全文获取类型
收费全文 | 8448篇 |
免费 | 793篇 |
国内免费 | 3篇 |
专业分类
9244篇 |
出版年
2023年 | 61篇 |
2022年 | 118篇 |
2021年 | 271篇 |
2020年 | 150篇 |
2019年 | 229篇 |
2018年 | 275篇 |
2017年 | 216篇 |
2016年 | 341篇 |
2015年 | 543篇 |
2014年 | 522篇 |
2013年 | 592篇 |
2012年 | 706篇 |
2011年 | 678篇 |
2010年 | 429篇 |
2009年 | 344篇 |
2008年 | 486篇 |
2007年 | 458篇 |
2006年 | 405篇 |
2005年 | 360篇 |
2004年 | 333篇 |
2003年 | 336篇 |
2002年 | 282篇 |
2001年 | 102篇 |
2000年 | 104篇 |
1999年 | 98篇 |
1998年 | 83篇 |
1997年 | 42篇 |
1996年 | 39篇 |
1995年 | 34篇 |
1994年 | 27篇 |
1993年 | 28篇 |
1992年 | 50篇 |
1991年 | 33篇 |
1990年 | 39篇 |
1989年 | 39篇 |
1988年 | 42篇 |
1987年 | 20篇 |
1986年 | 37篇 |
1985年 | 23篇 |
1984年 | 14篇 |
1983年 | 20篇 |
1982年 | 19篇 |
1981年 | 19篇 |
1980年 | 15篇 |
1979年 | 18篇 |
1978年 | 14篇 |
1977年 | 18篇 |
1974年 | 17篇 |
1973年 | 17篇 |
1967年 | 10篇 |
排序方式: 共有9244条查询结果,搜索用时 15 毫秒
61.
Escherichia coli RNA polymerase associated with the sigma54 factor (RNAP*sigma54) is a holoenzyme form that transcribes a special class of promoters not recognized by the standard RNA polymerase*sigma70 com plex. Promoters for RNAP*sigma54 vary in their overall 'strength' and show differences in their response to the presence of DNA curvature between enhancer and promoter. In order to examine whether these effects are related to the promoter affinity, we have determined the equilibrium dissociation constant K(d) for the binding of RNAP*sigma54 to the three promoters glnAp2, nifH and nifL. Binding studies were conducted by monitoring the changes in fluorescence anisotropy upon titrating RNAP*sigma54 to carboxyrhodamine-labeled DNA duplexes. For the glnAp2 and nifH promoters similar values of K(d) = 0.94 +/- 0.55 nM and K(d) = 0.85 +/- 0.30 nM were determined at physiological ionic strength, while the nifL promoter displayed a significantly weaker affinity with K(d) = 8.5 +/- 1.9 nM. The logarithmic dependence of K(d) on the ionic strength I was -Deltalog(K(d))/Deltalog(I) = 6.1 +/- 0.5 for the glnAp2, 5.2 +/- 1.2 for the nifH and 2.1 +/- 0.1 for the nifL promoter. This suggests that the polymerase can form fewer ion pairs with the nifL promoter, which would account for its weaker binding affinity. 相似文献
62.
63.
Pragl B Koschak A Trieb M Obermair G Kaufmann WA Gerster U Blanc E Hahn C Prinz H Schütz G Darbon H Gruber HJ Knaus HG 《Bioconjugate chemistry》2002,13(3):416-425
Hongotoxin(1) (HgTX(1)), a 39-residue peptide recently isolated from the venom of Centruroides limbatus, blocks the voltage-gated K+ channels K(v)1.1, K(v)1.2, and K(v)1.3 at picomolar toxin concentrations (Koschak, A., Bugianesi, R. M., Mitterdorfer, J., Kaczorowski, G. J., Garcia, M. L., and Knaus, H. G. (1998) J. Biol. Chem. 273, 2639-2644). In this report, we determine the three-dimensional structure of HgTX(1) using NMR spectroscopy (PDB-code: 1HLY). HgTX(1) was found to possess a structure similar to previously characterized K+ channel toxins (e.g. margatoxin) consisting of a three-stranded antiparallel beta-sheet (residues 2-4, 26-30, and 33-37) and a helical conformation (part 3(10) helix and part alpha helix; residues 10-20). Due to the importance of residue Lys-28 for high-affinity interaction with the respective channels, lysine-reactive fluorescence dyes cannot be used to label wild-type HgTX(1). On the basis of previous studies (see above) and our NMR data, a HgTX(1) mutant (HgTX(1)-A19C) was engineered, expressed, and purified. HgTX(1)-A19C-SH was labeled using sulfhydryl-reactive Cy3-, Cy5-, and Alexa-dyes. Pharmacological characterization of fluorescently labeled HgTX(1)-A19C in radioligand binding studies indicated that these hongotoxin(1) analogues retain high-affinity for voltage-gated K+ channels and a respective pharmacological profile. Cy3- and Alexa-dye-labeled hongotoxin(1) analogues were used to investigate the localization of K+ channels in brain sections. The distribution of toxin binding closely follows the distribution of K(v)1.2 immunoreactivity with the highest expression levels in the cerebellar Purkinje cell layer. Taken together, these results demonstrate that fluorescently labeled HgTX(1) analogues comprise novel probes to characterize a subset of voltage-gated K+ channels. 相似文献
64.
