全文获取类型
收费全文 | 20196篇 |
免费 | 1592篇 |
国内免费 | 11篇 |
专业分类
21799篇 |
出版年
2023年 | 81篇 |
2022年 | 195篇 |
2021年 | 361篇 |
2020年 | 219篇 |
2019年 | 310篇 |
2018年 | 373篇 |
2017年 | 338篇 |
2016年 | 548篇 |
2015年 | 928篇 |
2014年 | 1053篇 |
2013年 | 1316篇 |
2012年 | 1516篇 |
2011年 | 1418篇 |
2010年 | 939篇 |
2009年 | 866篇 |
2008年 | 1122篇 |
2007年 | 1056篇 |
2006年 | 1078篇 |
2005年 | 1047篇 |
2004年 | 983篇 |
2003年 | 858篇 |
2002年 | 904篇 |
2001年 | 254篇 |
2000年 | 203篇 |
1999年 | 241篇 |
1998年 | 269篇 |
1997年 | 199篇 |
1996年 | 221篇 |
1995年 | 198篇 |
1994年 | 214篇 |
1993年 | 205篇 |
1992年 | 183篇 |
1991年 | 144篇 |
1990年 | 120篇 |
1989年 | 133篇 |
1988年 | 132篇 |
1987年 | 94篇 |
1986年 | 92篇 |
1985年 | 114篇 |
1984年 | 139篇 |
1983年 | 101篇 |
1982年 | 131篇 |
1981年 | 103篇 |
1980年 | 98篇 |
1979年 | 82篇 |
1978年 | 77篇 |
1977年 | 72篇 |
1976年 | 48篇 |
1974年 | 48篇 |
1973年 | 56篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
José Antonio Fiz Raimon Jané Abel Torres Josep Morera Batxi Galdiz Joaquín Gea Alejandro Grassino 《BMC pulmonary medicine》2004,4(1):1-9
Background
The predictive role of many cytokines has not been well defined in Acute Respiratory Distress Syndrome (ARDS).Methods
We measured prospectively IL-4, IL-6, IL-6 receptor, IL-8, and IL-10, in the serum and bronchoalveolar lavage fluid (BALF) in 59 patients who were admitted to ICU in order to identify predictive factors for the course and outcome of ARDS. The patients were divided into three groups: those fulfilling the criteria for ARDS (n = 20, group A), those at risk for ARDS and developed ARDS within 48 hours (n = 12, group B), and those at risk for ARDS but never developed ARDS (n = 27, group C).Results
An excellent negative predictive value for ARDS development was found for IL-6 in BALF and serum (100% and 95%, respectively). IL-8 in BALF and IL-8 and IL-10 serum levels were higher in non-survivors in all studied groups, and were associated with a high negative predictive value. A significant correlation was found between IL-8 and APACHE score (r = 0.60, p < 0.0001). Similarly, IL-6 and IL-6r were highly correlated with PaO2/FiO2 (r = -0.27, p < 0.05 and r = -0.55, p < 0.0001, respectively).Conclusions
BALF and serum levels of the studied cytokines on admission may provide valuable information for ARDS development in patients at risk, and outcome in patients either in ARDS or in at risk for ARDS. 相似文献992.
Background
Streptomyces coelicolor is a bacterium with a vast repertoire of metabolic functions and complex systems of cellular development. Its genome sequence is rich in genes that encode regulatory proteins to control these processes in response to its changing environment. We wished to apply a recently published bioinformatic method for identifying novel regulatory sequence signals to gain new insights into regulation in S. coelicolor. 相似文献993.
994.
995.
Kienberger Ferry Zhu Rong Moser Rosita Rankl Christian Blaas Dieter Hinterdorfer Peter 《Biological procedures online》2004,6(1):120-128
Dynamic force microscopy (DFM) allows imaging of the structure and the assessment of the function of biological specimens
in their physiological environment. In DFM, the cantilever is oscillated at a given frequency and touches the sample only
at the end of its downward movement. Accordingly, the problem of lateral forces displacing or even destroying bio-molecules
is virtually inexistent as the contact time and friction forces are reduced. Here, we describe the use of DFM in studies of
human rhinovirus serotype 2 (HRV2) weakly adhering to mica surfaces. The capsid of HRV2 was reproducibly imaged without any
displacement of the virus. Release of the genomic RNA from the virions was initiated by exposure to low pH buffer and snapshots
of the extrusion process were obtained. In the following, the technical details of previous DFM investigations of HRV2 are
summarized.
Published: June 29, 2004. 相似文献
996.
