全文获取类型
收费全文 | 6079篇 |
免费 | 573篇 |
国内免费 | 97篇 |
专业分类
6749篇 |
出版年
2024年 | 17篇 |
2023年 | 78篇 |
2022年 | 94篇 |
2021年 | 118篇 |
2020年 | 170篇 |
2019年 | 211篇 |
2018年 | 242篇 |
2017年 | 164篇 |
2016年 | 167篇 |
2015年 | 181篇 |
2014年 | 280篇 |
2013年 | 374篇 |
2012年 | 130篇 |
2011年 | 300篇 |
2010年 | 331篇 |
2009年 | 351篇 |
2008年 | 455篇 |
2007年 | 407篇 |
2006年 | 372篇 |
2005年 | 349篇 |
2004年 | 278篇 |
2003年 | 263篇 |
2002年 | 199篇 |
2001年 | 100篇 |
2000年 | 90篇 |
1999年 | 98篇 |
1998年 | 118篇 |
1997年 | 89篇 |
1996年 | 68篇 |
1995年 | 86篇 |
1994年 | 64篇 |
1993年 | 60篇 |
1992年 | 45篇 |
1991年 | 41篇 |
1990年 | 32篇 |
1989年 | 31篇 |
1988年 | 30篇 |
1987年 | 24篇 |
1986年 | 16篇 |
1985年 | 24篇 |
1984年 | 57篇 |
1983年 | 34篇 |
1982年 | 39篇 |
1981年 | 26篇 |
1980年 | 24篇 |
1979年 | 11篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1972年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有6749条查询结果,搜索用时 15 毫秒
71.
Quercetin 3-O-(6″-O-galloyl)-β-d-glucoside has been identified as a constituent of Tellima grandiflora (Saxifragaceae). In all, twelve gallates were encountered: two isomeric gallates of quercetin 3-O-glucoside and two of quercetin 3-O-galactoside, a similar set involving kaempferol, and a similar set involving myricetin. 相似文献
72.
All caspases evolved from a common ancestor and subsequently developed into two general classes, inflammatory or apoptotic caspases. The caspase-hemoglobinase fold has been conserved throughout nearly one billion years of evolution and is utilized for both the monomeric and dimeric subfamilies of apoptotic caspases, called initiator and effector caspases, respectively. We compared the folding and assembly of procaspase-3b from zebrafish to that of human effector procaspases in order to examine the conservation of the folding landscape. Urea-induced equilibrium folding/unfolding of procaspase-3b showed a minimum three-state folding pathway, where the native dimer isomerizes to a partially folded dimeric intermediate, which then unfolds. A partially folded monomeric intermediate observed in the folding landscape of human procaspase-3 is not well-populated in zebrafish procaspase-3b. By comparing effector caspases from different species, we show that the effector procaspase dimer undergoes a pH-dependent conformational change, and that the conformational species in the folding landscape exhibit similar free energies. Together, the data show that the landscape for the caspase-hemoglobinase fold is conserved, yet it provides flexibility for species-specific stabilization or destabilization of folding intermediates resulting in changes in stability. The common pH-dependent conformational change in the native dimer, which yields an enzymatically inactive species, may provide an additional, albeit reversible, mechanism for controlling caspase activity in the cell. 相似文献
73.
Methylation and partial acid hydrolysis of xylans from the bast and core of kenaf (Hibiscus cannabinus) showed that the main chain of these xylans consists of (1 → 4)-linked β-d-xylopyranosyl (Xylp) residues, some of which carry a -1,2-linked 4-O-methyl-glucopyranosyluronic acid (Me-GlcAp) and glucopyranosyluronic acid (GlcAp) residues as side chains. Partial hydrolysis of kenaf xylans afforded two series of aldouronic acids from aldobio- to aldotetraouronic acids. The acids of the first series composed of 4-O-Me-d-GlcAp and d-Xylp residues: 4-O-Me-GlcA-Xyl3, 4-O-Me-GlcA-Xyl2 and 4-O-Me-GlcA-Xyl. The second series composed of d-GlcAp and d-Xylp: GlcA-Xyl3, GlcA-Xyl2 and GlcA-Xyl.
In addition to these acids, another aldobiouronic acid, 4-O-(-d-GalAp)-d-Xyl was found to be present in the partial hydrolysate.
The molar ratio of GalA, GlcA, 4-O-Me-GlcA, and Xyl residues was calculated to be 1.0:2.0:9.4:119 for the bast xylan and 1.0:1.3:7.9:99.4 for the core xylan. 相似文献
74.
Lynell C. MartinezGeorge J. Turner 《生物化学与生物物理学报:生物膜》2002,1564(1):91-98
A high-throughput screening method has been developed which enables functional analysis of bacteriorhodpsin in whole cell pastes. Reflectance spectra, from as little as 5 ml of Halobacterium salinarum cells, show close correspondence to that obtained from the purified purple membrane (PM), containing bacteriorhodopsin (BR) as the sole protein component. We demonstrate accurate quantification of BR accumulation by ratiometric analysis of BR (Amax 568) and a membrane-bound cytochrome (Amax 410). In addition, ground-state light- and dark-adapted (LA and DA, respectively) spectral differences were determined with high accuracy and precision. Using cells expressing the BR mutant D85N, we monitored transitions between intermediate-state homologues of the reprotonation phase of the light-activated proton pumping mechanism. We demonstrate that phenotypes of three mutants (D85N/T170C, D85N/D96N, and D85N/R82Q) previously characterized for their effect on photocycle transitions are reproduced in the whole cell samples. D85N/T170C stabilizes accumulation of the N state while D85N/D96N accumulates no N state. D85N/R82Q was found to have perturbed the pKa of M accumulation. These studies illustrate the correspondence between pH-dependent ground-state transitions accessed by D85N and the transitions accessed by the wild-type protein following photoexcitation. We demonstrate that whole cell reflectance spectroscopy can be used to efficiently characterize the large numbers of mutants generated by engineering strategies that exploit saturation mutagenesis. 相似文献
75.
