全文获取类型
收费全文 | 12731篇 |
免费 | 1179篇 |
国内免费 | 1846篇 |
出版年
2024年 | 51篇 |
2023年 | 221篇 |
2022年 | 523篇 |
2021年 | 765篇 |
2020年 | 570篇 |
2019年 | 615篇 |
2018年 | 620篇 |
2017年 | 465篇 |
2016年 | 596篇 |
2015年 | 950篇 |
2014年 | 1016篇 |
2013年 | 1104篇 |
2012年 | 1316篇 |
2011年 | 1151篇 |
2010年 | 637篇 |
2009年 | 655篇 |
2008年 | 687篇 |
2007年 | 628篇 |
2006年 | 593篇 |
2005年 | 479篇 |
2004年 | 352篇 |
2003年 | 281篇 |
2002年 | 254篇 |
2001年 | 138篇 |
2000年 | 134篇 |
1999年 | 135篇 |
1998年 | 89篇 |
1997年 | 82篇 |
1996年 | 72篇 |
1995年 | 67篇 |
1994年 | 60篇 |
1993年 | 46篇 |
1992年 | 69篇 |
1991年 | 55篇 |
1990年 | 40篇 |
1989年 | 33篇 |
1988年 | 27篇 |
1987年 | 29篇 |
1986年 | 22篇 |
1985年 | 23篇 |
1984年 | 13篇 |
1983年 | 11篇 |
1982年 | 12篇 |
1981年 | 7篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1973年 | 5篇 |
1972年 | 4篇 |
1971年 | 6篇 |
1970年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
Metabolic labelling by [14C]palmitic acid showed that growth of Streptococcus mutans LT11 in raffinose, an inducer of the msm operon, resulted in increased production of a 45-kDa lipoprotein corresponding to MsmE, which is believed to be a sugar-binding protein. MsmE was also labelled when an msmE clone was expressed in Escherichia coli. The presence of a lipid anchor on MsmE provides a likely explanation of how the sugar-binding protein component of the msm binding protein-dependent multiple sugar transport system is retained at the cell surface. 相似文献
62.
Bradykinin (BK) is a peptide hormone with sequence Arg1-Pro2-Pro3-Gly4-Phe5-Ser6-Pro7-Phe8-Arg9 and has been implicated in a multitude of pathophysiological processes such as the ability to lower systemic blood pressure and stimulate pain. BK analogues having bulky, β-branched D -aliphatic residues at position 7 combined with bulky L -aliphatic residues at position 8 have now been observed to be strong antagonists. Conformational studies based on two-dimensional nmr experiments in methanol/water (80/20 v/v) were carried out on several such active antagonists in a polar solvent. Included in this study were the very active antagonists, [D -Arg0, Hyp3, Thi5, D -Cpg7, Cpg8]-BK [Cpg: α-cyclo-pentyl-glycine; Hyp: trans-4-hydroxy-L -proline; Thi: β-(2-thienyl)-L -alanine] ( I ), [D -Arg0, Hyp3, D -Cpg7, Cpg8] -BK ( II ), as well as its variant with D -Cpg7 replaced by Cpg7, namely [D -Arg0, Hyp3, Cpg7, Cpg8]-BK ( III ). A turn-like structure, which coexists with the extended conformation, was observed between residues 2 and 5 for the most active antagonists I and II , in direct correlation with the peptide activities. No turn-like structure was found for residues 6–9. In peptide III , a turn-like structure was not identified. The existence of a turn at the C-terminal end of bradykinin and its analogues has been predicted by empirical calculations and supported by nmr measurements. But the present nmr study on the most active antagonists ( I , II ) does not support this hypothesis. Instead, the data suggest that a turn-like structure between residues 2 and 5 could be important for antagonist activity. Finally, one weak inhibitor [D -Cpg7]-BK ( IV ) showed no defined secondary structure. © 1993 John Wiley & Sons, Inc. 相似文献
63.
