全文获取类型
收费全文 | 2418篇 |
免费 | 154篇 |
国内免费 | 1篇 |
专业分类
2573篇 |
出版年
2023年 | 3篇 |
2022年 | 8篇 |
2021年 | 31篇 |
2020年 | 15篇 |
2019年 | 19篇 |
2018年 | 29篇 |
2017年 | 30篇 |
2016年 | 64篇 |
2015年 | 98篇 |
2014年 | 101篇 |
2013年 | 141篇 |
2012年 | 218篇 |
2011年 | 169篇 |
2010年 | 96篇 |
2009年 | 112篇 |
2008年 | 160篇 |
2007年 | 162篇 |
2006年 | 132篇 |
2005年 | 145篇 |
2004年 | 129篇 |
2003年 | 104篇 |
2002年 | 120篇 |
2001年 | 33篇 |
2000年 | 24篇 |
1999年 | 32篇 |
1998年 | 35篇 |
1997年 | 28篇 |
1996年 | 36篇 |
1995年 | 24篇 |
1994年 | 23篇 |
1993年 | 29篇 |
1992年 | 27篇 |
1991年 | 19篇 |
1990年 | 18篇 |
1989年 | 16篇 |
1988年 | 14篇 |
1987年 | 10篇 |
1986年 | 17篇 |
1985年 | 19篇 |
1984年 | 15篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 7篇 |
1945年 | 2篇 |
1866年 | 1篇 |
排序方式: 共有2573条查询结果,搜索用时 15 毫秒
81.
82.
Background
Recent studies indicate that 27% of Dutch blood donors have evidence of past infection with HEV. However, the low number of diagnosed HEV infections indicates either an asymptomatic course or under diagnosis.Objectives
We investigated whether HEV is a cause of acute hepatitis in Dutch patients and which diagnostic modality (serology or PCR) should be used for optimal detection.Study design
Serum samples were retrospectively selected from non-severely immuno-compromised patients from a university hospital population, suspected of having an infectious hepatitis. Criteria were: elevated alanine aminotransferase (ALT> 34 U/l) and request for antibody testing for CMV, EBV or Hepatitis A (HAV).Results
All samples were tested for HEV using ELISA and PCR. Ninety patients/sera were tested, of which 22% were HEV IgG positive. Only one serum was IgM positive. HEV PCR was positive in two patients: one patient was both HEV IgM and IgG positive, the other patient was only IgG positive. Both HEV RNA positive samples belonged to genotype 3. Evidence of recent infection with CMV, EBV and HAV was found in 13%, 10% and 3% respectively.Conclusions
Although our study is limited by small numbers, we conclude that HEV is a cause of acute hepatitis in hospital associated patients in The Netherlands. Moreover, in our study population the prevalence of acute HAV (3%) was almost similar to acute HEV (2%). We propose to incorporate HEV testing in panels for acute infectious hepatitis. Negative results obtained for HEV IgM in a HEV PCR positive patient, indicates that antibody testing alone may not be sufficient and argues for PCR as a primary diagnostic tool in hospital associated patients. The high percentage of HEV IgG seropositivity confirms earlier epidemiological studies. 相似文献83.
Towards the industrialization of new biosurfactants: Biotechnological opportunities for the lactone esterase gene from Starmerella bombicola 下载免费PDF全文
84.
Wim Hordijk 《Origins of life and evolution of the biosphere》2016,46(2-3):233-245
Several computational models of chemical reaction networks have been presented in the literature in the past, showing the appearance and (potential) evolution of autocatalytic sets. However, the notion of autocatalytic sets has been defined differently in different modeling contexts, each one having some shortcoming or limitation. Here, we review four such models and definitions, and then formally describe and analyze them in the context of a mathematical framework for studying autocatalytic sets known as RAF theory. The main results are that: (1) RAF theory can capture the various previous definitions of autocatalytic sets and is therefore more complete and general, (2) the formal framework can be used to efficiently detect and analyze autocatalytic sets in all of these different computational models, (3) autocatalytic (RAF) sets are indeed likely to appear and evolve in such models, and (4) this could have important implications for a possible metabolism-first scenario for the origin of life. 相似文献
85.
