全文获取类型
收费全文 | 4536篇 |
免费 | 420篇 |
国内免费 | 4篇 |
专业分类
4960篇 |
出版年
2022年 | 61篇 |
2021年 | 108篇 |
2020年 | 54篇 |
2019年 | 71篇 |
2018年 | 78篇 |
2017年 | 77篇 |
2016年 | 110篇 |
2015年 | 194篇 |
2014年 | 190篇 |
2013年 | 227篇 |
2012年 | 254篇 |
2011年 | 309篇 |
2010年 | 173篇 |
2009年 | 171篇 |
2008年 | 189篇 |
2007年 | 184篇 |
2006年 | 166篇 |
2005年 | 151篇 |
2004年 | 183篇 |
2003年 | 161篇 |
2002年 | 161篇 |
2001年 | 60篇 |
2000年 | 60篇 |
1999年 | 68篇 |
1998年 | 37篇 |
1997年 | 42篇 |
1996年 | 43篇 |
1995年 | 34篇 |
1994年 | 31篇 |
1993年 | 32篇 |
1992年 | 48篇 |
1991年 | 41篇 |
1990年 | 47篇 |
1989年 | 51篇 |
1988年 | 41篇 |
1987年 | 47篇 |
1986年 | 42篇 |
1985年 | 28篇 |
1984年 | 45篇 |
1983年 | 46篇 |
1982年 | 51篇 |
1981年 | 38篇 |
1980年 | 45篇 |
1979年 | 26篇 |
1978年 | 28篇 |
1977年 | 28篇 |
1974年 | 26篇 |
1973年 | 42篇 |
1972年 | 24篇 |
1970年 | 25篇 |
排序方式: 共有4960条查询结果,搜索用时 15 毫秒
1.
2.
A review of in vitro mutagenesis assessment of metal compounds in mammalian and nonmammalian test systems has been compiled.
Prokaryotic assays are ineffective or inconsistent in their detection of most metals as mutagens, with the notable exception
of hexavalent chromium. Mammalian assay systems appear to be similarly inappropriate for the screening of metal compounds
based upon the limited number of studies that have employed those compounds having known carcinogenic activity. Although of
limited value as screening tests for the detection of potentially carcinogenic metal compounds, the well-characterized in
vitro mutagenesis systems may prove to be of significant value as a means to elucidate mechanisms of metal genotoxicity. 相似文献
3.
Robert Blake II Elizabeth A. Shute James Waskovsky Arthur P. Harrison JR. 《Geomicrobiology journal》2013,30(3-4):173-192
Abstract Microorganisms capable of aerobic respiration on ferrous ions are spread throughout eubacterial and archaebacterial phyla. Phylogenetically distinct organisms were shown to express spectrally distinct redox‐active biomolecules during autotrophic growth on soluble iron. A new iron‐oxidizing eubacterium, designated as strain Funis, was investigated. Strain Funis was judged to be different from other known iron‐oxidizing bacteria on the bases of comparative lipid analyses, 16S rRNA sequence analyses, and cytochrome composition studies. When grown autotrophically on ferrous ions, Funis produced conspicuous levels of a novel acid‐stable, acid‐soluble yellow cytochrome with a distinctive absorbance peak at 579 nm in the reduced state. Stopped‐flow spectrophotometric kinetic studies were conducted on respiratory chain components isolated from cell‐free extracts of Thiobacillus ferrooxidans. Experimental results were consistent with a model where the primary oxidant of ferrous ions is a highly aggregated c‐type cytochrome that then reduces the periplasmic rusticyanin. The Fe(II)‐dependent, cytochrome c‐catalyzed reduction of the rusticyanin possessed three kinetic properties in common with corresponding intact cells that respire on iron: the same anion specificity, a similar dependence of the rate on the concentration of ferrous ions, and similar rates at saturating concentrations of ferrous ions 相似文献
4.
M.S. Blake K.H. Johnston G.J. Russell-Jones E.C. Gotschlich 《Analytical biochemistry》1984,136(1):175-179
A rapid, sensitive method has been developed to detect antibody-antigen complexes on “Western blots.” The methods of H. Towbin, T. Staehlin, and J. Gordon were used to separate and blot the antigens onto nitrocellulose. The remaining sites of attachment were blocked and the nitrocellulose was washed with polyoxyethylenesorbitan monolaurate (Tween 20). The blot was then reacted with the antiserum or hybridoma supernate to be tested. After the antigen-antibody reaction was completed, the blot was washed and treated with anti-antibody which has been conjugated to alkaline phosphatase. The alkaline phosphatase was detected by the reduction of the tetrazolium salt to diformazan by the hydrogen ions released in the formation of indigo by the reaction of the phosphatase on the indoxyl phosphate. The advantages of this method over previously described techniques are (1) use of Tween 20 allows the blot to be stained with Coomassie blue, (2) the substrates of the alkaline phosphatase reaction are stable for long periods of time, (3) the reaction products form an intense blue color which does not fade, (4) the resolution is extremely good with little to no band broadening, (5) the reaction is sensitive to picogram quantities of antigen, and (6) the reaction is quantitative. 相似文献
5.