Hindered diffusion of high molecular weight compounds in brain extracellular microenvironment measured with integrative optical imaging. 总被引:4,自引:0,他引:4 下载免费PDF全文
This paper describes the theory of an integrative optical imaging system and its application to the analysis of the diffusion of 3-, 10-, 40-, and 70-kDa fluorescent dextran molecules in agarose gel and brain extracellular microenvironment. The method uses a precisely defined source of fluorescent molecules pressure ejected from a micropipette, and a detailed theory of the intensity contributions from out-of-focus molecules in a three-dimensional medium to a two-dimensional image. Dextrans tagged with either tetramethylrhodamine or Texas Red were ejected into 0.3% agarose gel or rat cortical slices maintained in a perfused chamber at 34 degrees C and imaged using a compound epifluorescent microscope with a 10 x water-immersion objective. About 20 images were taken at 2-10-s intervals, recorded with a cooled CCD camera, then transferred to a 486 PC for quantitative analysis. The diffusion coefficient in agarose gel, D, and the apparent diffusion coefficient, D*, in brain tissue were determined by fitting an integral expression relating the measured two-dimensional image intensity to the theoretical three-dimensional dextran concentration. The measurements in dilute agarose gel provided a reference value of D and validated the method. Values of the tortuosity, lambda = (D/D*)1/2, for the 3- and 10-kDa dextrans were 1.70 and 1.63, respectively, which were consistent with previous values derived from tetramethylammonium measurements in cortex. Tortuosities for the 40- and 70-kDa dextrans had significantly larger values of 2.16 and 2.25, respectively. This suggests that the extracellular space may have local constrictions that hinder the diffusion of molecules above a critical size that lies in the range of many neurotrophic compounds. 相似文献
64.
本文对11例肺癌患者胸水13种游离氨基酸作了分析,并与28例正常人血浆游离氨基酸水平作了对照,结果表明:肺癌患者胸水的必需及非必需氨基酸普遍高于正常人血浆游离氨基酸,但其胸水谷氨酰胺水平则明显低于正常人血浆水平。 相似文献
65.
66.
厌氧氨氧化菌(anaerobic ammonia-oxidizing bacteria, AnAOB)的代谢多样性,使得该菌群能够在海洋、湿地和陆地等不同的自然生态系统中广泛分布,甚至在一些极热和极寒环境中也检测到了该菌群的存在。本文回顾并总结了厌氧氨氧化菌在不同生态系统中的发现、分布及脱氮贡献等方面的研究,分析了厌氧氨氧化菌分布的主要环境影响因素。该综述将帮助我们更好地理解全球氮循环中厌氧氨氧化菌的实际角色和功能,并基于厌氧氨氧化(anaerobicammoniaoxidation,anammox)过程,探究能与其进行协作的新型生物脱氮工艺,以期为这些工艺的研发和推广提供生态学基础和新的思考,从而实现脱氮工艺的技术变革。 相似文献
67.
蛋白分泌作为细胞之间传递信号的途径之一,在微生物生存竞争中也扮演着重要的角色。革兰氏阴性菌可以通过Ⅵ型分泌系统(type Ⅵ secretion system, T6SS)将效应蛋白传递至胞外或原核和真核微生物中,从而介导微生物间的竞争或宿主-细菌的相互作用,最终建立竞争优势。本文主要总结了T6SS的结构与组成,并重点对效应蛋白的装配以及其与免疫蛋白的作用机制的研究进展进行阐述,为以后靶向T6SS抗菌药物的研制提供新思路。 相似文献
68.
69.
Crevice-forming mutants in the rigid core of bovine pancreatic trypsin inhibitor: crystal structures of F22A, Y23A, N43G, and F45A. 总被引:4,自引:4,他引:0 下载免费PDF全文
A. T. Danishefsky D. Housset K. S. Kim F. Tao J. Fuchs C. Woodward A. Wlodawer 《Protein science : a publication of the Protein Society》1993,2(4):577-587
Crystal structures of four mutants of bovine pancreatic trypsin inhibitor (F22A, Y23A, N43G, and F45A), engineered to alter their stability properties, have been determined. The mutated residues, which are highly conserved among Kunitz-type inhibitors, are located in the rigid core of the molecule. Replacement of the partially buried bulky residues of the wild-type protein with smaller residues resulted in crevices open to the exterior of the molecule. The overall three-dimensional structure of these mutants is very similar to that of the wild-type protein and only small rearrangements are observed among the atoms lining the crevices. 相似文献
70.