全文获取类型
收费全文 | 682篇 |
免费 | 20篇 |
国内免费 | 1篇 |
专业分类
化学 | 496篇 |
晶体学 | 1篇 |
力学 | 5篇 |
数学 | 79篇 |
物理学 | 122篇 |
出版年
2023年 | 8篇 |
2022年 | 10篇 |
2021年 | 9篇 |
2020年 | 6篇 |
2019年 | 9篇 |
2018年 | 3篇 |
2017年 | 5篇 |
2016年 | 10篇 |
2015年 | 10篇 |
2014年 | 12篇 |
2013年 | 37篇 |
2012年 | 29篇 |
2011年 | 46篇 |
2010年 | 24篇 |
2009年 | 18篇 |
2008年 | 35篇 |
2007年 | 35篇 |
2006年 | 40篇 |
2005年 | 32篇 |
2004年 | 29篇 |
2003年 | 38篇 |
2002年 | 26篇 |
2001年 | 3篇 |
2000年 | 7篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1996年 | 13篇 |
1995年 | 10篇 |
1994年 | 7篇 |
1993年 | 6篇 |
1992年 | 11篇 |
1991年 | 10篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 17篇 |
1981年 | 13篇 |
1980年 | 3篇 |
1979年 | 11篇 |
1978年 | 13篇 |
1977年 | 9篇 |
1976年 | 10篇 |
1975年 | 8篇 |
1974年 | 7篇 |
1973年 | 2篇 |
排序方式: 共有703条查询结果,搜索用时 640 毫秒
661.
Oman TJ Lupoli TJ Wang TS Kahne D Walker S van der Donk WA 《Journal of the American Chemical Society》2011,133(44):17544-17547
The two-peptide lantibiotic haloduracin is composed of two post-translationally modified polycyclic peptides that synergistically act on gram-positive bacteria. We show here that Halα inhibits the transglycosylation reaction catalyzed by PBP1b by binding in a 2:1 stoichiometry to its substrate lipid II. Halβ and the mutant Halα-E22Q were not able to inhibit this step in peptidoglycan biosynthesis, but Halα with its leader peptide still attached was a potent inhibitor. Combined with previous findings, the data support a model in which a 1:2:2 lipid II:Halα:Halβ complex inhibits cell wall biosynthesis and mediates pore formation, resulting in loss of membrane potential and potassium efflux. 相似文献
662.
663.
Kraft SJ Williams UJ Daly SR Schelter EJ Kozimor SA Boland KS Kikkawa JM Forrest WP Christensen CN Schwarz DE Fanwick PE Clark DL Conradson SD Bart SC 《Inorganic chemistry》2011,50(20):9838-9848
Uranium compounds supported by redox-active α-diimine ligands, which have methyl groups on the ligand backbone and bulky mesityl substituents on the nitrogen atoms {(Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr], where Ar = 2,4,6-trimethylphenyl (Mes)}, are reported. The addition of 2 equiv of (Mes)DAB(Me), 3 equiv of KC(8), and 1 equiv of UI(3)(THF)(4) produced the bis(ligand) species ((Mes)DAB(Me))(2)U(THF) (1). The metallocene derivative, Cp(2)U((Mes)DAB(Me)) (2), was generated by the addition of an equimolar ratio of (Mes)DAB(Me) and KC(8) to Cp(3)U. The bond lengths in the molecular structure of both species confirm that the α-diimine ligands have been doubly reduced to form ene-diamide ligands. Characterization by electronic absorption spectroscopy shows weak, sharp transitions in the near-IR region of the spectrum and, in combination with the crystallographic data, is consistent with the formulation that tetravalent uranium ions are present and supported by ene-diamide ligands. This interpretation was verified by U L(III)-edge X-ray absorption near-edge structure (XANES) spectroscopy and by variable-temperature magnetic measurements. The magnetic data are consistent with singlet ground states at low temperature and variable-temperature dependencies that would be expected for uranium(IV) species. However, both complexes exhibit low magnetic moments at room temperature, with values of 1.91 and 1.79 μ(B) for 1 and 2, respectively. Iodomethane was used to test the reactivity of 1 and 2 for multielectron transfer. While 2 showed no reactivity with CH(3)I, the addition of 2 equiv of iodomethane to 1 resulted in the formation of a uranium(IV) monoiodide species, ((Mes)DAB(Me))((Mes)DAB(Me2))UI {3; (Mes)DAB(Me2) = [ArN═C(Me)C(Me(2))NAr]}, which was characterized by single-crystal X-ray diffraction and U M(4)- and M(5)-edge XANES. Confirmation of the structure was also attained by deuterium labeling studies, which showed that a methyl group was added to the ene-diamide ligand carbon backbone. 相似文献
664.
