全文获取类型
收费全文 | 2461篇 |
免费 | 264篇 |
国内免费 | 5篇 |
出版年
2022年 | 25篇 |
2021年 | 49篇 |
2020年 | 19篇 |
2019年 | 18篇 |
2018年 | 53篇 |
2017年 | 27篇 |
2016年 | 49篇 |
2015年 | 109篇 |
2014年 | 104篇 |
2013年 | 142篇 |
2012年 | 154篇 |
2011年 | 153篇 |
2010年 | 96篇 |
2009年 | 105篇 |
2008年 | 139篇 |
2007年 | 121篇 |
2006年 | 113篇 |
2005年 | 110篇 |
2004年 | 129篇 |
2003年 | 119篇 |
2002年 | 109篇 |
2001年 | 45篇 |
2000年 | 48篇 |
1999年 | 33篇 |
1998年 | 41篇 |
1997年 | 33篇 |
1996年 | 20篇 |
1995年 | 21篇 |
1994年 | 21篇 |
1993年 | 22篇 |
1992年 | 21篇 |
1991年 | 25篇 |
1990年 | 24篇 |
1989年 | 18篇 |
1988年 | 23篇 |
1987年 | 20篇 |
1986年 | 16篇 |
1985年 | 23篇 |
1984年 | 19篇 |
1983年 | 14篇 |
1982年 | 19篇 |
1981年 | 24篇 |
1980年 | 17篇 |
1978年 | 24篇 |
1977年 | 16篇 |
1976年 | 19篇 |
1975年 | 11篇 |
1973年 | 11篇 |
1971年 | 15篇 |
1968年 | 12篇 |
排序方式: 共有2730条查询结果,搜索用时 47 毫秒
1.
Michael Donnelly 《BMJ (Clinical research ed.)》1953,1(4807):438-439
2.
3.
4.
5.
Synthesis of 7-mercaptoheptanoylthreonine phosphate and its activity in the methylcoenzyme M methylreductase system 总被引:9,自引:0,他引:9
The structure of component B of the methylcoenzyme M methylreductase of Methanobacterium thermoautotrophicum was recently assigned as 7-mercaptoheptanoylthreonine phosphate (HS-HTP) (Noll, K. M., Rinehart, K. L., Jr., Tanner, R.S., and Wolfe, R.S. (1986) (Proc. Natl. Acad. Sci. U.S.A. 83, 4238-4242). We report here the chemical synthesis and biochemical activity of this compound. Thiourea and 7-bromoheptanoic acid were used to to synthesize 7,7'-dithiodiheptanoic acid. This disulfide was then condensed with DL-threonine phosphate using N-hydroxysuccinimide and dicyclohexylcarbodiimide. The product was reduced with dithiothreitol to give HS-HTP. It could be oxidized in air in the presence of 2-mercaptoethanol to give the compound as it was isolated from cell extracts. The resulting product was identical to the authentic compound by 1H NMR spectroscopy, mass spectrometry, and coelution using high performance liquid chromatography. The synthetic compound is active in the in vitro methanogenic assay at concentrations comparable to the authentic compound. This confirms the structure of component B as HS-HTP and provides a means to synthesize quantities sufficient for studies of the methylreductase system. 相似文献
6.
Sheila Polakoff 《BMJ (Clinical research ed.)》1987,294(6578):1031-1032
7.
Isolation and expression of an anther-specific gene from tomato 总被引:31,自引:0,他引:31
David Twell Rod Wing Judy Yamaguchi Sheila McCormick 《Molecular & general genetics : MGG》1989,217(2-3):240-245
8.
Robert M. Levin Penelope A. Longhurst Sheila S. Levin Niels Haugaard Alan J. Wein 《Molecular and cellular biochemistry》1990,97(2):153-159
The urinary bladder depends on intracellular ATP for the support of a number of essential intracellular processes including contraction. The concentration of ATP is maintained constant primarily via the rapid transfer of a phosphate from creatine phosphate (CP) to ADP catalyzed by the enzyme creatine kinase (CK). Since muscular pathologies associated with diabetes are in part related to intracellular alterations in metabolism, we have characterized the CK activity in both skeletal muscle and urinary bladder from control and streptozotocin-diabetic rats.The following is a summary of the results: 1) Bladder tissue from control rats showed linear kinetics with a Vmax = 390 nmoles/mg protein/min, and a Km = 275 µM. 2) Urinary bladder tissue isolated from diabetic rats displayed biphasic kinetics with Vmax = 65 and 324 nmoles/mg protein/min, and Km's = 10 µM and 190 µM respectively. 3) Skeletal muscle isolated from control rats showed linear kinetics with an approximate Vmax of 800 nmoles/mg protein/min and a Km of 280 µM CP. 4) Homogenates of skeletal muscle from diabetic rats showed complex kinetics not separable into distict component forms. 5) The Km for ADP for both skeletal muscle and bladder was approximately 10 µM.These studies demonstrate that whereas bladders isolated from both control and diabetic rats possess a low-affinity isomer(s) of CK with similar maximum enzymatic activity, there is a high affinity isomer present within the urinary bladder muscle of diabetic rats that is not present in bladder tissue isolated from control rats. Skeletal muscle isolated from both diabetic and control rats exhibited a maximal activity 2 to 3 times higher than that of the bladder. 相似文献
9.
Sallie W. Chisholm Sheila L. Frankel Ralf Goericke Robert J. Olson Brian Palenik John B. Waterbury Lisa West-Johnsrud Erik R. Zettler 《Archives of microbiology》1992,157(3):297-300
Several years ago, prochlorophyte picoplankton were discovered in the N. Atlantic. They have since been found to be abundant within the euphotic zone of the world's tropical and temperate oceans. The cells are extremely small, lack phycobiliproteins, and contain divinyl chlorophyll a and b as their primary photosynthetic pigments. Phylogenies constructed from DNA sequence data indicate that these cells are more closely related to a cluster of marine cyanobacteria than to their prochlorophyte relatives Prochlorothrix and Prochloron. Several strains of this organism have recently been brought into culture, and herewith are given the name Prochlorococcus marinus. 相似文献
10.
Environment-specific amino acid substitution tables: tertiary templates and prediction of protein folds. 总被引:4,自引:0,他引:4 下载免费PDF全文
J Overington D Donnelly M S Johnson A Sali T L Blundell 《Protein science : a publication of the Protein Society》1992,1(2):216-226
The local environment of an amino acid in a folded protein determines the acceptability of mutations at that position. In order to characterize and quantify these structural constraints, we have made a comparative analysis of families of homologous proteins. Residues in each structure are classified according to amino acid type, secondary structure, accessibility of the side chain, and existence of hydrogen bonds from the side chains. Analysis of the pattern of observed substitutions as a function of local environment shows that there are distinct patterns, especially for buried polar residues. The substitution data tables are available on diskette with Protein Science. Given the fold of a protein, one is able to predict sequences compatible with the fold (profiles or templates) and potentially to discriminate between a correctly folded and misfolded protein. Conversely, analysis of residue variation across a family of aligned sequences in terms of substitution profiles can allow prediction of secondary structure or tertiary environment. 相似文献