全文获取类型
收费全文 | 18945篇 |
免费 | 1645篇 |
国内免费 | 1693篇 |
出版年
2024年 | 39篇 |
2023年 | 193篇 |
2022年 | 442篇 |
2021年 | 747篇 |
2020年 | 574篇 |
2019年 | 768篇 |
2018年 | 770篇 |
2017年 | 579篇 |
2016年 | 818篇 |
2015年 | 1226篇 |
2014年 | 1477篇 |
2013年 | 1461篇 |
2012年 | 1832篇 |
2011年 | 1719篇 |
2010年 | 1108篇 |
2009年 | 961篇 |
2008年 | 1219篇 |
2007年 | 1122篇 |
2006年 | 893篇 |
2005年 | 799篇 |
2004年 | 695篇 |
2003年 | 604篇 |
2002年 | 570篇 |
2001年 | 297篇 |
2000年 | 260篇 |
1999年 | 227篇 |
1998年 | 153篇 |
1997年 | 114篇 |
1996年 | 75篇 |
1995年 | 59篇 |
1994年 | 58篇 |
1993年 | 37篇 |
1992年 | 58篇 |
1991年 | 45篇 |
1990年 | 43篇 |
1989年 | 35篇 |
1988年 | 26篇 |
1987年 | 19篇 |
1986年 | 20篇 |
1985年 | 20篇 |
1984年 | 13篇 |
1983年 | 11篇 |
1982年 | 11篇 |
1979年 | 13篇 |
1978年 | 6篇 |
1976年 | 5篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1970年 | 4篇 |
1968年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Two soluble serine proteases Do and So from Escherichia coli were found to distinctively cleave the purified, 39 kDa Ada protein into fragments with sizes of 12-31 kDa. Protease So appears to generate a C-terminal 19 kDa polypeptide, similarly to OmpT protease. In addition, the purified 19 kDa C-terminal half of Ada protein can be further processed mainly to an 18 kDa fragment by protease So and to a 12 kDa by protease Do. These results suggest that proteases Do and So are involved in endogenous cleavage of Ada protein, which may play a role in down-regulating the adaptive response to alkylating agents. 相似文献
112.
113.
The complex sterol mixture isolated from was found to contain a low level of Δ4-3-keto steroids, 5β-stanols and 4α-methyl sterols in addition to regular (4-demethyl) sterols. The following new marine sterols were isolated and identified using MS and 360 MHz NMR: 5β-cholest-22E-en-3β-ol, 24S-methyl-5β-cholest-22E-en-3β-ol, 24-methylene-5β-cholestan-3β-ol, both epimers at C-24 of 4α-methyl-24-ethyl-5α-cholest-22E-en-3β-ol, 4α, 22ξ, 23ξ-(or 24ξ-)trimethyl-5α-cholest-8(14)-en-3β-ol and (22S, 23S, 24S)-4α-24-dimethyl-22, 23-methylene-5α-cholestan-3β-ol. The latter sterol and 23-demethylgorqosterol have opposite configurations at C-22, C-23, and C-24; the Δ8(14) sterol has an unprecedented side chain. 相似文献
114.
B. H. Min 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1980,183(4)
A method is described for measuring a trimethyl prostaglandin E2 analog, TM-PGE2, in human plasma. Trideuterated and monofluorinated analogs of TM-PGE2 are added to plasma as internal standard and carrier, respectively. The plasma is adjusted to pH 3.0 and is extracted with a mixture of benzene—dichloromethane (9:1). The residue, following removal of the extracting solvent, is reacted consecutively with pentafluorobenzyl bromide and bistrimethylsilyltrifluoroacetamide. The excess derivatizing reagents are removed by evaporation, and an aliquot of the reconstituted residue is analyzed by capillary column gas chromatography using methane as the carrier gas. A quadrupole mass spectrometer is set to monitor in the gas chromatographic effluent the (M − C7H2F5)− fragmention of TM-PGE2 (m/e 449) and trideuterated TM-PGE2 (m/e 452) generated by methane negative chemical ionization. Quantitation of unknowns is based on a comparison of the m/e 449 to m/e 452 ion ratio in each unknown to that obtained from the analysis of control plasma spiked with known amounts of TM-PGE2 and fixed amounts of internal standard and carrier. The sensitivity limit of the assay is approximately 100 pg ml−1, which is equivalent to 1 pg injected. The assay was used to measure the concentration of TM-PGE2 in the plasma of two subjects following a single 10 μg kg−1 oral dose of the drug. 相似文献
115.