Alen Alexanderian Matthias K. Gobbert K. Renee Fister Holly Gaff Suzanne Lenhart Elsa Schaefer 《Nonlinear Analysis: Real World Applications》2011,12(6):3483-3498
Occasional outbreaks of cholera epidemics across the world demonstrate that the disease continues to pose a public health threat. Traditional models for the spread of infectious diseases are based on systems of ordinary differential equations. Since disease dynamics such as vaccine efficacy and the risk for contracting cholera depend on the age of the humans, an age-structured model offers additional insights and the possibility of studying the effects of treatment options. The model investigated is given as a system of hyperbolic (first-order) partial differential equations in combination with ordinary differential equations. First, using a representation from the method of characteristics and a fixed point argument, we prove the existence and uniqueness of a solution to our nonlinear system. Then we present a finite difference approximation to the model and study the effect of high and low rates of shedding of cholera vibrios on the dynamics of the spread of the disease. The simulations demonstrate the explosive nature of cholera outbreaks that is observed in reality. The contrast of results for high and low rates of shedding of vibrios suggest a possible underlying cause for this effect. 相似文献
665.
Edward Mausolf Erik Johnstone Frederic Poineau Suzanne Nguyen Steven Jones Thomas Hartmann Edgar Buck Kenneth Czerwinski 《Journal of Radioanalytical and Nuclear Chemistry》2013,298(2):1315-1321
Amorphous TcO2 and NH4TcO4 solubilized into 12 M HCl will spontaneously convert to hexachlorotechnetate (TcCl6 2?). This process is accelerated upon heating but species lower than Tc(IV) are not generated by this action. TcCl6 2? is kinetically unstable with regards to formation in solutions of low concentrations of HCl and will spontaneously convert back to soluble and insoluble forms of Tc(IV) in water. TcCl6 2? in 12 M HCl placed in contact with the reducing metal Zn at elevated temperatures (90 °C) forms a black precipitate that contains amorphous Tc metal, TcO2, and oxy-chlorides of Tc. Powder X-ray diffraction indicates the presence of Tc metal after thermal treatment where X-ray absorption fine structure spectroscopy indicates the presence of hexagonal Tc metal and amorphous TcO2 in the precipitate after rinsing with 12 HCl but before thermal treatment. The resulting solution contains a mixture of Tc chlorides and oxy-chlorides following reduction where TcCl6 2? is completely consumed resulting primarily in Tc2OCl10 4? dominating the UV–visible spectra. Reducing the solution volume and reconstituting the products into 12 M HCl while boiling the mixed solution (>24 h) will slowly convert all soluble Tc back to TcCl6 2?. Expanding on previous efforts made in this laboratory to recover Tc metal from aqueous solution, we investigate its synthesis when Tc(IV) and Tc(VII) in 12 M HCl is placed in contact with the reducing metal (i.e., Zn) at elevated temperatures. 相似文献
666.
J. N. Kasimos T. E. Merchant L. W. Gierke T. Glonek 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4)
Abstract Phosphatic metabolic profiles of nineteen malignant and normal human colon specimens were analyzed by techniques of perchloric acid extraction and 31P MR spectroscopy at 202.4 MHz. Thirty-one individual phosphorus-containing intermediaries of metabolism were identified and their relative concentrations in mole-percentages of phosphorus were computed. Metabolic indices representing ratios and sums of individual or grouped metabolites were concurrently computed. The individual metabolite concentrations and metabolic indices were statistically compared revealing significant differences between malignant and normal tissues (see table). Malignant tissues demonstrate significant increases in the phosphomonoesters and diesters along with corresponding decreases in the high-energy phosphate compounds. While these changes may be relative, the increases in the PME and PDE bands most likely represent an increase in cell membrane activity ind P-lipid metabolism. P-31 MR is effective in differentiating malignant from normal tissues. 相似文献
667.