Designing new materials from wheat protein 总被引:4,自引:0,他引:4
Woerdeman DL Veraverbeke WS Parnas RS Johnson D Delcour JA Verpoest I Plummer CJ 《Biomacromolecules》2004,5(4):1262-1269
We recently discovered that wheat gluten could be formed into a tough, plasticlike substance when thiol-terminated, star-branched molecules are incorporated directly into the protein structure. This discovery offers the exciting possibility of developing biodegradable high-performance engineering plastics and composites from renewable resources that are competitive with their synthetic counterparts. Wheat gluten powder is available at a cost of less than dollars 0.5/lb, so if processing costs can be controlled, an inexpensive alternative to synthetic polymers may be possible. In the present work, we demonstrate the ability to toughen an otherwise brittle protein-based material by increasing the yield stress and strain-to-failure, without compromising stiffness. Water absorption results suggest that the cross-link density of the polymer is increased by the presence of the thiol-terminated, star-branched additive in the protein. Size-exclusion high performance liquid chromatography data of molded tri-thiol-modified gluten are consistent with that of a polymer that has been further cross-linked when compared directly with unmodified gluten, handled under identical conditions. Remarkably, the mechanical properties of our gluten formulations stored in ambient conditions were found to improve with time. 相似文献
997.
For lintners with negligible amylose retrogradation, crystallinity related inversely to starch amylose content and, irrespective of starch source, incomplete removal of amorphous material was shown. The latter was more pronounced for B-type than for A-type starches. The two predominant lintner populations, with modal degrees of polymerization (DP) of 13-15 and 23-27, were best resolved for amylose-deficient and A-type starches. Results indicate a more specific hydrolysis of amorphous lamellae in such starches. Small-angle X-ray scattering showed a more intense 9-nm scattering peak for native amylose-deficient A-type starches than for their regular or B-type analogues. The experimental evidence indicates a lower contrasting density within the "crystalline" shells of the latter starches. A higher density in the amorphous lamellae, envisaged by the lamellar helical model, explains the relative acid resistance of linear amylopectin chains with DP > 20, observed in lintners of B-type starches. Because amylopectin chain length distributions were similar for regular and amylose-deficient starches of the same crystal type, we deduce that the more dense (and ordered) packing of double helices into lamellar structures in amylose-deficient starches is due to a different amylopectin branching pattern. 相似文献
998.
999.
Barinka C Sácha P Sklenár J Man P Bezouska K Slusher BS Konvalinka J 《Protein science : a publication of the Protein Society》2004,13(6):1627-1635
Glutamate carboxypeptidase II (GCPII) is a membrane peptidase expressed in the prostate, central and peripheral nervous system, kidney, small intestine, and tumor-associated neovasculature. The GCPII form expressed in the central nervous system, termed NAALADase, is responsible for the cleavage of N-acetyl-L-aspartyl-L-glutamate (NAAG) yielding free glutamate in the synaptic cleft, and is implicated in various pathologic conditions associated with glutamate excitotoxicity. The prostate form of GCPII, termed prostate-specific membrane antigen (PSMA), is up-regulated in cancer and used as an effective prostate cancer marker. Little is known about the structure of this important pharmaceutical target. As a type II membrane protein, GCPII is heavily glycosylated. In this paper we show that N-glycosylation is vital for proper folding and subsequent secretion of human GCPII. Analysis of the predicted N-glycosylation sites also provides evidence that these sites are critical for GCPII carboxypeptidase activity. We confirm that all predicted N-glycosylation sites are occupied by an oligosaccharide moiety and show that glycosylation at sites distant from the putative catalytic domain is critical for the NAAG-hydrolyzing activity of GCPII calling the validity of previously described structural models of GCPII into question. 相似文献
1000.
Giménez-Abián JF Sumara I Hirota T Hauf S Gerlich D de la Torre C Ellenberg J Peters JM 《Current biology : CB》2004,14(13):1187-1193
Sister chromatid separation in anaphase depends on the removal of cohesin complexes from chromosomes. In vertebrates, the bulk of cohesin is already removed from chromosome arms during prophase and prometaphase, whereas cohesin remains at centromeres until metaphase, when cohesin is cleaved by the protease separase. In unperturbed mitoses, arm cohesion nevertheless persists throughout metaphase and is principally sufficient to maintain sister chromatid cohesion. How arm cohesion is maintained until metaphase is unknown. Here we show that small amounts of cohesin can be detected in the interchromatid region of metaphase chromosome arms. If prometaphase is prolonged by treatment of cells with microtubule poisons, these cohesin complexes dissociate from chromosome arms, and arm cohesion is dissolved. If cohesin dissociation in prometaphase-arrested cells is prevented by depletion of Plk1 or inhibition of Aurora B, arm cohesion is maintained. These observations imply that, in unperturbed mitoses, small amounts of cohesin maintain arm cohesion until metaphase. When cells lacking Plk1 and Aurora B activity enter anaphase, chromatids lose cohesin. This loss is prevented by proteasome inhibitors, implying that it depends on separase activation. Separase may therefore be able to cleave cohesin at centromeres and on chromosome arms. 相似文献