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   815篇
  免费   28篇
  国内免费   1篇
化学   585篇
晶体学   11篇
力学   15篇
数学   31篇
物理学   202篇
  2021年   5篇
  2020年   8篇
  2019年   13篇
  2018年   7篇
  2016年   10篇
  2015年   15篇
  2014年   14篇
  2013年   28篇
  2012年   29篇
  2011年   49篇
  2010年   38篇
  2009年   32篇
  2008年   46篇
  2007年   52篇
  2006年   44篇
  2005年   47篇
  2004年   37篇
  2003年   28篇
  2002年   29篇
  2001年   9篇
  2000年   10篇
  1999年   13篇
  1998年   6篇
  1996年   12篇
  1995年   8篇
  1994年   9篇
  1993年   11篇
  1992年   15篇
  1991年   17篇
  1990年   12篇
  1989年   7篇
  1988年   9篇
  1987年   15篇
  1986年   11篇
  1985年   23篇
  1984年   9篇
  1983年   12篇
  1982年   11篇
  1981年   12篇
  1980年   13篇
  1979年   8篇
  1978年   9篇
  1977年   5篇
  1976年   2篇
  1975年   8篇
  1974年   6篇
  1973年   15篇
  1970年   2篇
  1969年   3篇
  1968年   2篇
排序方式: 共有844条查询结果,搜索用时 93 毫秒
821.
The crystal structures of octaisopropylcyclotetrasilane [i-Pr2Si]4 (1) and octakis(trimethylsilylmethyl)cyclotetrasilane [(Me3SiCH2)2Si]4 (2) have been determined by means of X-ray diffraction analysis. Various crystallographic and structural data for the two compounds were recorded. The Si4 rings of the compounds are nonplanar with quite large dihedral angles of 37.1° in (1) and 36.6° in (2), being comparable to that (36.8)° for [t-BuMeSi]4 reported previously and other characteristic features in the structures of (1) and (2) were described. Some structural properties of the cyclic catenation systems, [R1R2Si]n (n = 3–6), including (1) and (2) were also discussed from a comparative viewpoint with respect to the ring shape and the relationship between ring size and Si-Si bond length.  相似文献   
822.
823.
824.
Protein-based fluorescent biosensors with sufficient sensing specificity are useful analytical tools for detection of biologically important substances in complicated biological systems. Here, we present the design of a hybrid biosensor, specific for a bis-phosphorylated peptide, based on a natural phosphoprotein binding domain coupled with an artificial fluorescent chemosensor. The hybrid biosensor consists of a phosphoprotein binding domain, the WW domain, into which has been introduced a fluorescent stilbazole having Zn(II)-dipicolylamine (Dpa) as a phosphate binding motif. It showed strong binding affinity and high sensing selectivity toward a specific bis-phosphorylated peptide in the presence of various phosphate species such as the monophosphorylated peptide, ATP, and others. Detailed fluorescence titration experiments clearly indicate that the binding-induced fluorescence enhancement and the sensing selectivity were achieved by the cooperative action of both binding sites of the hybrid biosensor, i.e., the WW domain and the Zn(II)-Dpa chemosensor unit. Thus, it is clear that the tethered Zn(II)-Dpa-stilbazole unit operated not only as a fluorescence signal transducer, but also as a sub-binding site in the hybrid biosensor. Taking advantage of its selective sensing property, the hybrid biosensor was successfully applied to real-time and label-free fluorescence monitoring of a protein kinase-catalyzed phosphorylation.  相似文献   
825.
L-Leu hexapeptide containing alpha-aminoisobutyric acid (Aib) forms a right-handed (P) 3(10)-helix, whereas that containing cyclic alpha,alpha-disubstituted amino acid Ac(5)c(dOM) assumes a right-handed (P) alpha-helix in the solid state.  相似文献   
826.