Sylvia Boyd René Sitters Suzanne van der Ster Leen Stougie 《Mathematical Programming》2014,144(1-2):227-245
We study the traveling salesman problem (TSP) on the metric completion of cubic and subcubic graphs, which is known to be NP-hard. The problem is of interest because of its relation to the famous 4/3-conjecture for metric TSP, which says that the integrality gap, i.e., the worst case ratio between the optimal value of a TSP instance and that of its linear programming relaxation (the subtour elimination relaxation), is 4/3. We present the first algorithm for cubic graphs with approximation ratio 4/3. The proof uses polyhedral techniques in a surprising way, which is of independent interest. In fact we prove constructively that for any cubic graph on $n$ vertices a tour of length $4n/3-2$ exists, which also implies the 4/3-conjecture, as an upper bound, for this class of graph-TSP. Recently, Mömke and Svensson presented an algorithm that gives a 1.461-approximation for graph-TSP on general graphs and as a side result a 4/3-approximation algorithm for this problem on subcubic graphs, also settling the 4/3-conjecture for this class of graph-TSP. The algorithm by Mömke and Svensson is initially randomized but the authors remark that derandomization is trivial. We will present a different way to derandomize their algorithm which leads to a faster running time. All of the latter also works for multigraphs. 相似文献
668.
The purpose of the present quasi‐experimental study was to examine the impact of a horseshoe crab citizen science program on student achievement and science, technology, engineering, and mathematics (STEM) career motivation with 86 (n = 86) eighth‐grade students. The treatment group conducted fieldwork with naturalists and collected data for a professional biologist studying horseshoe crab speciation and a mock survey. The comparison group studied curriculum related to horseshoe crabs in the science classroom. A series of measures related to self‐efficacy, interest, outcome expectations, choice goals, and content knowledge were given to participants before and after the intervention. It was hypothesized that students would report higher motivational beliefs regarding science and show higher levels of achievement following the intervention than the comparison group. Support was shown for most of the hypotheses. In addition, path analyses indicated that students' motivational beliefs influence content knowledge and outcome expectations, which in turn affect their career goals. These results have implications for incorporating authentic fieldwork within a formal school structure as an effective method for promoting student achievement and STEM career motivation. 相似文献
669.
Dr. Suzanne G. Rzuczek Dr. HaJeung Park Prof. Dr. Matthew D. Disney 《Angewandte Chemie (International ed. in English)》2014,53(41):10956-10959
Potent modulators of RNA function can be assembled in cellulo by using the cell as a reaction vessel and a disease‐causing RNA as a catalyst. When designing small molecule effectors of function, a balance between permeability and potency must be struck. Low molecular weight compounds are more permeable whereas higher molecular weight compounds are more potent. The advantages of both types of compounds could be synergized if low molecular weight molecules could be transformed into potent, multivalent ligands by a reaction that is catalyzed by binding to a target in cells expressing a genetic defect. It was shown that this approach is indeed viable in cellulo. Small molecule modules with precisely positioned alkyne and azide moieties bind adjacent internal loops in r(CCUG)exp, the causative agent of myotonic dystrophy type 2 (DM2), and are transformed into oligomeric, potent inhibitors of DM2 RNA dysfunction by a Huisgen 1,3‐dipolar cycloaddition reaction, a variant of click chemistry. 相似文献
670.
Adaptive Amphiphilic Dendrimer‐Based Nanoassemblies as Robust and Versatile siRNA Delivery Systems
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Xiaoxuan Liu Jiehua Zhou Tianzhu Yu Chao Chen Qiang Cheng Kheya Sengupta Yuanyu Huang Haitang Li Cheng Liu Yang Wang Paola Posocco Menghua Wang Qi Cui Suzanne Giorgio Maurizio Fermeglia Fanqi Qu Sabrina Pricl Yanhong Shi Zicai Liang Palma Rocchi John J. Rossi Ling Peng 《Angewandte Chemie (International ed. in English)》2014,53(44):11822-11827
siRNA delivery remains a major challenge in RNAi‐based therapy. Here, we report for the first time that an amphiphilic dendrimer is able to self‐assemble into adaptive supramolecular assemblies upon interaction with siRNA, and effectively delivers siRNAs to various cell lines, including human primary and stem cells, thereby outperforming the currently available nonviral vectors. In addition, this amphiphilic dendrimer is able to harness the advantageous features of both polymer and lipid vectors and hence promotes effective siRNA delivery. Our study demonstrates for the first time that dendrimer‐based adaptive supramolecular assemblies represent novel and versatile means for functional siRNA delivery, heralding a new age of dendrimer‐based self‐assembled drug delivery in biomedical applications. 相似文献