全文获取类型
收费全文 | 595篇 |
免费 | 50篇 |
专业分类
645篇 |
出版年
2022年 | 8篇 |
2021年 | 9篇 |
2019年 | 6篇 |
2018年 | 7篇 |
2016年 | 7篇 |
2015年 | 27篇 |
2014年 | 19篇 |
2013年 | 14篇 |
2012年 | 28篇 |
2011年 | 26篇 |
2010年 | 24篇 |
2009年 | 9篇 |
2008年 | 28篇 |
2007年 | 22篇 |
2006年 | 21篇 |
2005年 | 20篇 |
2004年 | 22篇 |
2003年 | 29篇 |
2002年 | 18篇 |
2001年 | 19篇 |
2000年 | 18篇 |
1999年 | 25篇 |
1997年 | 7篇 |
1992年 | 9篇 |
1991年 | 14篇 |
1990年 | 6篇 |
1989年 | 11篇 |
1988年 | 6篇 |
1987年 | 15篇 |
1986年 | 10篇 |
1985年 | 5篇 |
1984年 | 8篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1975年 | 10篇 |
1973年 | 5篇 |
1972年 | 5篇 |
1971年 | 7篇 |
1970年 | 7篇 |
1969年 | 5篇 |
1968年 | 6篇 |
1966年 | 5篇 |
1856年 | 8篇 |
1855年 | 5篇 |
1853年 | 5篇 |
1852年 | 5篇 |
排序方式: 共有645条查询结果,搜索用时 15 毫秒
41.
42.
Wang F Miles RW Kicska G Nieves E Schramm VL Angeletti RH 《Protein science : a publication of the Protein Society》2000,9(9):1660-1668
The rate and extent of hydrogen/deuterium (H/D) exchange into purine nucleoside phosphorylase (PNP) was monitored by electrospray ionization mass spectrometry (ESI-MS) to probe protein conformational and dynamic changes induced by a substrate analogue, products, and a transition state analogue. The genetic deficiency of PNP in humans is associated with severe T-cell immunodeficiency, while B-cell immunity remains functional. Inhibitors of PNP have been proposed for treatment of T-cell leukemia, to suppress the graft-vs.-host response, or to counter type IV autoimmune diseases without destroying humoral immunity. Calf spleen PNP is a homotrimer of polypeptide chains with 284 amino residues, molecular weight 31,541. Immucillin-H inhibits PNP with a Kd of 23 pM when only one of the three catalytic sites is occupied. Deuterium exchange occurs at 167 slow-exchange sites in 2 h when no catalytic site ligands are present. The substrate analogue and product prevented H/D exchange at 10 of the sites. Immucillin-H protected 32 protons from exchange at full saturation. When one of the three subunits of the homotrimer is filled with immucillin-H, and 27 protons are protected from exchange in all three subunits. Deuterium incorporation in peptides from residues 132-152 decreased in all complexes of PNP. The rate and/or extent of deuterium incorporation in peptides from residues 29-49, 50-70, 81-98, and 112-124 decreased only in the complex with the transition state analogue. The peptide-specific H/D exchange demonstrates that (1) the enzyme is most compact in the complex with immucillin-H, and (2) filling a single catalytic site of the trimer reduces H/D exchange in the same peptides in adjacent subunits. The peptides most highly influenced by the inhibitor surround the catalytic site, providing evidence for reduced protein dynamic motion caused by the transition state analogue. 相似文献
43.
Microsensors used in microbial ecology are reviewed with emphasis on new sensor developments (NO3
-, NO2
-, NH4
+, CO2, H2, H2S and CH4 microsensors as well as fiberoptical microsensors for O2, temperature and pH). Examples of microsensor applications in biofilms and activated sludge flocs are presented, where sulfate reduction and denitrification were studied. 相似文献
44.
