全文获取类型
收费全文 | 5373篇 |
免费 | 384篇 |
国内免费 | 4篇 |
出版年
2022年 | 30篇 |
2021年 | 63篇 |
2020年 | 38篇 |
2019年 | 59篇 |
2018年 | 65篇 |
2017年 | 63篇 |
2016年 | 118篇 |
2015年 | 189篇 |
2014年 | 238篇 |
2013年 | 313篇 |
2012年 | 387篇 |
2011年 | 349篇 |
2010年 | 203篇 |
2009年 | 227篇 |
2008年 | 317篇 |
2007年 | 322篇 |
2006年 | 301篇 |
2005年 | 314篇 |
2004年 | 291篇 |
2003年 | 272篇 |
2002年 | 242篇 |
2001年 | 69篇 |
2000年 | 71篇 |
1999年 | 73篇 |
1998年 | 80篇 |
1997年 | 54篇 |
1996年 | 55篇 |
1995年 | 46篇 |
1994年 | 40篇 |
1993年 | 40篇 |
1992年 | 63篇 |
1991年 | 41篇 |
1990年 | 45篇 |
1989年 | 34篇 |
1988年 | 36篇 |
1987年 | 33篇 |
1986年 | 26篇 |
1985年 | 27篇 |
1984年 | 46篇 |
1983年 | 21篇 |
1982年 | 41篇 |
1980年 | 22篇 |
1979年 | 25篇 |
1978年 | 23篇 |
1976年 | 19篇 |
1975年 | 27篇 |
1974年 | 18篇 |
1973年 | 22篇 |
1971年 | 18篇 |
1969年 | 21篇 |
排序方式: 共有5761条查询结果,搜索用时 15 毫秒
101.
The interaction of cholesterol with the glycerol backbone segments of phospholipids was studied in bilayers of phosphatidylethanolamine containing equimolar amounts of cholesterol. Glycerol selectively deuterated at various positions was supplied to the growth medium of Escherichia coli strain 131 GP which is defective in endogeneous glycerol synthesis. The procedure enables the stereospecific labeling of the three glycerol backbone segments of the membrane phospholipids. Phosphatidylethanolamine with wild-type fatty acid composition was purified from E. coli cells and deuterium magnetic resonance spectra were obtained either from dispersions of pure phosphatidylethanolamine or from equimolar mixtures of phosphatidylethanolamine with cholesterol. For comparative purposes 1,2-di[9,10-2H2]elaidoyl-sn-glycero-3-phosphoethanolamine and [3-α-2H]cholesterol were synthesized in order to monitor the behavior of the fatty acyl chains and of the cholesterol molecule itself. For all deuterated segments the deuterium quadrupole splittings as well as the deuterium spin-lattice (T1) relaxation times were measured as a function of temperature. The glycerol backbone was found to be a remarkably stable structural element of the phospholipid molecule. The quadrupole splittings of the backbone segments changed only by at most 2 kHz upon incorporation of 50 mol % cholesterol. This was in contrast to the fatty acyl chains where the same amount of cholesterol increased the quadrupole splitting by more than 20 kHz. The glycerol segments exhibited the shortest T1 relaxation times of all CH2 segments indicating that the glycerol backbone is the slowest motional moiety of the lipid molecule. Addition of cholesterol has no effect on the backbone motion but the fast reorientation rate of the trans-double bonds in 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine is increased dramatically. 相似文献
102.
A new approach for the reaction of Sepharose with cyanogen bromide is described, using triethylamine as a “cyano-transfer” reagent. An optimized procedure for activation at neutral pH was developed. This procedure requires only about 5% of the usual amount of cyanogen bromide. Activated resins are free of imidocarbonates and carbamates, containing only active cyanate esters. Extremely high coupling capacities (75 μmol ligand/g wet Sepharose 4B) can be obtained using this method. 相似文献
103.
104.
