全文获取类型
收费全文 | 1169篇 |
免费 | 124篇 |
出版年
2022年 | 12篇 |
2021年 | 24篇 |
2020年 | 20篇 |
2019年 | 16篇 |
2018年 | 23篇 |
2017年 | 18篇 |
2016年 | 22篇 |
2015年 | 39篇 |
2014年 | 47篇 |
2013年 | 50篇 |
2012年 | 71篇 |
2011年 | 73篇 |
2010年 | 40篇 |
2009年 | 34篇 |
2008年 | 44篇 |
2007年 | 48篇 |
2006年 | 54篇 |
2005年 | 38篇 |
2004年 | 45篇 |
2003年 | 36篇 |
2002年 | 34篇 |
2001年 | 26篇 |
2000年 | 22篇 |
1999年 | 22篇 |
1998年 | 12篇 |
1997年 | 16篇 |
1996年 | 10篇 |
1995年 | 13篇 |
1994年 | 8篇 |
1993年 | 9篇 |
1992年 | 18篇 |
1991年 | 26篇 |
1990年 | 20篇 |
1989年 | 19篇 |
1988年 | 10篇 |
1987年 | 16篇 |
1986年 | 19篇 |
1985年 | 17篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1980年 | 12篇 |
1979年 | 13篇 |
1978年 | 22篇 |
1977年 | 19篇 |
1976年 | 15篇 |
1975年 | 13篇 |
1974年 | 8篇 |
1973年 | 16篇 |
排序方式: 共有1293条查询结果,搜索用时 15 毫秒
41.
Emanuel D. Rudolph 《American journal of botany》1982,69(8):1346-1355
Botany was thought to be a suitable study for young women in schools and an amateur avocation in the Nineteenth Century. A surprisingly large number of American women identified themselves as being seriously interested in botany. For example, in the first published directory of American botanists in 1873, 13 percent of the 599 names are women's and that increased to 16 percent of the 982 names in 1878. In this paper, 1,185 women have been identified as a sample of those actively interested in botany during the century. Less than 2% of them were active before 1870, and most of them, 67%, resided in New England and the Middle Atlantic States. Almost three quarters of them were unmarried, and only 10% had higher educational degrees although 15% had some identifiable profession. Some particular individuals are noted for their contributions to science, their activity as plant collectors, and their support of botanical societies. Though few American women became professional botanists in the Nineteenth Century, they constitute an important overlooked constituency for the developing profession of botany. 相似文献
42.
Peter Miethe Ingeborg Jansen Uwe Niedermeyer Udo Kragl Regine Haftendorn Maria-Regina Kula Christian Wandrey Karl-Heinz Mohr Helmut-Walter Meyer 《Biocatalysis and Biotransformation》1992,7(1):61-73
The possibility of using the enzyme (R)-Oxynitrilase in a biphasic lyotropic liquid crystal/dibutylether system has been demonstrated. This reaction system is applicable for the continuous production of (R)-benzaldehydecyanohydrin in a fixed bed reactor. The optical purity was between 94 and 96% ee and independent of the flow rate. The space time yield was maximal (2650 g/(1*d)) at a flow rate of 1.6 ml/min. 相似文献
43.
A simple system is introduced to produce dipeptides continuously by enzyme catalyzed condensation of amino acid esters and amino acid amides. Synthesis of N-terminal free dipeptide-amides is achieved by means of carboxypeptidase Y. The peptide-amide is deamidated utilizing a newly isolated peptide-amide is deamidated utilizing a newly isolated peptide-amidase. Separation of substrates and products is accomplished by anion-exchange chromatography. Modeling of the reactions shows that the two reactions have to be carried out in a cascade of two reactors in order to prevent hydrolysis of the peptide by the carboxypeptidase. Continuous production of Kyotorphin (H-TyrArg-OH) with a space-time yield of 257 g/L . d shows the feasibility of this concept. 相似文献
44.
An optimized coupled enzyme assay for UDP-glucose pyrophosphorylase (EC 2.7.7.9) using UDP-glucose dehydrogenase (EC 1.1.1.22) is presented. This optimized assay was developed by a detailed investigation of the kinetics of the UDP-glucose dehydrogenase reaction. In addition the data provide a basis for the enzymatic synthesis of UDP-glucuronic acid. The results demonstrate that the two binding sites of the dehydrogenase differ since a different modulation of the enzyme activity and stability is observed after preincubation with UDP-glucose or NAD+ at various pH values. This is of general interest for the preparation of assay mixtures where UDP-glucose dehydrogenase is used as an auxiliary enzyme. 相似文献
45.
Emanuel Hackel 《American journal of human genetics》1990,47(4):762-763
46.