Homeostasis of cell composition during prolonged darkness 总被引:1,自引:0,他引:1
The chemical composition of organisms in relation to their environmental resource availability is an area of intense research activity. We studied the changes in cell composition of the cyanobacterium Phormidium autumnale in response to prolonged darkness. Cells allocated their internal resources in a homeostatic manner, oxidizing all the three major cellular constituents in a proportional way. This resulted in constant C/N and carbohydrates, lipids and proteins ratios that remained unaltered throughout the whole incubation period. We propose the maintenance of balanced cell composition (homeostasis) as an evolutionary strategy related to environmental transitory changes. 相似文献
76.
Light gradients and spectral regime were measured in Lactuca sativa L. cv. Grand Rapids achenes using fiber optic microsensors. The distribution of scattered light across lettuce achenes was linear for 660 and 730 nm and non-linear for 450 nm light. Spectra for scattered light within intact achenes also showed a non-linear increase with wavelength. The preferential attenuation of blue light by the pericarp and seed explains in part the relative ineffectiveness of blue light with respect to red in triggering germination of lettuce. Calculated action spectra for phytochrome-stimulated germination agree closely in the red with experimentally derived action spectra; however, there is little agreement within the blue. 相似文献
77.
Dennison SR Howe J Morton LH Brandenburg K Harris F Phoenix DA 《Biochemical and biophysical research communications》2006,347(4):1006-1010
The antimicrobial activity of the anionic peptide, AP1 (GEQGALAQFGEWL), was investigated. AP1 was found to kill Staphylococcus aureus with an MLC of 3 mM and to induce maximal surface pressure changes of 3.8 mN m−1 over 1200 s in monolayers formed from lipid extract of S. aureus membranes. FTIR spectroscopy showed the peptide to be α-helical (100%) in the presence of vesicles formed from this lipid extract and to induce increases in their fluidity (Δν circa 0.5 cm−1). These combined data show that AP1 is able to function as an α-helical antimicrobial peptide against Gram-positive bacteria and suggest that the killing mechanism used by the peptide involves interactions with the membrane lipid headgroup region. Moreover, this killing mechanism differs strongly from that previously reported for AP1 against Gram-negative bacteria, indicating the importance of considering the effects of membrane lipid composition when investigating the structure/function relationships of antimicrobial peptides. 相似文献
78.
Matthias Rüdt Philipp Vormittag Nils Hillebrandt Jürgen Hubbuch 《Biotechnology and bioengineering》2019,116(6):1366-1379
Virus-like particles (VLPs) have shown great potential as biopharmaceuticals in the market and in clinics. Nonenveloped, in vivo assembled VLPs are typically disassembled and reassembled in vitro to improve particle stability, homogeneity, and immunogenicity. At the industrial scale, cross-flow filtration (CFF) is the method of choice for performing reassembly by diafiltration. Here, we developed an experimental CFF setup with an on-line measurement loop for the implementation of process analytical technology (PAT). The measurement loop included an ultraviolet and visible (UV/Vis) spectrometer as well as a light scattering photometer. These sensors allowed for monitoring protein concentration, protein tertiary structure, and protein quaternary structure. The experimental setup was tested with three Hepatitis B core Antigen (HBcAg) variants. With each variant, three reassembly processes were performed at different transmembrane pressures (TMPs). While light scattering provided information on the assembly progress, UV/Vis allowed for monitoring the protein concentration and the rate of VLP assembly based on the microenvironment of Tyrosine-132. VLP formation was verified by off-line dynamic light scattering (DLS) and transmission electron microscopy (TEM). Furthermore, the experimental results provided evidence of aggregate-related assembly inhibition and showed that off-line size-exclusion chromatography does not provide a complete picture of the particle content. Finally, a Partial-Least Squares (PLS) model was calibrated to predict VLP concentrations in the process solution. values of 0.947–0.984 were reached for the three HBcAg variants. In summary, the proposed experimental setup provides a powerful platform for developing and monitoring VLP reassembly steps by CFF. 相似文献
79.
Adenosine is a neuromodulator, and it has been suggested that cerebral acetate metabolism induces adenosine formation. In the present study the effects that acetate has on cerebral intermediary metabolism, compared with those of glucose, were studied using the adenosine A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA) and antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). Fasted rats received an intravenous injection of CCPA, DPCPX, or vehicle. Fifteen minutes later either [1,2-13C]acetate or [1-13C]glucose was given intraperitoneally; after another 30 min the rats were decapitated. Cortical extracts were analyzed with 13C NMR spectroscopy and HPLC analysis. DPCPX affected neuronal and astrocytic metabolism. De novo synthesis of GABA from neuronal and astrocytic precursors was significantly reduced. De novo syntheses of glutamate and aspartate were at control levels, but their degradation was significantly elevated. In glutamine the anaplerotic activity and the amount of label in the position representing the second turn in the tricarboxylic acid cycle were significantly increased, suggesting elevated metabolic activity in astrocytes. CCPA did not influence GABA, aspartate, or glutamine synthesis. In glutamate the contribution from the astrocytic anaplerotic pathway was significantly decreased. In the present study the findings in the [1,2-13C]acetate and [1-13C]glucose control, CCPA, and DPCPX groups were complementary, and no adenosine A1 agonist effects arising from cerebral acetate metabolism were detected. 相似文献
80.