全文获取类型
收费全文 | 119248篇 |
免费 | 8369篇 |
国内免费 | 38篇 |
出版年
2023年 | 613篇 |
2022年 | 556篇 |
2021年 | 1261篇 |
2020年 | 1063篇 |
2019年 | 1150篇 |
2018年 | 2778篇 |
2017年 | 2498篇 |
2016年 | 3576篇 |
2015年 | 5313篇 |
2014年 | 5578篇 |
2013年 | 7324篇 |
2012年 | 8939篇 |
2011年 | 8409篇 |
2010年 | 5380篇 |
2009年 | 4091篇 |
2008年 | 6919篇 |
2007年 | 6850篇 |
2006年 | 6269篇 |
2005年 | 5864篇 |
2004年 | 5505篇 |
2003年 | 5141篇 |
2002年 | 4719篇 |
2001年 | 2291篇 |
2000年 | 2240篇 |
1999年 | 1990篇 |
1998年 | 982篇 |
1997年 | 728篇 |
1996年 | 642篇 |
1995年 | 690篇 |
1994年 | 656篇 |
1993年 | 545篇 |
1992年 | 1355篇 |
1991年 | 1243篇 |
1990年 | 1108篇 |
1989年 | 1028篇 |
1988年 | 971篇 |
1987年 | 828篇 |
1986年 | 749篇 |
1985年 | 833篇 |
1984年 | 737篇 |
1983年 | 612篇 |
1982年 | 492篇 |
1981年 | 469篇 |
1979年 | 615篇 |
1978年 | 477篇 |
1977年 | 438篇 |
1976年 | 407篇 |
1975年 | 447篇 |
1974年 | 485篇 |
1973年 | 479篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
861.
The Ca2+-binding properties of calmodulin purified from zucchini (Cucurbita pepo L.) has been determined. A value of 3.3 mol Ca2+ per mol of zucchini calmodulin was measured at pH 7.5 by equilibrium chromatography. The far-and near-UV circular-dichroic spectra of the Ca2+-and Mg2+-saturated as well as from the metal-free forms of zucchini calmodulin reveal that upon Ca2+-binding the -helix content increases. A comparison with the spectra of vertebrate calmodulin indicates that both calmodulin have a similar secondary structure, similar Ca2+-induced conformational changes and the same number of Ca2+-binding sites.Abbreviations CAPP
10-(3-aminopropyl)-2-chloro-phenothiazine
- EGTA
ethylene glycol-bis(-aminoethyl ether)-N,N,N,N-tetraacetic acid
- EDTA
ethylenediaminetetraacetic acid
Dedicated to Prof. Dr. Karl Decker on the occasion of his 60th birthday 相似文献
862.
The distribution of the two glutamate-synthase (GOGAT) activities known to exist in higher plants (NADH dependent, EC 2.6.1.53; and ferredoxin dependent, EC 1.4.7.1) was studied in non-chlorophyllous and chlorophyllous cultured tissue as well as in young leaves of Bouvardia ternifolia. The NADH-GOGAT was present in all three tissues. Using a sucrose gradient we found it in both the soluble and the plastid fraction of non-chlorophyllous and chlorophyllous tissue, but exclusively in the chloroplast fraction of the leaves. Ferredoxin-GOGAT was found only in green tissues and was confined to the chloroplasts. Ferredoxin-GOGAT activity increased in parallel with the chlorophyll content of the callus during the greening process in Murashige-Skoog medium (nitrate and ammonium as the nitrogen sources), while NADH-GOGAT was not affected by the greening process in this medium. Furthermore, both activities were differentially affected by either nitrate or ammonium as the sole nitrogen source in the medium during this process. It is suggested that each GOGAT activity is a different entity or is differently regulated.Abbreviations GOGAT
glutamate synthase
- MS
Murashige-Skoog (1962) medium
- PMSF
phenylmethylsulfonyl fluoride 相似文献
863.
864.
