首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1889篇
  免费   50篇
  国内免费   9篇
化学   1313篇
晶体学   7篇
力学   37篇
数学   260篇
物理学   331篇
  2023年   17篇
  2022年   19篇
  2021年   20篇
  2020年   26篇
  2019年   35篇
  2018年   28篇
  2017年   18篇
  2016年   47篇
  2015年   32篇
  2014年   39篇
  2013年   97篇
  2012年   119篇
  2011年   108篇
  2010年   64篇
  2009年   49篇
  2008年   87篇
  2007年   83篇
  2006年   100篇
  2005年   80篇
  2004年   79篇
  2003年   74篇
  2002年   57篇
  2001年   33篇
  2000年   42篇
  1999年   18篇
  1998年   14篇
  1997年   18篇
  1996年   31篇
  1995年   21篇
  1994年   18篇
  1993年   26篇
  1992年   22篇
  1991年   19篇
  1990年   18篇
  1989年   20篇
  1988年   17篇
  1987年   20篇
  1986年   11篇
  1985年   23篇
  1984年   28篇
  1983年   22篇
  1982年   20篇
  1981年   19篇
  1980年   14篇
  1979年   22篇
  1978年   14篇
  1977年   18篇
  1976年   22篇
  1974年   16篇
  1973年   16篇
排序方式: 共有1948条查询结果,搜索用时 10 毫秒
91.
The volatile fraction from aerial parts (flowers, stems and leaves) of Discaria americana Gillies & Hook (Rhamnaceae) was obtained by hydrodistillation and the chemical composition of this oil was determined by gas chromatography and gas chromatography-mass spectrometry. The major constituents resulted to be 4-methylphenol (15.5%), eugenol (11%), 3-methylindole (9.7%) and alpha-terpineol (6.2%). The essential oil of this plant displayed strong antioxidant activity (DPPH assay) that could be explained by the presence of active compounds like eugenol, 4-methylphenol, alpha-terpineol, linalool, thymol and cis-nerolidol.  相似文献   
92.
93.
94.