65.
66.
Yadvinder Malhi Timothy R. Baker Oliver L. Phillips Samuel Almeida Esteban Alvarez Luzmilla Arroyo Jerome Chave Claudia I. Czimczik Anthony Di Fiore Niro Higuchi Timothy J. Killeen Susan G. Laurance William F. Laurance Simon L. Lewis Lina María Mercado Montoya Abel Monteagudo David A. Neill Percy Núez Vargas Sandra Patio Nigel C.A. Pitman Carlos Alberto Quesada Rafael Salomo Jos Natalino Macedo Silva Armando Torres Lezama Rodolfo Vsquez Martínez John Terborgh Barbara Vinceti Jon Lloyd 《Global Change Biology》2004,10(5):563-591
The net primary production of tropical forests and its partitioning between long‐lived carbon pools (wood) and shorter‐lived pools (leaves, fine roots) are of considerable importance in the global carbon cycle. However, these terms have only been studied at a handful of field sites, and with no consistent calculation methodology. Here we calculate above‐ground coarse wood carbon productivity for 104 forest plots in lowland New World humid tropical forests, using a consistent calculation methodology that incorporates corrections for spatial variations in tree‐size distributions and wood density, and for census interval length. Mean wood density is found to be lower in more productive forests. We estimate that above‐ground coarse wood productivity varies by more than a factor of three (between 1.5 and 5.5 Mg C ha?1 a?1) across the Neotropical plots, with a mean value of 3.1 Mg C ha?1 a?1. There appear to be no obvious relationships between wood productivity and rainfall, dry season length or sunshine, but there is some hint of increased productivity at lower temperatures. There is, however, also strong evidence for a positive relationship between wood productivity and soil fertility. Fertile soils tend to become more common towards the Andes and at slightly higher than average elevations, so the apparent temperature/productivity relationship is probably not a direct one. Coarse wood productivity accounts for only a fraction of overall tropical forest net primary productivity, but the available data indicate that it is approximately proportional to total above‐ground productivity. We speculate that the large variation in wood productivity is unlikely to directly imply an equivalent variation in gross primary production. Instead a shifting balance in carbon allocation between respiration, wood carbon and fine root production seems the more likely explanation. 相似文献
67.
Sonya C. Bardswell Friederike Cuello Alexandra J. Rowland Sakthivel Sadayappan Jeffrey Robbins Mathias Gautel Jeffery W. Walker Jonathan C. Kentish Metin Avkiran 《The Journal of biological chemistry》2010,285(8):5674-5682
Protein kinase D (PKD), a serine/threonine kinase with emerging cardiovascular functions, phosphorylates cardiac troponin I (cTnI) at Ser22/Ser23, reduces myofilament Ca2+ sensitivity, and accelerates cross-bridge cycle kinetics. Whether PKD regulates cardiac myofilament function entirely through cTnI phosphorylation at Ser22/Ser23 remains to be established. To determine the role of cTnI phosphorylation at Ser22/Ser23 in PKD-mediated regulation of cardiac myofilament function, we used transgenic mice that express cTnI in which Ser22/Ser23 are substituted by nonphosphorylatable Ala (cTnI-Ala2). In skinned myocardium from wild-type (WT) mice, PKD increased cTnI phosphorylation at Ser22/Ser23 and decreased the Ca2+ sensitivity of force. In contrast, PKD had no effect on the Ca2+ sensitivity of force in myocardium from cTnI-Ala2 mice, in which Ser22/Ser23 were unavailable for phosphorylation. Surprisingly, PKD accelerated cross-bridge cycle kinetics similarly in myocardium from WT and cTnI-Ala2 mice. Because cardiac myosin-binding protein C (cMyBP-C) phosphorylation underlies cAMP-dependent protein kinase (PKA)-mediated acceleration of cross-bridge cycle kinetics, we explored whether PKD phosphorylates cMyBP-C at its PKA sites, using recombinant C1C2 fragments with or without site-specific Ser/Ala substitutions. Kinase assays confirmed that PKA phosphorylates Ser273, Ser282, and Ser302, and revealed that PKD phosphorylates only Ser302. Furthermore, PKD phosphorylated Ser302 selectively and to a similar extent in native cMyBP-C of skinned myocardium from WT and cTnI-Ala2 mice, and this phosphorylation occurred throughout the C-zones of sarcomeric A-bands. In conclusion, PKD reduces myofilament Ca2+ sensitivity through cTnI phosphorylation at Ser22/Ser23 but accelerates cross-bridge cycle kinetics by a distinct mechanism. PKD phosphorylates cMyBP-C at Ser302, which may mediate the latter effect. 相似文献
68.
