全文获取类型
收费全文 | 5048篇 |
免费 | 595篇 |
专业分类
5643篇 |
出版年
2023年 | 28篇 |
2022年 | 55篇 |
2021年 | 104篇 |
2020年 | 34篇 |
2019年 | 67篇 |
2018年 | 86篇 |
2017年 | 60篇 |
2016年 | 155篇 |
2015年 | 243篇 |
2014年 | 301篇 |
2013年 | 325篇 |
2012年 | 442篇 |
2011年 | 378篇 |
2010年 | 285篇 |
2009年 | 203篇 |
2008年 | 285篇 |
2007年 | 262篇 |
2006年 | 231篇 |
2005年 | 224篇 |
2004年 | 221篇 |
2003年 | 193篇 |
2002年 | 206篇 |
2001年 | 112篇 |
2000年 | 85篇 |
1999年 | 74篇 |
1998年 | 53篇 |
1997年 | 40篇 |
1996年 | 28篇 |
1995年 | 38篇 |
1994年 | 40篇 |
1993年 | 43篇 |
1992年 | 70篇 |
1991年 | 59篇 |
1990年 | 43篇 |
1989年 | 53篇 |
1988年 | 48篇 |
1987年 | 41篇 |
1986年 | 22篇 |
1985年 | 26篇 |
1984年 | 24篇 |
1983年 | 27篇 |
1982年 | 24篇 |
1980年 | 19篇 |
1979年 | 22篇 |
1978年 | 22篇 |
1977年 | 22篇 |
1974年 | 22篇 |
1973年 | 18篇 |
1972年 | 17篇 |
1970年 | 21篇 |
排序方式: 共有5643条查询结果,搜索用时 15 毫秒
71.
72.
Oxidoreductases Involved in Cell Carbon Synthesis of Methanobacterium thermoautotrophicum 总被引:35,自引:23,他引:12 下载免费PDF全文
Cell-free extracts of Methanobacterium thermoautotrophicum were found to contain high activities of the following oxidoreductases (at 60°C): pyruvate dehydrogenase (coenzyme A acetylating), 275 nmol/min per mg of protein; α-ketoglutarate dehydrogenase (coenzyme A acylating), 100 nmol/min per mg; fumarate reductase, 360 nmol/min per mg; malate dehydrogenase, 240 nmol/min per mg; and glyceraldehyde-3-phosphate dehydrogenase, 100 nmol/min per mg. The kinetic properties (apparent Vmax and KM values), pH optimum, temperature dependence of the rate, and specificity for electron acceptors/donors of the different oxidoreductases were examined. Pyruvate dehydrogenase and α-ketoglutarate dehydrogenase were shown to be two separate enzymes specific for factor 420 rather than for nicotinamide adenine dinucleotide (NAD), NADP, or ferredoxin as the electron acceptor. Both activities catalyzed the reduction of methyl viologen with the respective α-ketoacid and a coenzyme A-dependent exchange between the carboxyl group of the α-ketoacid and CO2. The data indicate that the two enzymes are similar to pyruvate synthase and α-ketoglutarate synthase, respectively. Fumarate reductase was found in the soluble cell fraction. This enzyme activity coupled with reduced benzyl viologen as the electron donor, but reduced factor 420, NADH, or NADPH was not effective. The cells did not contain menaquinone, thus excluding this compound as the physiological electron donor for fumarate reduction. NAD was the preferred coenzyme for malate dehydrogenase, whereas NADP was preferred for glyceraldehyde-3-phosphate dehydrogenase. The organism also possessed a factor 420-dependent hydrogenase and a factor 420-linked NADP reductase. The involvement of the described oxidoreductases in cell carbon synthesis is discussed. 相似文献
73.
The nucleoside triphosphate pools of Escherichia coli minicells are different from those in parental cells. The growth phase in which minicells accumulate significantly affects the pool sizes. 相似文献
74.
L Fuchs T Bintein P Laget 《Comptes rendus des séances de la Société de biologie et de ses filiales》1979,173(1):36-41
In the chick, as soon as hatching, the responses recorded in various areas in the "accessorius hyperstriatum" (Wulst) seem to be very localized in relation with the stimulated region of the contralateral tectum. Very similar results are obtained within the other striate structures. These observations suggest in first part, that the tectal efferents project very diffusely to various striate structures and, in other part, that it exists in these a rather precise tectotopy and a columnar organisation. These results appear very constant during the first month of the hatching. 相似文献
75.
A simple double-isotope procedure has been developed for making simultaneous measurements of bound Ca2+ and relative force in glycerinated rabbit psoas bundles containing two fibers. With this preparation it is possible to study Ca2+-troponin interactions coincident with MgATP-induced force development. Over the free [Ca2+] range 6 · 10?8–1.2 · 10?5 M the bound Ca2+ varied from 0.25 to 1.65 μmol/g protein. The free [Ca2+] at half-maximal Ca2+ saturation was 2 · 10?7 M while that a half-maximal force was 5 · 10?7 M. Half-maximal Ca2+ saturation was associated with 20% maximal force. The force-[Ca2+] saturation curve showed a steep rise in slope at greater than half saturation. The observed relationship was consistent with a model in which multiple occupancy of troponin Ca2+-binding sites is essential for initiation of cross-bridge cycling. 相似文献
76.
