全文获取类型
收费全文 | 337篇 |
免费 | 10篇 |
国内免费 | 3篇 |
专业分类
化学 | 277篇 |
力学 | 8篇 |
数学 | 43篇 |
物理学 | 22篇 |
出版年
2023年 | 2篇 |
2022年 | 18篇 |
2021年 | 6篇 |
2020年 | 8篇 |
2019年 | 4篇 |
2018年 | 9篇 |
2017年 | 2篇 |
2016年 | 15篇 |
2015年 | 17篇 |
2014年 | 9篇 |
2013年 | 21篇 |
2012年 | 20篇 |
2011年 | 21篇 |
2010年 | 8篇 |
2009年 | 10篇 |
2008年 | 20篇 |
2007年 | 20篇 |
2006年 | 19篇 |
2005年 | 17篇 |
2004年 | 25篇 |
2003年 | 10篇 |
2002年 | 14篇 |
2001年 | 5篇 |
2000年 | 1篇 |
1999年 | 4篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1973年 | 1篇 |
排序方式: 共有350条查询结果,搜索用时 0 毫秒
21.
DNA sequencing is performed in a multiplexed capillary electrophoresis system by UV absorption detection. Four individual electropherograms are obtained by simultaneously running the unlabeled DNA products of the four ddNTP-terminated reactions in the capillary array. The sequence of the template used in the cycle-sequencing reaction can be determined by overlaying the four electropherograms. Two internal standards are employed to adjust for the variance in migration times among the capillaries. After applying the correction algorithm, base calling can be done at a high level of confidence. 相似文献
22.
As taurine is a very important compound involved in a large number of metabolic processes, it is naturally present in the mammal tissues and is often deliberately added in some foods as a fortifying component. A detailed knowledge of taurine metabolic roles in biological systems can be obtained only if a sensitive, reliable and rapid analytical method is available. This article describes the successful application of high-performance anion-exchange chromatography coupled with integrated pulsed amperometric detection (HPAEC-IPAD) for taurine determination in egg white and yolk samples, as well extracts of human serum and urine. Applications are shown for determination of taurine in soft drinks and pharmaceutical preparations where the taurine content was evaluated by standard additions. These results were achieved without prior derivatization of taurine. 相似文献
23.
Carnosine (beta-alanyl-L-histidine) is a dipeptide regarded as an important molecular marker of the presence of processed animal proteins including meat and bone meal in animal feed. For its identification and quantification a sensitive and selective method by high-performance anion-exchange chromatography coupled with integrated pulsed amperometric detection (HPAEC-IPAD) was developed. The assay is based on isocratic elution with 100 mM NaOH as the mobile phase. Interferences of real matrices were efficiently removed; carnosine could be determined at the concentration ranges 0.1-100 microM with a rather low detection limit of 0.23 ng. Unlike feeds for dogs and cats, no carnosine peak was observed in all examined feeds for ruminants. The good analytical characteristics allowed camosine determination down to 5 microg/g of feed. 相似文献
24.
Donatella Diana Dr. Barbara Ziaco Dr. Guido Scarabelli Dr. Carlo Pedone Prof. Giorgio Colombo Dr. Luca D. D'Andrea Dr. Roberto Fattorusso Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(18):5400-5407
The analysis of the folding mechanism in peptides adopting well‐defined secondary structure is fundamental to understand protein folding. Herein, we describe the thermal unfolding of a 15‐mer vascular endothelial growth factor mimicking α‐helical peptide (QKL10A) through the combination of spectroscopic and computational analyses. In particular, on the basis of the temperature dependencies of QKL10A Hα chemical shifts we show that the first phase of the thermal helix unfolding, ending at around 320 K, involves mainly the terminal regions. A second phase of the transition, ending at around 333 K, comprises the central helical region of the peptide. The determination of high‐resolution QKL10A conformational preferences in water at 313 K allowed us to identify, at atomic resolution, one intermediate of the folding–unfolding pathway. Molecular dynamics simulations corroborate experimental observations detecting a stable central helical turn, which represents the most probable site for the helix nucleation in the folding direction. The data presented herein allows us to draw a folding–unfolding picture for the small peptide QKL10A compatible with the nucleation–propagation model. This study, besides contributing to the basic field of peptide helix folding, is useful to gain an insight into the design of stable helical peptides, which could find applications as molecular scaffolds to target protein–protein interactions. 相似文献
25.
The natural radioactivity in phosphate fertilizers used in central Italy has been measured by alpha and gamma spectrometry. The results show that the specific activities of 238U, 234U and 235U ranged within interval from < 0.5 to 2628.8, from < 0.5 to 2826.3 and from < 0.9 to 138.0 Bq kg− 1 respectively and those of 210Pb and 210Po ranged within interval from < 15.0 to 401.5 and < 0.5 to 351.6 Bq kg− 1 respectively. The 228Th, 230Th and 232Th concentrations found by alpha spectrometry ranged from < 0.5 to 50.2, from 0.9 to 2727.2 and from < 0.5 to 60.8 Bq kg− 1 respectively. 226Ra and 228Ra activity concentrations ranged from < 14.3 to 290.9 and from < 2.9 to 24.9 respectively. 40K specific activities ranged from 39.2 to 8263.7 Bq kg− 1. A very good correlation exists between the activity concentrations found by alpha spectrometry and those found by gamma spectrometry. 相似文献
26.