The kinetics of Z-(cis)/E-(trans) isomerization of enalapril was investigated by reversed phase high-performance liquid chromatography (RP-HPLC) using a monolith ODS column under a series of different temperature and pH conditions. At a neutral pH 7, the rate (k(obs)) of Z-(cis)/E-(trans) isomerization of enalapril at 4 degrees C (9.4 x 10(-3)min(-1)) is much lower than at 23 degrees C (1.8 x 10(-1)min(-1)), while the fractional concentration of Z-(cis) isomer is always higher than that of E-(trans) isomer in the pH range 2-7. The fractional concentration of the E-(trans) isomer becomes a maximum (about 40%) in the pH range 3-6, where enalapril exists as a zwitterion. The hydrophobicity (logP(O/W)) of both isomers was estimated by high-speed counter-current chromatography (HSCCC). Normal phase HSCCC separation using a tert-butyl methyl ether-acetonitrile-20mM potassium phosphate buffer (pH 5) two-phase solvent system (2:2:3, v/v/v) at 4 degrees C was effective in partially separating the isomers, and the partition coefficient (K) of each isomer was directly calculated from the retention volume (V(R)). The logP(O/W) values of Z-(cis) and E-(trans) isomers were -0.46 and -0.65, respectively.  相似文献   
827.
Han X  Ma X  Zhang T  Zhang Y  Liu Q  Ito Y 《Journal of chromatography. A》2007,1151(1-2):180-182
Following an initial clean-up step on silica, high-speed counter-current chromatography (HSCCC) was used to purify a flavone, casticin (5,3'-dihydroxy-3,6,7, 4'-tetramethoxyflavone), from an extract of the dried leaves of Artemisia annua L. The two-phase solvent system used was composed of n-hexane-ethyl acetate-methanol-water at an optimized volume ratio of 7:10:7:10 (v/v). HSCCC separation of 226.4 mg of crude sample (containing casticin at 16.5% purity after silica gel clean-up) yielded 36.3 mg of casticin with a purity of over 99% and 96.2% recovery. Identification of the target compound was performed by (1)H NMR, (13)C NMR, two-dimensional NMR, electrospray ionization MS, IR and UV.  相似文献   
828.
We introduce a model for ionic electrodiffusion and osmotic water flow through cells and tissues. The model consists of a system of partial differential equations for ionic concentration and fluid flow with interface conditions at deforming membrane boundaries. The model satisfies a natural energy equality, in which the sum of the entropic, elastic and electrostatic free energies is dissipated through viscous, electrodiffusive and osmotic flows. We discuss limiting models when certain dimensionless parameters are small. Finally, we develop a numerical scheme for the one-dimensional case and present some simple applications of our model to cell volume control.  相似文献   
829.
Nano-particulate coatings with high reflectance against solar irradiation can control undesirable thermal heating by sunlight absorption. It can reduce the energy consumption for air conditioning of houses and cars. For the objects covered by these coatings and subjected to human sight, e.g. roofing surfaces, high dazzle of reflected visible light can offend the human eyes and spoil the fine view of covered objects. The authors introduced a new optimization method in designing pigmented coatings which considers both thermal and aesthetic effects in previous studies. The optimization is possible by controlling the material, size and concentration of pigment particles. The proposed coatings maximize the reflectance of near infrared (NIR) region to care the thermal effects and minimize the visible (VIS) reflected energy to keep the dark tone because of aesthetic appeal. Two different types of copper oxide pigment particles namely cupric oxide (CuO) and cuprous oxide (Cu2O) were considered in this study. The optimum characteristics and performances are obtained and compared with titanium dioxide (TiO2) particle as a typical cool pigment. The results show that cupric oxide has much better performance for our objective.  相似文献   
830.
We describe and analyze a bistable reaction-diffusion (RD) model for two interconverting chemical species that exhibits a phenomenon of wave-pinning: a wave of activation of one of the species is initiated at one end of the domain, moves into the domain, decelerates, and eventually stops inside the domain, forming a stationary front. The second ("inactive") species is depleted in this process. This behavior arises in a model for chemical polarization of a cell by Rho GTPases in response to stimulation. The initially spatially homogeneous concentration profile (representative of a resting cell) develops into an asymmetric stationary front profile (typical of a polarized cell). Wave-pinning here is based on three properties: (1) mass conservation in a finite domain, (2) nonlinear reaction kinetics allowing for multiple stable steady states, and (3) a sufficiently large difference in diffusion of the two species. Using matched asymptotic analysis, we explain the mathematical basis of wave-pinning, and predict the speed and pinned position of the wave. An analysis of the bifurcation of the pinned front solution reveals how the wave-pinning regime depends on parameters such as rates of diffusion and total mass of the species. We describe two ways in which the pinned solution can be lost depending on the details of the reaction kinetics: a saddle-node or a pitchfork bifurcation.  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