全文获取类型
收费全文 | 801篇 |
免费 | 8篇 |
国内免费 | 2篇 |
专业分类
化学 | 403篇 |
晶体学 | 1篇 |
力学 | 5篇 |
数学 | 61篇 |
物理学 | 341篇 |
出版年
2020年 | 4篇 |
2019年 | 6篇 |
2018年 | 3篇 |
2015年 | 7篇 |
2014年 | 9篇 |
2013年 | 21篇 |
2012年 | 22篇 |
2011年 | 37篇 |
2010年 | 15篇 |
2009年 | 18篇 |
2008年 | 38篇 |
2007年 | 19篇 |
2006年 | 27篇 |
2005年 | 29篇 |
2004年 | 24篇 |
2003年 | 17篇 |
2002年 | 11篇 |
2001年 | 26篇 |
2000年 | 23篇 |
1999年 | 12篇 |
1998年 | 10篇 |
1997年 | 9篇 |
1996年 | 18篇 |
1995年 | 18篇 |
1994年 | 17篇 |
1993年 | 31篇 |
1992年 | 28篇 |
1991年 | 14篇 |
1990年 | 17篇 |
1989年 | 14篇 |
1988年 | 17篇 |
1987年 | 17篇 |
1986年 | 18篇 |
1985年 | 21篇 |
1984年 | 9篇 |
1983年 | 11篇 |
1982年 | 15篇 |
1981年 | 8篇 |
1980年 | 13篇 |
1979年 | 19篇 |
1978年 | 6篇 |
1977年 | 19篇 |
1976年 | 16篇 |
1975年 | 12篇 |
1974年 | 7篇 |
1973年 | 13篇 |
1972年 | 4篇 |
1971年 | 4篇 |
1969年 | 7篇 |
1968年 | 4篇 |
排序方式: 共有811条查询结果,搜索用时 15 毫秒
41.
42.
An inexpensive flow-injection instrument for determining low concentrations of dissolved reactive phosphorus in natural waters is reported. The unique feature is the use of an inexpensive detector consisting of a flow cell and a simple photometer that incorporates a super-bright light-emitting diode as the source and a photodiode as the detector. The tin(II) chloride-molybdate method was optimized using a modified simplex optimization method. Silicate interference up to 5 mg Si l?1 was removed by addition of 0.10% (w/v) tartaric acid. Using the tartaric acid-modified optimized reagents, a detection limit of 0.6 μg P l?1 was achieved. The method was linear over the range 0–100 μg P l?1 with an excellent precision (r.s.d. 2.9% at 2.0 and 0.5% at 50 μg P l?1). An in-line pre-concentration anion-exchange column was used to obtain an even lower detection limit of 0.1 μg P l?1 and applied to the analysis of real samples. 相似文献
43.
44.
The addition of lithium benzylthiolates to Nα,-formyl-α,β-dehydro-α-amino acid esters gave protected Nα-formyl-β-S-benzyl-β,β-dialkylcysteine esters. Hydrolysis and Nα-protection produced β,β-dialkyl-cysteines which can be used for peptide synthesis. 相似文献
45.
Radical S‐Adenosyl Methionine Epimerases: Regioselective Introduction of Diverse D‐Amino Acid Patterns into Peptide Natural Products
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)
Dr. Brandon I. Morinaka Dr. Anna L. Vagstad Maximilian J. Helf Dr. Muriel Gugger Dr. Carsten Kegler Dr. Michael F. Freeman Prof. Dr. Helge B. Bode Prof. Dr. Jörn Piel 《Angewandte Chemie (International ed. in English)》2014,53(32):8503-8507
PoyD is a radical S‐adenosyl methionine epimerase that introduces multiple D ‐configured amino acids at alternating positions into the highly complex marine peptides polytheonamide A and B. This novel post‐translational modification contributes to the ability of the polytheonamides to form unimolecular minimalistic ion channels and its cytotoxic activity at picomolar levels. Using a genome mining approach we have identified additional PoyD homologues in various bacteria. Three enzymes were expressed in E. coli with their cognate as well as engineered peptide precursors and shown to introduce diverse D ‐amino acid patterns into all‐L peptides. The data reveal a family of architecturally and functionally distinct enzymes that exhibit high regioselectivity, substrate promiscuity, and irreversible action and thus provide attractive opportunities for peptide engineering. 相似文献
46.