6.
Georg Steinhauser Johannes H. Sterba Karin Poljanc Max Bichler Karl Buchtela 《Journal of trace elements in medicine and biology》2006,20(3):119-153
In this study, 18 partly commercially available samples of rock salt from Austria, Germany, Pakistan, Poland, Switzerland, and Ukraine were investigated with respect to their content of trace elements using instrumental neutron activation analysis. Elements detected were Al, Ba, Br, Ca, Ce, Cl, Co, Cr, Cs, Eu, Fe, Hf, La, Mn, Na, Rb, Sb, Sc, Sm, Sr, Ta, Tb, Th, and Zn, some of them only in individual cases. An estimation of the bioavailability of these trace elements was performed by dissolving an equivalent of the sodium chloride samples in diluted hydrochloric acid (simulating stomach acid), filtering off the insoluble components, and analyzing the evaporated filtrate. It could be shown that in most cases bioactive trace elements like Fe can be found in rock salt in the form of almost insoluble compounds and are therefore not significantly bioavailable, whereas thorium, for example, was partly bioavailable in two cases. A significant contribution to the recommended daily intake of metal trace elements by using rock salt for nutrition can be excluded. 相似文献
7.
8.
Protein of the tobacco mosaic virus mutant E66 has lysine replacing asparagine of the type strain, vulgare, at position 140. Thus, E66 protein should have one more positive or one less net negative charge than vulgare at pH 6 to 7. To investigate the effect of charge, a comparative study of the polymerization of E66 and vulgare proteins at pH 6.0, 6.2, 6.4, 6.6, and 6.8 at ionic strengths 0.15, 0.10, and 0.05 was made by turbidimetry. Polymerization of E66 protein always proceeded at a lower temperature than vulgare. However, the extent of polymerization was much lower in E66, especially at the higher ionic strengths. Sedimentation velocity results paralleled those from turbidity measurements in that E66 protein polymerizes at lower temperatures than vulgare; the 20 S component is more abundant in E66 protein. Osmotic pressure measurements also show that E66 protein is more polymerized than vulgare, especially at lower pH values. Hydrogen ion titrations of E66 protein were carried out from pH 8 to 5 and back to pH 8 in 0.10 m KCl at three temperatures, 4, 10, and 15 °C. These titrations were reversible when carried out slowly. The isoionic point is near pH 5; thus the charge at pH 7.5 is ?3. The reversible titration results were correlated with the aggregates present at the various pH values and temperatures, determined from the areas under the schlieren peaks in sedimentation velocity experiments. It is found that hydrogen ion binding at the three pH values is correlated with the disappearance of the smallest aggregates and is independent of the type of higher polymer formed. To investigate the effect of ionic strength and pH on the characteristic temperature corresponding to an optical density increment of 0.01 by the method used previously for vulgare, two sets of turbidity measurements were carried out. In the first one the ionic strength was changed from 0.025 to 0.15 in increments of 0.025 at pH 6.0 and 6.4. In the other set, the ionic strength was kept constant at 0.10 and the pH changed from 5.9 to 6.7 in increments of 0.1 pH units. When the analysis of these data was carried out, was obtained. For the salting out constant a value of 1.7 was found, compared to 2.2 for vulgare, a result consistent with the fact that E66 should be less hydrophobic than vulgare. The electrical work term ΔWel also turns out to be about one-half that for vulgare, which is expected from the lower net negative charge on E66 protein. 相似文献
9.
J Blake D M Helmeste C H Li 《International journal of peptide and protein research》1985,25(6):575-579
Two analogs of human beta-endorphin (beta-EP) which contain cystine bridges, [Cys15-Cys26,Phe27,Gly31]-beta-EP (I) and [Cys16-Cys26,Phe27,Gly31]-beta-EP (II), were synthesized by the solid-phase method. Peptides I and II were shown to contain 2-2.5 times the opiate receptor binding activity of beta-endorphin. We also synthesized two analogs with reduced alkylated cysteine residues and these peptides, [Arg9,19,24,28,29 Cys(Cam)11,26,Phe27,Gly31] and [Arg9,19,24,28,29,Cys-(Cam)12,26,Phe27,Gly31], were shown to have approximately the same opiate receptor activity as beta-endorphin. 相似文献
10.