic and non-enzymic syntheses. The purified product was homogeneous in polyacrylamide-gel electrophoresis and could be crystallized. The biological activity in vivo of crystalline [B23-D-Ala]des-(B25-B30)-hexapeptide-insulin was determined as 58% of that of standard pig insulin (27 i.u./mg). 相似文献
107.
Karl-Anders Karlsson Bo E. Samuelsson Göran O. Steen 《The Journal of membrane biology》1971,5(2):169-184
Summary Ducks (Anas platyrhynchos) were fed hypertonic saline for eight days, resulting in an activation and hypertrophy of the salt gland. The Na+–K+-dependent adenosine triphosphatase, an enzyme generally assumed to be part of the active Na transport system, increased its specific activity by about 200% during this activation. Sulfatides, the major glycolipids of the salt gland, increased their concentration to the same extent. Cholesterol, cerebrosides, and six phospholipid classes showed an increase of 20–80%.A preliminary report on this work was given at the Second International Meeting of the International Society for Neurochemistry, Milan, September 1–5, 1969, and at the XIIIth International Conference on the Biochemistry of Lipids, Athens, September 7–12, 1969. 相似文献
108.
Intact ribonucleic acid (RNA) molecules in an extended form were extracted from purified influenza virus and observed in the electron microscope. For this study, the RNA extraction procedure and the Kleinschmidt protein monolayer technique were modified. The mean lengths of RNA from X7, X7-F1, and WSN strains of influenza virus were found to be 2.69, 2.55, and 2.37 mum, respectively. From these measurements, the corresponding estimated molecular weights would be 2.9, 2.8, and 2.5 x 10(6) daltons. X7 and WSN RNA preparations were exposed to pH 3 to disrupt intact molecules. Histograms of length measurements showed five peaks, which were interpreted to represent the five pieces of RNA reported to exist in the influenza virion. X7 RNA appeared to be more stable than WSN RNA when stored at 4 C. The profiles of histograms of incomplete virus RNA suggest that the high molecular-weight component is missing. In preliminary experiments on Newcastle disease virus RNA, molecules of various lengths were observed. 相似文献
109.
Synthesis of a peptide with full somatostatin activity 总被引:2,自引:0,他引:2
A peptide has been synthesized according to the structure proposed for somatostatin by the solid phase method. The synthetic product was assayed and found to possess full somatostatin activity as compared with the natural material. 相似文献
110.
Summary A microdensitometer system for point measurements in areas down to a few square micrometers is described. The detector system is connected to an integrating digital voltmeter, constituting a very sensitive system. The digital output signals and a digital identification are registered on a recorder and on a tape punch, thus facilitating computer analysis.The precision of the equipment varied from 1.1 per cent, for low signal levels, to 0.3 percent for the highest signal levels. Stray light, the only important source of systematic errors, was found to influence the output signal level. Concerning determinations of the densities, however, a variation of the order of 1 per cent was only obtained for small areas with densities differing from that of the background. 相似文献