Andrei Prodan Sultan Imangaliyev Henk S. Brand Martijn N. A. Rosema Evgeni Levin Wim Crielaard Bart J. F. Keijser Enno C. I. Veerman 《Metabolomics : Official journal of the Metabolomic Society》2016,12(9):147
Introduction
Understanding the changes occurring in the oral ecosystem during development of gingivitis could help improve prevention and treatment strategies for oral health. Erythritol is a non-caloric polyol proposed to have beneficial effects on oral health.Objectives
To examine the effect of experimental gingivitis and the effect of erythritol on the salivary metabolome and salivary functional biochemistry.Methods
In a two-week experimental gingivitis challenge intervention study, non-targeted, mass spectrometry-based metabolomic profiling was performed on saliva samples from 61 healthy adults, collected at five time-points. The effect of erythritol was studied in a randomized, controlled trial setting. Fourteen salivary biochemistry variables were measured with antibody- or enzymatic activity-based assays.Results
Bacterial amino acid catabolites (cadaverine, N-acetylcadaverine, and α-hydroxyisovalerate) and end-products of bacterial alkali-producing pathways (N-α-acetylornithine and γ-aminobutyrate) increased significantly during the experimental gingivitis. Significant changes were found in a set of 13 salivary metabolite ratios composed of host cell membrane lipids involved in cell signaling, host responses to bacteria, and defense against free radicals. An increase in mevalonate was also observed. There were no significant effects of erythritol. No significant changes were found in functional salivary biochemistry.Conclusions
The findings underline a dynamic interaction between the host and the oral microbial biofilm during an experimental induction of gingivitis.86.
E. R. Jasper Wubs Wim H. van der Putten Simon R. Mortimer Gerard W. Korthals Henk Duyts Roel Wagenaar T. Martijn Bezemer 《Ecology letters》2019,22(7):1145-1151
Recent demonstrations of the role of plant–soil biota interactions have challenged the conventional view that vegetation changes are mainly driven by changing abiotic conditions. However, while this concept has been validated under natural conditions, our understanding of the long‐term consequences of plant–soil interactions for above‐belowground community assembly is restricted to mathematical and conceptual model projections. Here, we demonstrate experimentally that one‐time additions of soil biota and plant seeds alter soil‐borne nematode and plant community composition in semi‐natural grassland for 20 years. Over time, aboveground and belowground community composition became increasingly correlated, suggesting an increasing connectedness of soil biota and plants. We conclude that the initial composition of not only plant communities, but also soil communities has a long‐lasting impact on the trajectory of community assembly. 相似文献
87.
88.
89.
There is strong interest in creating an enzyme that can deacylate natural cephalosporins such as cephalosporin C in order to efficiently acquire the starting compound for the industrial production of semisynthetic cephalosporin antibiotics. In this study, the active site of the glutaryl acylase from Pseudomonas SY-77 was randomized rationally. Several mutations that were found in previous studies to enhance the activity of the enzyme towards adipyl-7-aminodesacetoxycephalosporanic acid (ADCA) and cephalosporin C have now been combined, and libraries have been made in which random amino acid substitutions at these positions are joined. The mutants were expressed in a leucine-deficient Escherichia coli strain and subjected to growth selection with adipyl-leucine or amino-adipyl-leucine as sole leucine source. The mutants growing on these media were selected and purified, and their hydrolysis activities towards adipyl-7-ADCA and cephalosporin C were tested. Several mutants with highly improved activities towards the desired substrates were found in these rationally randomized libraries. The best mutant was selected from a library of totally randomized residues: 178, 266, and 375. This mutant comprises two mutations, Y178F + F375H, which synergistically improve the catalytic efficiency towards adipyl-7-ADCA 36-fold. The activity of this mutant towards adipyl-7-ADCA is 50% of the activity of the wild-type enzyme towards the preferred substrate glutaryl-7-aminocephalosporanic acid, and therefore the characteristics of this mutant approach those needed for industrial application. 相似文献
90.
Miao Yu Tiago Castanheira Silva Andries van Opstal Stefan Romeijn Hayley A. Every Wim Jiskoot Geert-Jan Witkamp Marcel Ottens 《Biophysical journal》2019,116(4):595-609
In this study, we developed a microfluidics method, using a so-called H-cell microfluidics device, for the determination of protein diffusion coefficients at different concentrations, pHs, ionic strengths, and solvent viscosities. Protein transfer takes place in the H-cell channels between two laminarly flowing streams with each containing a different initial protein concentration. The protein diffusion coefficients are calculated based on the measured protein mass transfer, the channel dimensions, and the contact time between the two streams. The diffusion rates of lysozyme, cytochrome c, myoglobin, ovalbumin, bovine serum albumin, and etanercept were investigated. The accuracy of the presented methodology was demonstrated by comparing the measured diffusion coefficients with literature values measured under similar solvent conditions using other techniques. At low pH and ionic strength, the measured lysozyme diffusion coefficient increased with the protein concentration gradient, suggesting stronger and more frequent intermolecular interactions. At comparable concentration gradients, the measured lysozyme diffusion coefficient decreased drastically as a function of increasing ionic strength (from zero onwards) and increasing medium viscosity. Additionally, a particle tracing numerical simulation was performed to achieve a better understanding of the macromolecular displacement in the H-cell microchannels. It was found that particle transfer between the two channels tends to speed up at low ionic strength and high concentration gradient. This confirms the corresponding experimental observation of protein diffusion measured via the H-cell microfluidics. 相似文献