I. Pinnau C. G. Casillas A. Morisato B. D. Freeman 《Journal of Polymer Science.Polymer Physics》1996,34(15):2613-2621
The gas permeation properties of poly(1-trimethylsilyl-1-propyne) (PTMSP), poly(1-phenyl-1-propyne) (PPP), and blends of PTMSP and PPP have been determined with hydrocarbon/hydrogen mixtures. For a glassy polymer, PTMSP has unusual gas permeation properties which result from its very high free volume. Transport in PPP is similar to that observed in conventional, low-free-volume glassy polymers. In experiments with n-butane/hydrogen gas mixtures, PTMSP and PTMSP/PPP blend membranes were more permeable to n-butane than to hydrogen. PPP, on the other hand, was more permeable to hydrogen than to n-butane. As the PTMSP composition in the blend increased from 0 to 100%, n-butane permeability increased by a factor of 2600, and n-butane/hydrogen selectivity increased from 0.4 to 24. Thus, both hydrocarbon permeability and hydrocarbon/hydrogen selectivity increase with the PTMSP content in the blend. The selectivities measured with gas mixtures were markedly higher than selectivities calculated from the corresponding ratio of pure gas permeabilities. The difference between mixed gas and pure gas selectivity becomes more pronounced as the PTMSP content in the blend increases. The mixed gas selectivities are higher than pure gas selectivities because the hydrogen permeability in the mixture is much lower than the pure hydrogen permeability. For example, the hydrogen permeability in PTMSP decreased by a factor of 20 as the relative propane pressure (p/psat) in propane/hydrogen mixtures increased from 0 to 0.8. This marked reduction in permanent gas permeability in the presence of a more condensable hydrocarbon component is reminiscent of blocking of permanent gas transport in microporous materials by preferential sorption of the condensable component in the pores. The permeability of PTMSP to a five-component hydrocarbon/hydrogen mixture, similar to that found in refinery waste gas, was determined and compared with published permeation results for a 6-Å microporous carbon membrane. PTMSP exhibited lower selectivities than those of the carbon membrane, but permeability coefficients in PTMSP were nearly three orders of magnitude higher. © 1996 John Wiley & Sons, Inc. 相似文献
47.
A. Morisato H. C. Shen S. S. Sankar B. D. Freeman I. Pinnau C. G. Casillas 《Journal of Polymer Science.Polymer Physics》1996,34(13):2209-2222
Pure gas and hydrocarbon vapor transport properties of blends of two glassy, polyacetylene-based polymers, poly(1-trimethylsilyl-1-propyne) [PTMSP] and poly(1-phenyl-1-propyne) [PPP], have been determined. Solid-state CP/MAS NMR proton rotating frame relaxation times were determined in the pure polymers and the blends. NMR studies show that PTMSP and PPP form strongly phase-separated blends. The permeabilities of the pure polymers and each blend were determined with hydrogen, nitrogen, oxygen, carbon dioxide, and n-butane. PTMSP exhibits unusual gas and vapor transport properties which result from its extremely high free volume. PTMSP is more permeable to large organic vapors, such as n-butane, than to small, permanent gases, such as hydrogen. PPP exhibits gas permeation characteristics of conventional low free volume glassy polymers; PPP is more permeable to hydrogen than to n-butane. In PTMSP/PPP blends, both n-butane permeability and n-butane/hydrogen selectivity increase as the PTMSP content of the blends increases. © 1996 John Wiley & Sons, Inc. 相似文献
48.
Wilson PF Freeman CG McEwan MJ Milligan DB Allardyce RA Shaw GM 《Rapid communications in mass spectrometry : RCM》2002,16(5):427-432
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
49.
50.