We have adapted a subtraction hybridization technique involving photoactivatable biotin, streptavidin binding, and organic extraction for solution hybridization analysis of mammalian genomic DNA. By combining maximal hybridization conditions of high salt, dextran sulfate, and formamide with successive hybridization steps and sequence enrichment by agarose gel electrophoresis, up to 97% of tracer DNA can be reproducibly driven to hybridize with photobiotinylated driver DNA. We demonstrate that the fractionation of hybridized from unhybridized sequences by this technique differs from hydroxyapatite chromatography with respect to the handling of nondenatured tracer, foldback sequences, and tracer-tracer hybrids. Strategies are presented to control the contribution of these species to the final subtracted product thereby making this technology a useful adjunct to solution hybridization approaches such as deletion cloning. 相似文献
47.
William S. Thayer Emanuel Rubin 《Biochimica et Biophysica Acta (BBA)/Molecular Cell Research》1982,721(4):328-335
Previous studies have established that hepatic mitochondria and submitochondrial particles from rats, fed ethanol chronically, display diminished respiratory activities and alterations in the contents of specific electron transfer chain components. The latter include a decrease of about 50% in cytochrome b content. Titrations of respiratory activity in submitochondrial particles with antimycin, a stoichiometric inhibitor of electron flow through the cytochrome b-c1 region of the respiratory chain, indicated a comparable decrease (35%) in the amount of antimycin required to elicit maximal inhibition (‘titer’) after chronic ethanol treatment. Measurements of antimycin binding to submitochondrial particles by fluorescence quenching demonstrated a similar diminution in the number of tight binding sites per mg protein. By contrast, hepatocytes isolated from control and ethanol-fed rats exhibited nearly identical rates of oxygen utilization under a variety of conditions. However, antimycin titrations of respiratory activity in isolated hepatocytes revealed a 60% decrease in the antimycin titer, but no change in the maximal extent of inhibition after chronic ethanol treatment. Direct measurements of cytochrome b which could be reduced in the presence of antimycin in hepatocytes confirmed a comparable decrease (42%) after chronic ethanol treatment. The results demonstrate that molecular alterations in the cytochrome b region of the respiratory chain caused by ethanol feeding are present in intact liver cells, but suggest that substrate accessibility, rather than the respiratory chain, limits the rate of oxygen utilization in isolated hepatocytes. The data also suggest that mitochondria account for at least 80% of total oxygen utilization by liver cells from both control and ethanol-fed rats. 相似文献
48.
Binding of non-substrate nucleotides to a restriction endonuclease: a model for the interaction of bam HI with its recognition sequence 总被引:1,自引:1,他引:0 下载免费PDF全文
The kinetic constants of the site-specific endonuclease Bam HI for various substrates were determined and binding of non-substrate nucleotides to the enzyme was studied. Agarose gel assays in combination with an integrated Michaelis-Menten equation were used for the evaluation of data. The turnover number was 2.2 min-1 at 37 degrees C with pJC80 DNA as the substrate. It depends on the conformation and base composition of the substrate. Michaelis constants also depend on substrate conformation. Non-substrate polynucleotides were found to inhibit Bam competitively with KI ranging from 10(-6) to > 10(-3) M depending on base composition, base pairing, and helix conformation. Dinucleotides showed sequence-specific, competitive inhibition with KIs ranging from 10(-5) to > 10(-3) M. Mononucleotides and -nucleosides acted noncompetitively. Binding was influenced by the extent of phosphorylation, but not by the nature of the base. KIs varied between 10(-3) and 10(-2) M. The results are discussed with respect to the recognition requirements of Bam HI. 相似文献
49.
Rolf Wichmann Christian Wandrey Andreas F. Bückmann Maria-regina Kula 《Biotechnology and bioengineering》1981,23(12):2789-2802
Multienzyme reaction systems with simultaneous coenzyme regeneration have been investigated in a continuously operated membrane reactor at bench scale. NAD(H) covalently bound to polyethylene glycol with a molecular weight of 104 [PEG-10,000-NAD(H)] was used as coenzyme. It could be retained in the membrane reactor together with the enzymes. L -leucine dehydrogenase (LEUDH) was used as catalyze for the reductive amination of α-ketoisocaproate (2-oxo-4-methylpentanoic acid) to L -leucine. Format dehydrogenease (FDH) was used for the regeneration of NADH. Kinetic experiments were carried out to obtain data which could be used in a kinetic model in order to predict the performance of an enzyme membrane reactor for the continuous production of L -leucine. The kinetic constants Vmax and Km of enzymes are all in the same range regardless of whether native NAD(H) or PEG-10,000-NAD(H) is used as coenzyme. L -leucine was produced continuously out of α-ketoisocaproate for 48 days; a maximal conversion of 99.7% was reached. The space-time yield was 324 mmol/L day (or 42.5 g/L day). 相似文献
50.