We examined feeding of blue catfish, Ictalurus furcatus, and flathead catfish, Pylodictis olivaris, collected from floodplain lake, secondary (side) river channel, and main river channel habitats in the lower Mississippi
River (LMR), U.S.A. We described the feeding ecology of two large river catfish species within the context of whether off-channel
habitats in the LMR (i.e., floodplain lakes and secondary channels) potentially provided energetic benefits to these fishes
as purported in contemporary theory on the ecology of large rivers. We used diet composition and associated caloric densities
of prey consumed as indicators of energetic benefit to catfishes. Differences in diet among habitats were strong for blue
catfish, but weak for flathead catfish; consumed foods generally differed among habitats in caloric (energy) content. Caloric
densities of consumed foods were generally greatest in floodplain lakes, least in the main river channel, and intermediate
in secondary river channels. Strong between-year variation in diet was observed, but only for blue catfish. Blue catfish fed
disproportionately on lower-energy zebra mussels in the main river channel during 1997, and higher-energy chironomids and
oligochaetes in floodplain lakes during 1998. Results suggested that although off-channel habitats potentially provided greater
energetic return to catfishes in terms of foods consumed, patterns of feeding and subsequent energy intake may vary annually.
Energetic benefits associated with off-channel habitats as purported under contemporary theory (e.g., the ‘flood-pulse concept’)
may not be accrued by catfishes every year in the LMR. 相似文献
45.
Daniel Pereira Maria Cristina Dreher Mansur Leandro D. S. Duarte Arthur Schramm de Oliveira Daniel Mansur Pimpão Cláudia Tasso Callil Cristián Ituarte Esperanza Parada Santiago Peredo Gustavo Darrigran Fabrizio Scarabino Cristhian Clavijo Gladys Lara Igor Christo Miyahira Maria Teresa Raya Rodriguez Carlos Lasso 《Hydrobiologia》2014,735(1):15-44
Based on literature review and malacological collections, 168 native freshwater bivalve and five invasive species have been recorded for 52 hydrographic regions in South America. The higher species richness has been detected in the South Atlantic, Uruguay, Paraguay, and Amazon Brazilian hydrographic regions. Presence or absence data were analysed by Principal Coordinate for Phylogeny-Weighted. The lineage Veneroida was more representative in hydrographic regions that are poorer in species and located West of South America. The Mycetopodidae and Hyriidae lineages were predominant in regions that are richest in species toward the East of the continent. The distribution of invasive species Limnoperna fortunei is not related to species richness in different hydrographic regions there. The species richness and its distribution patterns are closely associated with the geological history of the continent. The hydrographic regions present distinct phylogenetic and species composition regardless of the level of richness. Therefore, not only should the richness be considered to be a criterion for prioritizing areas for conservation, but also the phylogenetic diversity of communities engaged in services and functional aspects relevant to ecosystem maintenance. A plan to the management of this fauna according to particular ecological characteristics and human uses of hydrographic regions is needed. 相似文献
46.
Incubation of a rat adipose tissue homogenate causes a time and temperature dependent activation of glycogen synthetase (UDP glucose:glycogen 4-alpha-glucosyltransferase) and simultaneous inactivation of phosphorylase (1,4-alpha-D-glucan: orthophosphate alpha-glucosyltransferase, EC 2.4.1.1). Activation of glycogen synthetase at 15 and 23 degrees C was preceded by a lag period. The duration of the lag period could not be correlated with significant changes in phosphorylase activity. Addition of glucose and methylxanthines caused an increase in the rates of glycogen synthetase activation and phosphorylase inactivation. The effect on glycogen synthetase activation was mainly on the linear phase. Addition of AMP inhibited phosphorylase inactivation and accelerated glycogen synthetase activation. Addition of muscle phosphorylase alpha caused a prolongation of the lag period which lasted until phosphorylase alpha activity had decreased to the level originally present in the preparation. It is concluded that in adipose tissue activation of glycogen synthetase is not dependent on prior inactivation of phosphorylase and that other factors should be looked for to explain the lag period preceding glycogen synthetase activation. 相似文献
47.
