全文获取类型
收费全文 | 148篇 |
免费 | 0篇 |
国内免费 | 2篇 |
专业分类
系统科学 | 2篇 |
理论与方法论 | 1篇 |
现状及发展 | 41篇 |
研究方法 | 7篇 |
综合类 | 98篇 |
自然研究 | 1篇 |
出版年
2013年 | 1篇 |
2012年 | 6篇 |
2011年 | 4篇 |
2010年 | 1篇 |
2008年 | 4篇 |
2007年 | 5篇 |
2006年 | 7篇 |
2005年 | 8篇 |
2004年 | 6篇 |
2003年 | 3篇 |
2002年 | 2篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1994年 | 3篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 6篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 4篇 |
1972年 | 4篇 |
1971年 | 10篇 |
1969年 | 1篇 |
1968年 | 6篇 |
1967年 | 7篇 |
1966年 | 10篇 |
1965年 | 3篇 |
1964年 | 1篇 |
1959年 | 1篇 |
1955年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有150条查询结果,搜索用时 15 毫秒
71.
Zauderer BA Berger E Soderberg AM Loeb A Narayan R Frail DA Petitpas GR Brunthaler A Chornock R Carpenter JM Pooley GG Mooley K Kulkarni SR Margutti R Fox DB Nakar E Patel NA Volgenau NH Culverhouse TL Bietenholz MF Rupen MP Max-Moerbeck W Readhead AC Richards J Shepherd M Storm S Hull CL 《Nature》2011,476(7361):425-428
Active galactic nuclei, which are powered by long-term accretion onto central supermassive black holes, produce relativistic jets with lifetimes of at least one million years, and the observation of the birth of such a jet is therefore unlikely. Transient accretion onto a supermassive black hole, for example through the tidal disruption of a stray star, thus offers a rare opportunity to study the birth of a relativistic jet. On 25 March 2011, an unusual transient source (Swift J164449.3+573451) was found, potentially representing such an accretion event. Here we report observations spanning centimetre to millimetre wavelengths and covering the first month of evolution of a luminous radio transient associated with Swift J164449.3+573451. The radio transient coincides with the nucleus of an inactive galaxy. We conclude that we are seeing a newly formed relativistic outflow, launched by transient accretion onto a million-solar-mass black hole. A relativistic outflow is not predicted in this situation, but we show that the tidal disruption of a star naturally explains the observed high-energy properties and radio luminosity and the inferred rate of such events. The weaker beaming in the radio-frequency spectrum relative to γ-rays or X-rays suggests that radio searches may uncover similar events out to redshifts of z?≈?6. 相似文献
72.
Structural and serological evidence for a novel mechanism of antigenic variation in foot-and-mouth disease virus. 总被引:21,自引:0,他引:21
Changes resulting in altered antigenic properties of viruses nearly always occur on their surface and have been attributed to the substitution of residues directly involved in binding antibody. To investigate the mechanism of antigenic variation in foot-and-mouth disease virus (FMDV), variants that escape neutralization by a monoclonal antibody have been compared crystallographically and serologically with parental virus. FMDVs form one of the four genera of the Picornaviridae. The unenveloped icosahedral shell comprises 60 copies each of four structural proteins VP1-4. Representatives from each of the genera have similar overall structure, but differences in the external features. For example, human rhinovirus has a pronounced 'canyon' that is proposed to contain the cell attachment site, whereas elements of the attachment site for FMDV, which involves the G-H loop (residues 134-160) and C-terminus (200-213) of VP1, are exposed on the surface. Moreover, this G-H loop, which is a major antigenic site of FMDV, forms a prominent, highly accessible protrusion, a feature not seen in other picornaviruses. It is this loop that is perturbed in the variant viruses that we have studied. The amino acid mutations characterizing the variants are not at positions directly involved in antibody binding, but result in far-reaching perturbations of the surface structure of the virus. Thus, this virus seems to use a novel escape mechanism whereby an induced conformational change in a major antigenic loop destroys the integrity of the epitope. 相似文献
73.
74.
R. L. Hancock Patricia McFarland R. R. Fox 《Cellular and molecular life sciences : CMLS》1967,23(10):806-807
Résumé Utilisant SAM-14CH3 il a été démontré que le foie foetal a plus d'activité de sRNA methylase que le foie adulte.
This investigation was supported in part by an allocation from a Public Health Service General Research Support Grant No. SO 1 FR-05545-03, and an American Cancer Society Grant No. IN 19F to The Jackson Laboratory, Bar Harbor, Maine, and by Public Health Service Research Grants No. HD-01496 from the National Institute of Child Health and Human Development and No. FR-00251 from the Division of Research Facilities and Resources. 相似文献
This investigation was supported in part by an allocation from a Public Health Service General Research Support Grant No. SO 1 FR-05545-03, and an American Cancer Society Grant No. IN 19F to The Jackson Laboratory, Bar Harbor, Maine, and by Public Health Service Research Grants No. HD-01496 from the National Institute of Child Health and Human Development and No. FR-00251 from the Division of Research Facilities and Resources. 相似文献
75.
Summary Triethyltin (TET) and acetazolamide (ACTZ) both produce marked anticonvulsant effects as evaluated by the maximal electroshock seizure test. However, the mechanism responsible for the anticonvulsant effect of TET and ACTZ is probably not the same since TET does not inhibit the activity of carbonic anhydrase isozyme C in vitro.This study was supported by NIEHS Toxicology Training grant No. ES07 090, NIOSH grant No. OH07085 and a University of Texas Biomedical Research grant. We wish to thank Rebecca Crenshaw for her excellent technical assistance. 相似文献
76.
M W Fox 《Experientia》1966,22(2):111-112
77.
78.
Zusammenfassung
Dendraster laevis bildet ein grünes, wasserlösliches Pigment (farblos bei pH unter 5) und kein Echinochrom.Dendraster excentricus dagegen erzeugt zwei purpurrote, wasserlösliche und zwei echinochromähnliche Pigmente.
Contribution from the Scripps Institution of Oceanography, University of California, New Series No 773. 相似文献
Contribution from the Scripps Institution of Oceanography, University of California, New Series No 773. 相似文献
79.
80.
Proline isomerism in staphylococcal nuclease characterized by NMR and site-directed mutagenesis 总被引:2,自引:0,他引:2
Nuclear magnetic resonance (NMR) studies have shown that two distinct folded conformations of staphylococcal nuclease coexist in solution and that these two states can interconvert directly without passing through an unfolded state. These experiments have also revealed that the two forms have very different folding kinetics, although the possibility that one component is an obligatory intermediate for the folding of the other form could be discounted. Here we report NMR data which show that alternative unfolded states are also distinguishable. These observations led us to hypothesize that cis/trans isomerism at a single peptide bond between a proline and its preceding residue might be the origin of the conformational multiplicity. Proline 117 was identified as a likely candidate for the site concerned and a mutant protein, in which Pro 117 was replaced by Gly, was constructed in order to test this. Alternative conformations are not observed in the spectrum of this mutant, lending powerful support to this hypothesis. 相似文献