首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2514篇
  免费   164篇
  2022年   13篇
  2021年   32篇
  2020年   14篇
  2019年   24篇
  2018年   39篇
  2017年   29篇
  2016年   52篇
  2015年   66篇
  2014年   69篇
  2013年   115篇
  2012年   130篇
  2011年   133篇
  2010年   86篇
  2009年   74篇
  2008年   147篇
  2007年   128篇
  2006年   134篇
  2005年   154篇
  2004年   115篇
  2003年   113篇
  2002年   128篇
  2001年   90篇
  2000年   75篇
  1999年   74篇
  1998年   33篇
  1997年   35篇
  1996年   28篇
  1995年   30篇
  1994年   25篇
  1993年   19篇
  1992年   49篇
  1991年   45篇
  1990年   43篇
  1989年   52篇
  1988年   42篇
  1987年   20篇
  1986年   31篇
  1985年   18篇
  1984年   18篇
  1983年   11篇
  1982年   11篇
  1981年   10篇
  1980年   8篇
  1979年   11篇
  1978年   16篇
  1976年   7篇
  1975年   17篇
  1974年   8篇
  1973年   7篇
  1971年   9篇
排序方式: 共有2678条查询结果,搜索用时 15 毫秒
81.
The heat treatment of recombinant mesophilic cells having heterologous thermophilic enzymes results in the denaturation of indigenous mesophilic enzymes and the elimination of undesired side reactions; therefore, highly selective whole-cell catalysts comparable to purified enzymes can be readily prepared. However, the thermolysis of host cells leads to the heat-induced leakage of thermophilic enzymes, which are produced as soluble proteins, limiting the exploitation of their excellent stability in repeated and continuous reactions. In this study, Escherichia coli cells having the thermophilic fumarase from Thermus thermophilus (TtFTA) were treated with glutaraldehyde to prevent the heat-induced leakage of the enzyme, and the resulting cells were used as a whole-cell catalyst in repeated and continuous reactions. Interestingly, although electron microscopic observations revealed that the cellular structure of glutaraldehyde-treated E. coli was not apparently changed by the heat treatment, the membrane permeability of the heated cells to relatively small molecules (up to at least 3 kDa) was significantly improved. By applying the glutaraldehyde-treated E. coli having TtFTA to a continuous reactor equipped with a cell-separation membrane filter, the enzymatic hydration of fumarate to malate could be operated for more than 600 min with a molar conversion yield of 60% or higher.  相似文献   
82.
We report here that Tyrophagus similis and Tyrophagus putrescentiae (Astigmata: Acaridae) have the ability to biosynthesize linoleic acid [(9Z, 12Z)-9, 12-octadecadienoic acid] via a Δ12-desaturation step, although animals in general and vertebrates in particular appear to lack this ability. When the mites were fed on dried yeast enriched with d31-hexadecanoic acid (16:0), d27-octadecadienoic acid (18:2), produced from d31-hexadecanoic acid through elongation and desaturation reactions, was identified as a major fatty acid component of phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs) in the mites. The double bond position of d27-octadecadienoic acid (18:2) of PCs and PEs was determined to be 9 and 12, respectively by dimethyldisulfide (DMDS) derivatization. Furthermore, the GC/MS retention time of methyl 9, 12-octadecadienoate obtained from mite extracts agreed well with those of authentic linoleic acid methyl ester. It is still unclear whether the mites themselves or symbiotic microorganisms are responsible for inserting a double bond into the Δ12 position of octadecanoic acid. However, we present here the unique metabolism of fatty acids in the mites.  相似文献   
83.
The submerged tree volume and the projection area of mangroves play a significant role in damping tsunami inundation flow with a distinct root formation above ground. We modeled the stilt root morphology of the Rhizophora sp., especially to incorporate into a hydraulic drag of tsunami inundation simulation. The equivalent Manning’s roughness coefficient has been used as the hydraulic drag of mangroves for the computation of inundation flow [Yanagisawa et al. (Coast Shelf Sci 81: 27–37, 2009)], but it could not elucidate the effectiveness under different tree conditions. The field data from 18 sample trees in Ranong Province, Thailand, were measured. The total number of primary roots, the root height at trunk, and the root-spread distance, the root diameter, and the vertical root angle from trunk could be estimated with the diameter of the breast height. The quadratic equation expressed the root curve of the primary stilt root, and functions to estimate root volume and projected area were derived by the integration of the equation that will be used to calculate drag force in tsunami simulation.  相似文献   
84.