Isolation and partial characterization of the messenger RNA encoding the B880 holochrome protein of Rhodospirillum rubrum 总被引:2,自引:0,他引:2
The B880 holochrome messenger RNA was extracted from cultures of the photosynthetic bacterium Rhodospirillum rubrum. It was purified by chromatography on Sepharose 4B followed by sucrose density gradient centrifugation. The purified fractions were shown to program an Escherichia coli cell-free system into synthesizing both the alpha and the beta polypeptides of the holochrome. The translation products were identified by immunoprecipitation with specific antibodies raised against these polypeptides. The latter are effective competitors with the translation products for antigen-antibody complex formation. The purest mRNA preparations contained approximately 33% holochrome messenger RNA activity. Its most probable size, as determined by agarose gel electrophoresis in the presence of 6 M urea or methylmercuric hydroxide, is approximately 620 nucleotides. Since the combined sizes of the alpha and beta polypeptides add up to only 106 amino acid residues, we conclude that the holochrome mRNA is most probably polycistronic. 相似文献
865.
Isolation and structure elucidation of an alpha-amylase inhibitor, AI-3688, from Streptomyces aureofaciens 总被引:2,自引:0,他引:2
A novel polypeptide inhibitor, AI-3688, which acts upon human pancreatic alpha-amylase, was isolated from fermentation broth of Streptomyces aureofaciens. The purified peptide contains no unusual amino acids. Its Mr is 3936. The primary structure of AI-3688 was elucidated by automatic Edman degradation of the native or modified inhibitor. Two intramolecular cysteines form a disulphide bridge, thus creating a ring structure consisting of 17 amino acids. Strong sequence homology also exists to another microbial alpha-amylase inhibitor, tendamistat (HOE 467). This paper discusses the role of a common partial sequence, -Gln-Ser-Trp-Arg-Tyr-, present in the loop of both inhibitors as the active site of microbial peptide alpha-amylase inhibitors. 相似文献
866.
Polyphosphoinositides were quantified in metaphase II-arrested eggs of the amphibian Xenopus laevis and 8-10 min later in eggs activated by pricking. The content of phosphatidylinositol 4,5-biphosphate (PIP2) was remarkably high in metaphase II-arrested eggs with respect to that of phosphatidylinositol 4-phosphate (PIP). It was found to drop dramatically at activation. In contrast PIP content did not change significantly. 相似文献
867.
868.
Cellular receptors for type beta transforming growth factor. Ligand binding and affinity labeling in human and rodent cell lines 总被引:22,自引:0,他引:22
Type beta transforming growth factor (beta TGF) purified from human platelets to homogeneity as judged by NH2-terminal amino acid sequence analysis has been labeled with 125I to characterize its interaction with cellular receptors. Binding of 125I-beta TGF to target cells is temperature- and time-dependent, specific, saturable, and reversible. About 1.6-1.9 X 10(4) binding sites/cell with high affinity for beta TGF (Kd = 5.6-7.8 X 10(-11) M and 9.1-14 X 10(-11) M, respectively) are found in NRK-49F and BALB/c 3T3 cells. beta TGF receptors do not appear to undergo acute down-regulation by the ligand. Specific binding of 125I-beta TGF has been observed in several human, rat, and mouse fibroblast lines and in some, but not all, tumor-derived cell lines examined. 125I-beta TGF has been cross-linked to intact cells and isolated membrane preparations using disuccinimidyl suberate. Cells and isolated membranes from human, rat, and mouse origin affinity labeled with 125I-beta TGF exhibit a major labeled species of approximately 280 kilodaltons that has the properties of high affinity and specificity expected from a physiologically relevant beta TGF receptor. Minor labeled species of 70-90 kilodaltons are also labeled by 125I-beta TGF, but they correspond to molecular species with low apparent affinity (Kd approximately 10(-8) M) for 125I-beta TGF. 相似文献
869.
Liquid and lyophilized samples of Lachesis muta venom were stored at different temperatures and for different periods of time, and analyzed by polyacrylamide gel electrophoresis (PAGE) and immunoelectrophoresis. Only slight variations were evident when three pools of freeze-dried venom, that had been kept at -30 degrees C for several months were compared with fresh venom. These results suggest that L. muta venom is not altered drastically when stored under these conditions. 相似文献
870.
The respiratory effects of naloxone (200 micrograms X kg-1) on dogs, was studied measuring the parameters breath rate, inspiratory volume, breath minute volume, inspiratory time, total time of cycle, inspiratory volume/inspiratory time ratio, and inspiratory time/total time of cycle ratio. The statistical study was made by using the analysis of variance. Naloxone increases all the parameters studied as well as the response to CO2 while it decreases inspiratory time. The results suggest that there are opioid neurons in the respiratory centre. The blockade of the opioid receptors by naloxone almost explains the respiratory effects observed. 相似文献