Background  

Minichromosome maintenance proteins (Mcm) 2, 3, 4, 5, 6 and 7 are related by sequence and form a variety of complexes that unwind DNA, including Mcm4/6/7. A Mcm4/6/7 trimer forms one half of the Mcm2-7 hexameric ring and can be thought of as the catalytic core of Mcm2-7, the replicative helicase in eukaryotic cells. Oligomeric analysis of Mcm4/6/7 suggests that it forms a hexamer containing two Mcm4/6/7 trimers, however, under certain conditions trimeric Mcm4/6/7 has also been observed. The functional significance of the different Mcm4/6/7 oligomeric states has not been assessed. The results of such an assessment would have implications for studies of both Mcm4/6/7 and Mcm2-7.  相似文献   
95.
Peptide epitope affinity chromatography is a powerful technique for the purification of antibodies. This study aims to demonstrate the versatility of the technique and to show how biophysical techniques such as circular dichroism (CD) and fluorescence quenching (FQ) can aid the rational design of affinity ligands and characterization of antibody-based reagents. The performance of a number of peptide ligands for the purification of a range of different antibodies and recombinant fragments is investigated by automated fast-protein liquid chromatography. Purified products are analyzed for purity by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. They are then radiolabelled and the immunoreactivity is determined. Ligands are analyzed for secondary structural characteristics by CD and for binding affinity by FQ. Finally, a study is performed to investigate the thermal stability of a recombinant antibody fragment by CD analysis. It is found that simple ligand modifications such as the introduction of a C-terminal cysteine residue can improve purification performance. The FQ studies show that the modified peptide has a higher affinity for antibody. The CD analysis shows that it has a tendency to dimerize because of the formation of disulfide bonds. The versatility of epitope affinity is demonstrated through the purification of a recombinant diabody (dbFv) and by the use of a separate peptide matrix for the purification of an unrelated antibody. All studies result in antibody preparations of high purity and immunoreactivity. The CD analysis of the dbFv shows that it is denatured at 37 degrees C and is therefore unsuitable as a targeting reagent for use in humans in its present form. It is concluded that epitope affinity chromatography coupled with biophysical analyses plays an important role in the production and characterization of antibody-based reagents for targeted diagnosis and therapy of human diseases.  相似文献   
96.
We report measurements of residual vapour levels of xylenes and trimethylbenzenes, present following a floor re-surfacing procedure, using the technique of selected ion flow tube mass spectrometry (SIFT-MS). A subject exposed to controlled amounts of xylene and mesitylene was monitored by direct breath exhalation over a 4-hour period after exposure to the volatile organic compounds (VOCs) had stopped. The headspace gases above 5 mL blood samples taken over this period were also monitored. The decays of the solvent levels with time were fitted to a two-compartment model with residence times for xylene and mesitylene of 0.37 h and 0.38 h, respectively (compartment one) and 2.5 h and 2.8 h, respectively (compartment two).  相似文献   
97.
A new synthesis of (8‐quinolyl)‐5‐methoxysalicylaldimine (Hqsal‐5‐OMe) is reported and its crystal structure is presented. Two FeIII complexes, [Fe(qsal‐5‐OMe)2]Cl ? solvent (solvent=2 MeOH ? 0.5 H2O ( 1 ) and MeCN ? H2O ( 2 )) have been prepared and their structural, electronic and magnetic properties studied. [Fe(qsal‐5‐OMe)2] Cl ? 2 MeOH ? 0.5 H2O ( 1 ) exhibits rare crystallographically independent high‐spin and low‐spin FeIII centres at 150 K, whereas [Fe(qsal‐5‐OMe)2]Cl ? MeCN ? H2O ( 2 ) is low spin at 100 K. In both structures there are extensive π–π and C? H???π interactions. SQUID magnetometry of 2 reveals an unusual abrupt stepped‐spin crossover with T1/2=245 K and 275 K for steps 1 and 2, respectively, with a slight hysteresis of 5 K in the first step and a plateau of 15 K between the steps. In contrast, 1 is found to undergo an abrupt half‐spin crossover also with a hysteresis of 10 K. The two compounds are the first FeIII complexes of a substituted qsal ligand to exhibit abrupt spin crossover. These conclusions are supported by 57Fe Mössbauer spectroscopy. Both complexes exhibit reversible reduction to FeII at ?0.18 V and irreversible oxidation of the coordinated qsal‐5‐OMe ligand at +1.10 V.  相似文献   
98.
Polypyridyl complexes of Co decorated with 350-Da polyether chains (Co(350)(2+)) form molten phases of nucleic acids when paired with DNA counterions (Co(350)DNA) or 25-mer oligonucleotides. Analysis of voltammetry and chronoamperometry of mixtures of these phases with complexes having ClO(4)(-) counterions (Co(350)(ClO(4))(2)) and no other diluent provides charge transport rates from the oxidation and reduction currents for the complexes. As the mole fraction of the Co(350)(ClO(4))(2) complex in the mixture is varied from ca. 0.25 to 1, the physical diffusion constants derived from the Co(III/II) wave increase from 1 x 10(-11) cm(2)/s to 5 x 10(-10) cm(2)/s, and apparent diffusion constants dominated by the Co(II/I) electron self-exchange increase from 1 x 10(-10) cm(2)/s to 2 x 10(-8) cm(2)/s. Pure Co(350)DNA melts, containing no Co(350)(ClO(4))(2) complex, do not exhibit recognizable voltammetric waves; DNA suppresses the Co(II/I) electron transfer reactions of Co complexes for which it is the counterion. There are therefore two microscopically distinct kinds of Co(350) complexes, those with DNA and those with ClO(4)(-) counterions, with respect to their Co(II/I) electron-transfer dynamics, leading to percolative behavior in their mixtures. The electron-transfer rates of the Co(II/I) couple are controlled by the diffusive relaxation of the ionic atmosphere around the reaction pair, and the inactivity of the bound Co complexes can be attributed to the very low mobility of the anionic phosphate groups in the DNA counterion. Substitution of sulfonated polystyrene for DNA produced similar results, suggesting that this phenomenon is general to other polymer counterions of low mobility. We conclude that the measured Co(II/I) charge transport and electron-transfer rate constants reflect more the diffusive mobility of the perchlorate counterion than the intrinsic Co(II/I) electron hopping rate.  相似文献   
99.
This paper describes reactions in which ligands are exchanged and metals are transferred between monolayer-protected metal clusters (MPCs) that are in different phases (heterophase exchange) or are in the same phase. For example, contact of toluene solutions of alkanethiolate-coated gold MPCs with aqueous solutions of tiopronin-coated gold MPCs yields toluene-phase MPCs that have some tiopronin ligands and aqueous-phase MPCs that have some alkanethiolate ligands. In a second example, heterophase transfer reactions occur between toluene solutions of alkanethiolate-coated gold MPCs and aqueous solutions of tiopronin-coated silver MPCs, in which tiopronin ligands are transferred to the former and gold metal to the latter phase. These ligand and metal exchange reactions are inhibited when conducted under N(2). The results implicate participation of an oxidized form of Au (such as a Au(I) thiolate, Au(I)-SR) as both a ligand and metal carrier in the exchange reactions. Au(I)-SR is demonstrated to be an exchange catalyst.  相似文献   
100.
Abstract

The Accelerated solvent extraction (ASE) of PAHs (23 2- to 6-ring species) spiked onto glass fibre filters (GFFs) was studied as a function of variable extraction solvents, pressure, temperature and extraction times. Acceptable recoveries (85% ± 15%) were obtained for certain combinations of conditions and a tentative method (1500 psi, 150°C, 70:30 hexane:acetone mixture, 7 min heat-up time, 5 min static extraction time, 60% flush volume, 2 static cycles was selected for further testing. However, this method did not prove as effective as the traditional Soxhlet method of extraction when these parameters were used to extract native PAHs from ambient atmospheric particulate matter collected on a GFF by Integrated Atmospheric Deposition Network (IADN) sampling protocols. The extraction recovery study for spiked GFFs was repeated using slightly different extraction conditions: 2000 psi, 100°C, 70:30 hexane:acetone, 5 min heat-up time, 5 min static extraction time, 150% flush volume, 3 static cycles. When this method was applied to the extraction of native PAHs from ambient atmospheric particulate matter collected on GFFs, the results showed equivalent or better recoveries to that of the Soxhlet method. The total time of extraction was 25 min requiring only 30 mL of solvent. This ASE method is presently used to quantitatively determine PAHs in IADN particle-phase samples.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号