85.
86.
The isolation of chloramphenicol resistant strains from Corynebacterium hydrocarboclastus KY 4339 (rough type) was examined to seek a good source of corynecins (analogs of chloramphenicol). Various mutants resistant to chloramphenicol were isolated in the range from 50 to 1000 µg/ml by adaptation or induced mutagenesis by N-methyl-N′-nitro-N-nitro-soguanidine. Productivities of mutants related apparently to the degree of resistance from 50 to 500 µg/ml. Highly resistant mutants capable of growing in the presence of 1000 µg of chloramphenicol per ml showed decreased productivity which might be related to their lower growth rate in the fermentation medium.

Further attempts to derive resistant mutants to structural analogs of aromatic amino acids resulted in only a slight improvement of productivity, indicating that aromatic amino acids might play minor regulatory roles in corynecins synthesis.

The increase in productivity of corynecins by the best strain was about 4.5 fold of the parental strain.  相似文献   
87.
An NAD linked formate dehydrogenating enzyme which catalyzed the last step of methanol oxidation system was extracted from the methanol-grown Kloeckera sp. No. 2201. The specific activity of the enzyme in the extract of methanol-grown cells was found to be considerably higher than that of the glucose-grown cells. The enzyme was purified about 35-fold from the extract of methanol-grown cells by heat treatment, column chromatographies on DEAE-cellulose and on hydroxylapatite, and Sephadex G-200 gel filtration. The purified enzyme was shown to be homogeneous by analyses with electrophoresis and ultracentrifuga-tion. The purified enzyme was a kind of NAD: formate oxidoreductase (EC, 1.2.1.2) which catalyzed specifically the oxidation of formate to carbon dioxide. The Km values were 22 mm for formate and 0.1 mm for NAD. The enzyme was inactivated by potassium cyanide, sodium azide, and p-chloromercuribenzoate but not by any metal-chelating reagents tested. Other general properties of the enzyme were also investigated.  相似文献   
88.
We studied the inhibitory effects of isorhamnetin on mushroom tyrosinase by inhibition kinetics and computational simulation. Isorhamnetin reversibly inhibited tyrosinase in a mixed-type manner at K i=0.235 ± 0.013 mM. Measurements of intrinsic and 1-anilinonaphthalene-8-sulfonate(ANS)-binding fluorescence showed that isorhamnetin did not induce significant changes in the tertiary structure of tyrosinase. To gain insight into the inactivation process, the kinetics were computed via time-interval measurements and continuous substrate reactions. The results indicated that inactivation induced by isorhamnetin was a first-order reaction with biphasic processes. To gain further insight, we simulated docking between tyrosinase and isorhamnetin. Simulation was successful (binding energies for Dock6.3: ?32.58 kcal/mol, for AutoDock4.2: ?5.66 kcal/mol, and for Fred2.2: ?48.86 kcal/mol), suggesting that isorhamnetin interacts with several residues, such as HIS244 and MET280. This strategy of predicting tyrosinase interaction in combination with kinetics based on a flavanone compound might prove useful in screening for potential natural tyrosinase inhibitors.  相似文献   
89.
The effects of mutant cell division cycle 25 homolog B (CDC25B) overexpression on the generation of cells producing a monoclonal antibody were investigated in Chinese hamster ovary (CHO) cells. Mutant CDC25B (m-CDC25B) expression plasmids were transfected into CHO DG44-derived cells producing a monoclonal antibody, and the frequency of highly producing cells was assessed following gene amplification in the presence of 250 nM methotrexate. Most of the clones obtained from the m-CDC25B-overexpressing cells had higher antibody titers than did mock-transfected control cells. This arose from either higher transgene copy numbers or higher mRNA expression levels for the antibody. However, the high mRNA expression levels were not always accompanied by increases in transgene copy numbers. Our results suggest that cells producing high levels of a monoclonal antibody can be selected efficiently using m-CDC25B overexpression.  相似文献   
90.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号