77.
J Moreno D A Vignali F Nadimi S Fuchs L Adorini G J H?mmerling 《Journal of immunology (Baltimore, Md. : 1950)》1991,147(10):3306-3313
Class II MHC molecules on the surface of an APC present immunogenic peptides derived mainly from exogenous proteins to CD4+ T cells. During its transport to the cell surface, class II molecules intersect the endocytic pathway where they acquire peptides derived from endocytosed proteins. However, class II-restricted presentation of endogenously derived peptides can also occur. The current studies were undertaken to examine the ability of different types of APC to generate and present four different T cell determinants derived from an endogenous, nonsecreted, truncated form of hen-egg white lysozyme (HEL[1-80]-Kk). This was compared with the ability of these APC to generate the same determinants from exogenous HEL. All the peptides derived from endogenous HEL[1-80]-Kk tested, were presented by B cells to HEL-specific T cell hybridomas with an efficiency similar to presentation of the same determinants from exogenous HEL. In contrast, an I-Ak-bearing rat fibroblast was unable to generate the HEL peptide 25-43 from exogenous HEL, but could efficiently produce it from endogenous HEL[1-80]-Kk. The results indicate first, that peptides derived from an endogenous Ag can be presented by MHC class II molecules with an efficiency comparable to that of the presentation of the exogenous Ag. Second, that Ag-presenting B cells can generate the same repertoire of antigenic peptides from endogenous Ag as those generated from the exogenous protein. And third, that in contrast to B cells, certain "nonprofessional" APC can generate, from an endogenous protein, T cell determinants distinct from those generated after endocytosis of the exogenous protein. These results suggest that processing of exogenous and endogenous Ag by different APC take place in different intracellular compartments. 相似文献
78.
J Leighton A Sette J Sidney E Appella C Ehrhardt S Fuchs L Adorini 《Journal of immunology (Baltimore, Md. : 1950)》1991,147(1):198-204
We have analyzed the interaction of the hen egg-white lysozyme (HEL) peptide 107-116 with the MHC class II molecule I-Ek, using truncated and single residue substitution analogues to measure activation of I-Ek-restricted, 107-116-specific T cell hybridomas and competition for Ag presentation by I-Ek molecules. These results have been compared with previous findings on the interaction of the same peptide with the I-Ed molecule. Stimulation of T cell hybridomas by truncated peptides defines the sequence 108-116 as the minimum epitope necessary for activation of both I-Ek- and I-Ed-restricted T cell hybridomas. Substitution analysis pinpoints three residues (V109, A110, and K116) in the sequence 108-116 as being critical for binding to I-Ek molecules and demonstrates the involvement of most other residues in recognition by T cells. Results previously obtained for binding of HEL 107-116 to I-Ed molecules indicated that peptide residues R112, R114, and K116 were critical for interaction with I-Ed. Comparison of these results indicates a difference in the likely MHC contact residues between the HEL sequence 108-116 and I-Ed or I-Ek molecules, suggesting that the same HEL peptide assumes a different conformation in the binding site of these two MHC molecules. This in turn affects residues interacting with the specific T cell receptor. According to the hypothetical tridimensional structure predicted for class II molecules, the difference in MHC contact residues observed within the sequence 108-116 can be related to polymorphic amino acids in the binding site of I-Ek and I-Ed molecules. A search through published binding data for a common pattern in this and other I-Ek-binding peptides has permitted us to derive a possible motif for predicting peptide binding to I-Ek molecules. This putative motif was tested by determining binding to I-Ek of an unbiased panel of about 150 synthetic peptides. Binding data indeed demonstrate the presence of this motif in the majority of good binders to I-Ek molecules. 相似文献
79.
P Sodano K V Chary O Bj?rnberg A Holmgren B Kren J A Fuchs K Wüthrich 《European journal of biochemistry》1991,200(2):369-377
Escherichia coli glutaredoxin (85 amino acid residues, Mr = 9100), the glutathione-dependent hydrogen donor for ribonucleotide reductase, was purified from an inducible lambda PL, expression system both with a natural isotope content and with uniform 15N labelling. This material was used for obtaining sequence-specific 1H magnetic resonance assignments and the identification of regular secondary structures in the oxidized form of the protein, which contains the redox-active disulfide Cys11-Pro-Tyr-Cys14. Oxidized glutaredoxin contains a four-stranded beta-sheet, with the peripheral strand 32-37 arranged parallel to the strand 2-7, which further combines with the two additional strands 61-64 and 67-69 in an antiparallel fashion. The protein further contains three helices extending approximately from residues 13-28, 45-54 and 72-84. 相似文献
80.