全文获取类型
收费全文 | 505044篇 |
免费 | 53495篇 |
国内免费 | 1076篇 |
专业分类
559615篇 |
出版年
2018年 | 14858篇 |
2017年 | 13596篇 |
2016年 | 11715篇 |
2015年 | 7161篇 |
2014年 | 7900篇 |
2013年 | 11313篇 |
2012年 | 16253篇 |
2011年 | 24743篇 |
2010年 | 19845篇 |
2009年 | 15646篇 |
2008年 | 20348篇 |
2007年 | 22230篇 |
2006年 | 10913篇 |
2005年 | 10767篇 |
2004年 | 10860篇 |
2003年 | 10885篇 |
2002年 | 10277篇 |
2001年 | 19972篇 |
2000年 | 20090篇 |
1999年 | 15763篇 |
1998年 | 5490篇 |
1997年 | 6059篇 |
1996年 | 5754篇 |
1995年 | 5370篇 |
1994年 | 5262篇 |
1993年 | 5415篇 |
1992年 | 13382篇 |
1991年 | 13229篇 |
1990年 | 12582篇 |
1989年 | 12384篇 |
1988年 | 11304篇 |
1987年 | 10947篇 |
1986年 | 10214篇 |
1985年 | 10166篇 |
1984年 | 8513篇 |
1983年 | 7363篇 |
1982年 | 5651篇 |
1981年 | 5029篇 |
1980年 | 4853篇 |
1979年 | 8006篇 |
1978年 | 6405篇 |
1977年 | 5829篇 |
1976年 | 5492篇 |
1975年 | 6040篇 |
1974年 | 6221篇 |
1973年 | 6151篇 |
1972年 | 5781篇 |
1971年 | 5368篇 |
1970年 | 4266篇 |
1969年 | 4108篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
881.
882.
The effect of substrate modification on binding of porcine pancreatic alpha amylase: hydrolysis of modified amylose containing D-allose residues 总被引:1,自引:0,他引:1
A modified amylose containing 10% of tritiated D-allose residues has been hydrolyzed by porcine pancreatic alpha amylase (PPA). This reaction produced a number of radioactive oligosaccharides of low molecular weight, including modified mono-, di-, and tri-saccharides, as well as larger products. Analysis of these products by chemical and enzymic methods identified D-allose, two isomers of modified maltose, and isomers of modified maltotriose. These results may be interpreted in terms of current PPA models to indicate that D-allose residues may be productively bound at all five subsites of the active site of the enzyme. The distribution of modified residues in these products, however, further suggests that productive binding of D-allose at the subsite where catalytic attack occurs (subsite 3) is less favorable than binding of D-glucose. These results are compared with results of a series of PPA substrates having modifications at C-3 and at other positions. Trends observed in enzyme hydrolysis of these modified substrates reflect factors that contribute to PPA catalysis, with respect to steric, electronic, and hydrogen-bonding interactions between enzyme and substrate. 相似文献
883.
Measurement of the volume change by a rapid density method upon sequential addition of calcium ion to calmodulin showed relatively large, nonuniform increases for the first 4 moles Ca2+ per mole calmodulin. Substantially larger volume increases (approximately 15 ml/mol protein) were observed upon addition of the second and fourth moles Ca2+ relative to the first and third moles added per mole calmodulin. A total volume increase of approximately 170 ml/mol protein attended the addition of 4 moles Ca2+, as expected for multidentate carboxylate coordination to metal ion. Marginal changes in volume were observed upon further additions, the data showing a remarkably sharp transition after [Ca2+]/[calmodulin] = 4. The results are consistent with an ordered binding of Ca2+ in which pair-wise additions produce similar volume changes; the volume change behavior, however, does not indicate an absence of distinct conformational states for a Ca2+(1)-calmodulin and a Ca2+(3)-calmodulin complex as has been proposed on the basis of 1H-NMR evidences. 相似文献
884.
885.
T. R. FRASIER T. RASTOGI M. W. BROWN P. K. HAMILTON S. D. KRAUS B. N. WHITE 《Molecular ecology resources》2006,6(4):1025-1029
A North Atlantic right whale (Eubalaena glacialis) genomic library was developed and screened with a (GATA)8 probe to identify tetranucleotide microsatellite loci. Sixteen characterized loci were polymorphic in North Atlantic and/or South Atlantic (Eubalaena australis) right whales, 12 being polymorphic in E. glacialis, and 15 in E. australis. Fourteen of these were combined with 21 other previously identified loci for a suite of 35 loci which can be used to increase resolution of genetic analyses of these species. Multiplex reactions were developed for genotyping samples at these loci, providing a method that is rapid, reliable and cost‐effective. 相似文献
886.
Distinguishing between cryptic species is a perennial problem for biologists. Bombus ruderatus and Bombus hortorum are two species of bumblebee, which can be indistinguishable from their morphology. The former species is in decline, whereas the latter is ubiquitous. In the UK, isolated records of B. ruderatus occur amongst many for B. hortorum. For ecological studies of B. ruderatus to be feasible, the two species need to be reliably distinguishable. We present a diagnostic tool for quick and reliable identification of problematic individuals based on a restriction enzyme digest of the cytochrome b region of mitochondrial DNA. 相似文献
887.
A general in vitro cloning system was established for four Helleborus species: H. argutifolius, H. foetidus, H. niger and H. orientalis. The plant material was introduced in vitro from axillary buds. A Murashige and Skoog (MS)—based medium (Murashige and Skoog
1962) was used supplemented with 2% (w/v) sucrose, 2-isopentenyladenine (2-iP) and 6-benzylaminopurine (BA). Multiplication
rates depended on the genotype and varied from 1.3 for H. foetidus till 3.8 for H. niger. The first results showed that the rooting phase could be done ex vitro. Rooting was induced by a drench for one week in
a solution of indole-3-butyric acid (IBA -3 mg l−1) and 1-naphthaleneacetic acid (NAA-1 mg l−1) at 5°C. 相似文献
888.
889.
890.
Chien-Hung Liu Wen-Ming Chen Jo-Shu Chang 《World journal of microbiology & biotechnology》2007,23(5):633-640
Acidic lipase finds its commercial values in medical applications and bioremediation of food wastes. In this work, approaches
for rapid screening of lipase-producing bacteria were developed and the feasibility assessment of the screening methods was
performed. From food waste samples, the proposed screening procedures allowed isolation of sixteen pure bacterial strains
expressing higher lipase activity at acidic pH (pH 6.0) than at alkaline pH (pH 9.0). To enhance the accuracy of lipase activity
determination under acidic conditions, a novel assay procedure was also developed by deactivating lipase activity by microwave
treatment prior to back titration. This additional step could minimize interferences arising from residual lipase activity
during conventional direct back-titration methods in measuring lipase activity at acidic pH. Using the four strategies proposed
in this work, the best acidic-lipase-producing isolate was obtained by strategy C (SSC) and was identified as Aeromonas sp. C14, displaying an optimal lipase activity of 0.7 U/ml at an acidic pH of 6.0. 相似文献