Thirteen Weddell seals ( Leptonychotes weddellii ) were collected at Vestkapp, eastern Weddell Sea coast, in austral spring 1986. All stomachs contained partially digested food. The mean wet weight of stomach contents was 7.5 kg, 3.3% of the. mean body weight of the collected seals. Twelve fish species and three cephalopod species were identified from 372 left otoliths and 25 lower beaks, representing 58.4% of 679 total prey items obtained. Composition by number of total prey was: Chionodraco myersi (15.8%), Trematomus eulepidotus (10.0%), Pagetopsis maculatus (9.7%), Racovitzia glacialis (9.6%) and Cryodraco antarcticus (4.1%). Otoliths of the seven other fish species and beaks of the three cephalopod species together represented 9.1% of total prey numbers. The pooled wet weights calculated from 13 prey species (regressions for two octopod species were not available) amounted to 43.5 kg food mass and represented 44.7% of the combined food mass in all stomachs. Composition by mass was: C. myersi (44.5%), T. eulepidotus (19.8%), squid Psychroteuthis glacialis (8.5%), P. maculates (7.9%), C. antarcticus (7.1%) and R. glacialis (6.2%). The remaining 7 fish species together represented 5.8% by mass. Temporal variation in food availability was apparent. Midwater fish Pleuragramma antarcticum was the staple food of Weddell seals from the same area during the 1985 summer, whereas it was absent in the samples taken in spring 1986. Estimates of fish biomass from net hauls demonstrate a highly variable availability of pelagic food resources for top predators in the Vestkapp area. 相似文献
105.
Glasshouse trials were performed to investigate the control of the parasitic weed Striga hermonthica by Fusarium nygamai and the performance of the host plant sorghum (Sorghum bicolor) using different inoculum substrates and inoculum amounts of the fungus. Optimal constant and alternating temperatures for the growth of the fungus were 25°C and 30/20°C, respectively. Striga incidence was decreased up to 100% when the fungus was incorporated into the soil preplanting. Emerged Striga plants at different stages of growth up to the flowering stage were killed by the fungus when the fungus was applied postemergent. In root-chamber trials none of the Striga seeds germinated when 10 ml inoculum suspension of 8 × 106 spores/ml of F. nygamai was applied on seeds of the parasitic weed sprinkled on the surface of filter paper. F. nygamai has potential as a bioherbicide for Striga control. Further studies regarding its performance under field conditions and its safety to the environment and humans should be assessed. 相似文献
106.
107.
Dimitrios Tsikas Joachim Fauler Jürgen C. Frlich 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1992,574(2)
A simple and rapid method is described for the preparation of a stable isotope oxygen-18 labelled leukotriene E4 (LTE4). Oxygen-18 labelling of LTE4 methyl ester in oxygen-18 water catalysed by a pig liver esterase resulted in the incorporation of two oxygen-18 atoms in the carboxylic group of LTE4 to the extent of 89.8% ([18O2]LTE4) and one oxygen-18 atom to the extent of 9.4% ([16O18O]LTE4), with only 0.7% remaining unchanged ([16O2]LTE4). [18O2]LTE4 was found not to back-exchange following incubation in acidified urine (pH 4.0) at 4°C for up to 20 h. [18O2]LTE4 was demonstrated to be a useful internal standard in a method for the quantitative determination of LTE4 in human urine involving high-performance liquid chromatography and gas chromatography with negative-ion chemical ionization tandem mass spectrometry: the concentration of LTE4 in a 24-h urine sample of a healthy subject was determined to be 68.1 pg/ml. 相似文献
108.
109.
110.
Thermal Denaturation of Native Striatal Tyrosine Hydroxylase: Increased Thermolability of the Phosphorylated Form of the Enzyme 总被引:5,自引:5,他引:0
Mitchell A. Lazar Roger J. W. Truscott Joachim D. Raese Jack D. Barchas 《Journal of neurochemistry》1981,36(2):677-682
Tyrosine hydroxylase was purified from bovine corpus striatum. The native enzyme had a half-life of 15 +/- 3 min at 50 degrees C. Phosphorylation of tyrosine hydroxylase with protein kinase purified from both corpus striatum and heart activated the enzyme, but activity was rapidly lost with additional preincubation of the enzyme at 30 degrees C. Thermal denaturation studies indicated that phosphorylated tyrosine hydroxylase had a half-life of 5 +/- 2 min at 50 degrees C 相似文献