Antibodies against purified ( from the rectal gland of Squalus acanthias, as well as against its catalytic subunit, inhibited ouabain binding by as much as 50%. However, antibodies against the glycoprotein subunit did not inhibit ouabain binding. These data suggest that binding of antibody against the catalytic subunit to the enzyme either covers the ouabain binding site or destroys its conformation, while binding of antibody against the glycoprotein has no such effect. 相似文献
116.
J P Leroux J C Marchand R Hong Tuan Ha P Cartier 《European journal of biochemistry》1975,58(2):367-373
Viable human polymorphonuclear leukocytes isolated from peripheral blood were incubated for 1 h at 37 degrees C with variable concentrations of insulin in a saline medium buffered at pH 7.4. The hormone increased glucose consumption by about 40% without influencing the permeability of the membranes to glucose, whose uptake followed a passive diffusion process. The measurement of intermediates localized activation of glycolysis by insulin, down to 0.36 nM, at the phosphofructokinase step. However, the spectrophotometric measurement showed no activation of phosphofructokinase after preincubation with insulin of either intact granulocytes or crude or ultracentrifuged homogenates. The level of cyclic AMP, which is known to activate phosphofructokinase, was not modified by insulin; cyclic GMP did not activate the enzyme in the granulocyte extracts: neither of the two nucleotides can therefore be considered as a direct messenger of the action of insulin on phosphofructokinase. An important fraction of the extra glucose consumed under the influence of insulin was recovered as neither glycogen nor lactate, nor was it oxidized in the Krebs cycle. It might be assumed to have been converted into glycerolipids. However, insulin produced no detectable accumulation of triglycerides and activated neither the pentose phosphate pathway nor oxidative decarboxylation of pyruvate. The fate of the extra glucose consumed under the influence of insulin therefore remains questionable. 相似文献
117.
Galium procurrens is described as a new diploid relic species from Montenegro/N. Albania and SW. Bulgaria. It is related to the tetraploidG. laevigatum and other diploid and polyploid taxa of theG. sylvaticum-group inhabiting European deciduous forests. 相似文献
118.
119.
Hyo Je Cho Kyungsun Kim Seo Yean Sohn Ha Yeon Cho Kyung Jin Kim Myung Hee Kim Dockyu Kim Eungbin Kim Beom Sik Kang 《The Journal of biological chemistry》2010,285(45):34643-34652
A meta-cleavage pathway for the aerobic degradation of aromatic hydrocarbons is catalyzed by extradiol dioxygenases via a two-step mechanism: catechol substrate binding and dioxygen incorporation. The binding of substrate triggers the release of water, thereby opening a coordination site for molecular oxygen. The crystal structures of AkbC, a type I extradiol dioxygenase, and the enzyme substrate (3-methylcatechol) complex revealed the substrate binding process of extradiol dioxygenase. AkbC is composed of an N-domain and an active C-domain, which contains iron coordinated by a 2-His-1-carboxylate facial triad motif. The C-domain includes a β-hairpin structure and a C-terminal tail. In substrate-bound AkbC, 3-methylcatechol interacts with the iron via a single hydroxyl group, which represents an intermediate stage in the substrate binding process. Structure-based mutagenesis revealed that the C-terminal tail and β-hairpin form part of the substrate binding pocket that is responsible for substrate specificity by blocking substrate entry. Once a substrate enters the active site, these structural elements also play a role in the correct positioning of the substrate. Based on the results presented here, a putative substrate binding mechanism is proposed. 相似文献
120.