Giorgio Abbiati Alessandro Contini Donatella Nava Elisabetta Rossi 《Tetrahedron》2009,65(24):4664-3418
The cycloaddition reactions of ‘all-carbon’ 1,3-diazabuta-1,3-dienes with a few conjugated and unconjugated alkynyl ketenes are described. The reactions provide some interesting azetidinones and dihydropyrimidinones bearing an alkynyl moiety. The regiochemistry of cycloadduct is related with the degree of conjugation of the alkynyl ketene. Moreover, two alternative approaches to ‘all-carbon’ 1,3-diazabuta-1,3-dienes are reported. 相似文献
27.
Donatella Giomi 《Tetrahedron》2009,65(34):7048-9541
A peculiar thermal behaviour of hydroxyallylpyridyl derivatives, likely associated to the weak acidity of the ‘picoline-type’ hydrogen atom and responsible for the formation of allyl inversion products, has been reported. The ‘mobility’ of the same hydrogen atom allowed the unprotected title compound to behave regioselectively as C-1, C-2 or C-3 carbon nucleophile depending on the thermal or base-promoted experimental conditions and on the kind of electrophile; moreover, the corresponding Hantzsch-type pyridine tautomer displayed a biomimetic ability to transfer hydrogen to aromatic and heteroaromatic nitro derivatives. 相似文献
28.
Licchelli M Linati L Orbelli Biroli A Perani E Poggi A Sacchi D 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(22):5161-5169
The new quadridentate bischelating ligands 2 and 3 display in solution the typical absorption and emission properties expected for naphthalenediimide derivatives. Spectrophotometric studies show that systems 2 and 3 interact with Zn(II), Cd(II) and Cu(I) in CHCl3 or MeCN according to an apparent 1:1 stoichiometry. Molecular modelling, ESI-MS and 1H NMR experiments indicate that the complex species formed in the presence of stoichiometric amounts of metal ion are nonhelical [2 + 2] adducts. The metal-induced self-assembling process is signalled by an intense excimer-type emission caused by the intramolecular interaction of two naphthalenediimide subunits that face each other in the [2 + 2] adduct, as shown by molecular modelling studies. In the presence of excess metal ion, a disassembling process takes place, leading to a dinuclear complex with a 2:1 metal/ligand stoichiometry, in which the intramolecular excimer is no longer allowed to form because the interaction between the naphthalenediimide subunits has been lost. Thus, the overall metal-induced assembling/disassembling process is signalled by the appearance and disappearance of the excimer band in the emission spectrum. 相似文献
29.
Bernardi A Potenza D Capelli AM García-Herrero A Cañada FJ Jiménez-Barbero J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(20):4598-4612
As a step to delineate a strategy of ligand design for cholera toxin (CT), NMR studies were performed on several mimics of the GM1 ganglioside oligosaccharide. The conformation of these analogues was investigated first in solution and then upon binding to cholera toxin by transferred nuclear Overhauser effect (TR-NOE) measurements. It was demonstrated that CT selects a conformation similar to the global minima of the free saccharides from the ensemble of presented conformations. No evidence of major conformational distortions was obtained, but one or two of the available conformers of the hydroxyacid side chain appear to be selected in the bound state. The NMR data were interpreted with the aid of computer models, generated and analyzed by using a combination of different approaches (MacroModels' MC/EM and MC/SD, Autodock, and GRID). Analysis of the NMR data supported by computational studies allowed us to interpret the experimental observations and to derive workable models of the ligand:toxin complexes. These models suggest that the higher affinity of the (R)-lactic acid derivative 3 may stem from lipophilic interactions with a hydrophobic area in the toxin binding site located in the vicinity of the sialic acid side chain binding region of the CT:GM1 complex, and formed by the side chain of Ile-58 and Lys-34. Thus, the models obtained have allowed us to make useful design suggestions for the improvement of ligand affinity. 相似文献
30.
Fabbrizzi L Foti F Licchelli M Maccarini PM Sacchi D Zema M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(21):4965-4972
A cyclam-like macrocycle has been synthesized with a pendant arm containing a dansylamide group. In the corresponding nickel(II) complex, binding of the pendant arm to the metal is pH controlled. In particular, at pH 4.3, the sulfonamide group deprotonates and coordinates the NiII center, giving rise to a complex of trigonal bipyramidal stereochemistry, as shown by X-ray diffraction studies performed on the crystalline complex salt. At pH 7.5, an OH- ion binds the metal and a six-coordinate species forms. The binding-detachment of the pendant arm to/from the NiII center is signaled by changes in the emission properties of the dansyl subunit in the side chain; the fluorescence of this side chain is high when the pendant arm is not coordinated and low when the sulfonamide group is bound to the metal. The system investigated represents the prototype of a light-emitting molecular machine, driven by a pH change. 相似文献