Alexandra S. Solovyova Jonathan A. Pointon Paul R. Race Wendy D. Smith Michael A. Kehoe Mark J. Banfield 《European biophysics journal : EBJ》2010,39(3):469-480
Adhesion of the serotype M1 Streptococcus pyogenes strain SF370 to human tonsil explants and cultured keratinocytes requires extended polymeric surface structures called pili.
In this important human pathogen, pili are assembled from three protein subunits: Spy0125, Spy0128 and Spy0130 through the
action of sortase enzymes. For this study, the structural properties of these pili proteins have been investigated in solution.
Spy0125 and Spy0128 display characteristics of globular, folded proteins. Circular dichroism suggests a largely β-sheet composition
for Spy0128 and Spy0125; Spy0130 appears to contain little secondary structure. Each of the proteins adopts a monodisperse,
monomeric state in solution as assessed by analytical ultracentrifugation. Further, small-angle X-ray scattering curves for
Spy0125, Spy0128 and Spy0130 suggest each protein adopts an elongated shape, likely comprised of two domains, with similar
maximal dimensions. Based on the scattering data, dummy atom models of each of the pili subunits have been reconstructed ab
initio. This study provides the first insights into the structure of Streptococcus pyogenes minor pili subunits, and possible implications for protein function are discussed. 相似文献
69.
Coronavirus envelope (E) protein is a small integral membrane protein with multi-functions in virion assembly, morphogenesis and virus-host interaction. Different coronavirus E proteins share striking similarities in biochemical properties and biological functions, but seem to adopt distinct membrane topology. In this report, we study the membrane topology of the SARS-CoV E protein by immunofluorescent staining of cells differentially permeabilized with detergents and proteinase K protection assay. It was revealed that both the N- and C-termini of the SARS-CoV E protein are exposed to the cytoplasmic side of the membranes (N(cyto)C(cyto)). In contrast, parallel experiments showed that the E protein from infectious bronchitis virus (IBV) spanned the membranes once, with the N-terminus exposed luminally and the C-terminus exposed cytoplasmically (N(exo(lum)-)C(cyto)). Intriguingly, a minor proportion of the SARS-CoV E protein was found to be modified by N-linked glycosylation on Asn 66 and inserted into the membranes once with the C-terminus exposed to the luminal side. The presence of two distinct membrane topologies of the SARS-CoV E protein may provide a useful clue to the pathogenesis of SARS-CoV. 相似文献
70.
To investigate the effects of Procambarus clarkii on macroinvertebrate diversity, we conducted a mesocosm experiment simulating small pools in rice field pads after the rice
season. We hypothesized that crayfish predation would negatively impact macroinvertebrate diversity, and the magnitude of
this impact should vary with the size of P. clarkii. We conducted a short-term mesocosm experiment to determine macroinvertebrate diversity in the presence of three size classes
and in the absence of crayfish, as well as the diet composition of crayfish from the three size classes. At the end of the
experiments, the diet of crayfish was composed of the most available taxa (Culicidae, Chironomus, Tanytarsini and Orthocladinae). These results also show evidence that, in confined areas, crayfish are important predators
of major rice pests such as rice Chironominae larvae. Macroinvertebrate diversity was negatively affected by crayfish presence,
but the effect was inversely proportional to crayfish size. The highest diversity index was obtained in the absence of P. clarkii, and juvenile crayfish significantly reduced macroinvertebrate diversity. Thus, the impact of P. clarkii on aquatic macroinvertebrates is size dependent and may be relevant in small pools formed in rice field pads from early autumn
to late winter. Overall, our findings suggest that the negative effects of P. clarkii on macroinvertebrate diversity may be particularly strong in local natural assemblages confined to puddles of water or small
ponds in wetland areas. 相似文献