Analysis of Stomatal and Nonstomatal Components in the Environmental Control of CO(2) Exchange in Leaves of Welwitschia mirabilis 总被引:2,自引:3,他引:2
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In well-watered plants of Welwitschia mirabilis, grown in the glass-house under high irradiance conditions, net CO2 assimilation was almost exclusively observed during the daytime. The plants exhibited a very low potential for Crassulacean acid metabolism, which usually resulted in reduced rates of net CO2 loss for several hours during the night. In leaves exposed to the diurnal changes in temperature and humidity typical of the natural habitats, CO2 assimilation rates in the light were markedly depressed under conditions resembling those occurring during midday, when leaf temperatures and the leaf-air vapor pressure differences were high (36°C and 50 millibars bar−1, respectively). Studies on the relationship between CO2 assimilation rate and intercellular CO2 partial pressure at various temperatures and humidities showed that this decrease in CO2 assimilation was largely due to stomatal closure. The increase in the limitation of photosynthesis by CO2 diffusion, which is associated with the strong decline in stomatal conductance in Welwitschia exposed to midday conditions, may significantly contribute to the higher 13C content of Welwitschia compared to the majority of C3 species. 相似文献
48.
Hillyer P Mane VP Schramm LM Puig M Verthelyi D Chen A Zhao Z Navarro MB Kirschman KD Bykadi S Jubin RG Rabin RL 《Immunology and cell biology》2012,90(8):774-783
Recent genome-wide association studies suggest distinct roles for 12 human interferon-alpha (IFN-α) and 3 IFN-λ subtypes that may be elucidated by defining the expression patterns of these sets of genes. To overcome the impediment of high homology among each of the sets, we designed a quantitative real-time PCR assay that incorporates the use of molecular beacon and locked nucleic acid (LNA) probes, and in some instances, LNA oligonucleotide inhibitors. We then measured IFN subtype expression by human peripheral blood mononuclear cells and by purified monocytes, myeloid dendritic cells (mDC), plasmacytoid dendritic cells (pDC), and monocyte-derived macrophages (MDM), and -dendritic cells (MDDC) in response to poly I:C, lipopolysaccharide (LPS), imiquimod and CpG oligonucleotides. We found that in response to poly I:C and LPS, monocytes, MDM and MDDC express a subtype pattern restricted primarily to IFN-β and IFN-λ1. In addition, while CpG elicited expression of all type I IFN subtypes by pDC, imiquimod did not. Furthermore, MDM and mDC highly express IFN-λ, and the subtypes of IFN-λ are expressed hierarchically in the order IFN-λ1 followed by IFN-λ2, and then IFN-λ3. These data support a model of coordinated cell- and ligand-specific expression of types I and III IFN. Defining IFN subtype expression profiles in a variety of contexts may elucidate specific roles for IFN subtypes as protective, therapeutic or pathogenic mediators. 相似文献
49.
The concept of a competitive enzyme immunoassay that utilizes simultaneously the bound and the free analyte-enzyme conjugate (heterobifunctional conjugate) for signal generation in response to varying analyte concentrations in samples has been investigated. Two antigenic sites of the heterobifunctional conjugate are used in the assay for binding to immunoglobulins: the analyte derivative binds to an immobilized antibody, Ab(1), and the enzyme component binds to a spatially separated immobilized antibody, Ab(2). The analytical system is set up such that in the absence of analyte, the conjugate is predominantly bound in the compartment that contains Ab(1). With increasing concentration of native analyte in samples, an increasing concentration of native analyte in samples, an increasing amount of conjugate migrates to the second compartment that contains Ab(2). The enzyme bound in each compartment is used for signal generation. Mathematical models have been developed to determine the optimal conditions and to predict the performance of such dual-antibody systems. The theoretical predictions are supported by experimental results. The dual-antibody system has been compared with a conventional competitive enzyme immunoassay using the same reagents. 相似文献
50.
Elisangela Oliveira Freitas Dirlei Nico Marcus Vinícius Alves-Silva Alexandre Morrot Keith Clinch Gary B. Evans Peter C. Tyler Vern L. Schramm Clarisa B. Palatnik-de-Sousa 《PLoS neglected tropical diseases》2015,9(12)