全文获取类型
收费全文 | 33668篇 |
免费 | 1175篇 |
国内免费 | 28篇 |
专业分类
化学 | 22062篇 |
晶体学 | 242篇 |
力学 | 667篇 |
数学 | 4584篇 |
物理学 | 6135篇 |
无线电 | 1181篇 |
出版年
2023年 | 302篇 |
2022年 | 197篇 |
2021年 | 417篇 |
2020年 | 572篇 |
2019年 | 501篇 |
2018年 | 748篇 |
2017年 | 711篇 |
2016年 | 1305篇 |
2015年 | 1048篇 |
2014年 | 1108篇 |
2013年 | 2228篇 |
2012年 | 2426篇 |
2011年 | 2548篇 |
2010年 | 1501篇 |
2009年 | 1205篇 |
2008年 | 2183篇 |
2007年 | 2097篇 |
2006年 | 1840篇 |
2005年 | 1657篇 |
2004年 | 1368篇 |
2003年 | 1047篇 |
2002年 | 936篇 |
2001年 | 689篇 |
2000年 | 601篇 |
1999年 | 437篇 |
1998年 | 325篇 |
1997年 | 235篇 |
1996年 | 360篇 |
1995年 | 250篇 |
1994年 | 228篇 |
1993年 | 289篇 |
1992年 | 251篇 |
1991年 | 170篇 |
1990年 | 160篇 |
1989年 | 140篇 |
1988年 | 136篇 |
1987年 | 133篇 |
1986年 | 127篇 |
1985年 | 191篇 |
1984年 | 162篇 |
1983年 | 132篇 |
1982年 | 120篇 |
1981年 | 116篇 |
1980年 | 97篇 |
1979年 | 108篇 |
1978年 | 97篇 |
1977年 | 83篇 |
1976年 | 91篇 |
1975年 | 77篇 |
1973年 | 89篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
Youness Ouahid María C. Zaccaro Gloria Zulpa Mónica Storni Ana M. Stella Juan C. Bossio 《International journal of environmental analytical chemistry》2013,93(6):525-536
Microcystis is one of the most common bloom-forming cyanobacteria genera in diverse ecosystems. More than 80% of its strains are toxic, mainly due to their ability to produce metabolites known as microcystins (MC). Here we report on a M. aeruginosa bloom that appeared in the summer of 2001 at a site of the Río de la Plata, within the city of Buenos Aires. The symptoms in mice indicated that the bloom was hepatotoxic. LC-PDA analysis revealed a similar high concentration (0.9–1?mg?g–1?d w) of just one MC in the bloom biomass during the 3-month study period. During this period most of the MC (ca. 98 %) was found intracellularly, and the content remained almost the same. The molecular mass of the bloom MC was 1036?Da, as deduced from LC-ESI-MS data. Fragment ion analysis by LC-IT-MS-MS allowed identifying 6 out of the 7 amino acids, as well as their position in the molecule. The molecular mass of the unidentified amino acid residue was 155?Da. According to the data obtained, the MC under study was MC-XR, X representing the unidentified amino acid. This is the first report both on the characterisation of MCs in an urban site of the Río de la Plata waters, and on an Argentinean bloom exhibiting only one MC variant. 相似文献
172.
In this study, electron paramagnetic resonance of γ-irradiated single crystals of N-Trifluoroacetyl-α-amino isobutyric acid (F3Ac-Aib-OH) was investigated at room temperature and analyzed for different orientations of the crystal in the magnetic field. The paramagnetic species in N-Trifluoroacetyl-α-aminoisobutyric acid was attributed to the ?F2-R radical (R= CONHC(CH3)COOH). Hyperfine coupling constants and g value were also determined. In addition, the single crystal of F3Ac-Aib-OH was UV-irradiated and paramagnetic species formed was studied at room temperature. The effects of gamma irradiation on fluoroamino acid and stability were discussed. 相似文献
173.
José L. Domínguez M. Carmen Villaverde Fredy Sussman 《Journal of computer-aided molecular design》2013,27(5):403-417
In this work we propose a protocol for estimating the effect of pH on the docking performance to BACE-1, which affords the charge state of the inhibitor as well as the protonation state of all ionisable residues in the protein at a given pH value. To the best of our knowledge, this is the first report of a protocol predicting the BACE-1 ligand docking poses not only at the neutral pH at which most crystallographic structures were obtained, but also at the optimal pH of the enzyme (in the acidic range), at which most of the BACE-1 binding affinity assays are performed. We have applied this protocol to a set of 23 fragment-like BACE-1 ligands that span four orders of magnitude in their binding affinities. The pK a values of the BACE-1 acidic residues deviate substantially from the estimates for model compounds in solution and display a ligand dependent variability, especially in the case of the catalytic Asp dyad residues. This outcome should have a strong bearing on the design of protocols for docking based BACE-1 screening campaigns. Finally, we were able to find an explanation for the poor docking success rate of some fragments based on the availability of anchoring points, a rationale that could help to improve hit rates in BACE-1 screening campaigns. 相似文献
174.
J. A. López Sastre J. F. Rodríguez Amo J. M. Báñez Sanz J. Molina Molina M. C. Romero-Avila A. Sanz Tejedor 《Journal of carbohydrate chemistry》2013,32(3):291-308
Abstract The reaction of 2,3-O-isopropylidene-D-glyceraldehyde with different dialkyl and diarylthiomethyl sulfoxides occurs with a high selectivity and produces the corresponding 1-alkyl(or l-aryl)-sulfynil-l-alkyl(or 1-aryl) thio-3,4-O-isopropylidene-D-tetroses as pure diastereoisomers. We have determined the absolute configuration of the three chiral centers formed in these reactions. 相似文献
175.
María José Camarasa Ana San-Félix María Jesús Pérez-Pérez Sonsoles Velázquez Rosa Alvarez Cristina Chamorro 《Journal of carbohydrate chemistry》2013,32(4-5):451-469
1. INTRODUCTION AIDS will still be one of the most important challenges for the Scientific Community in the approaching new century. Since the identification, in 1983-84,1,2 of human immunodeficiency virus (HIV) as the etiological agent of AIDS, significant progress has been made in the treatment of HIV-infected patients. This has been in part due to the discovery and clinical use of an increasing number of anti-HIV drugs. However, while highly active antiretroviral therapy (HAART)3 approaches have reduced the morbidity and mortality, the intertwined problems of drug induced viral resistance, poor compliance with complex regimens and therapy failure continue. Therefore, there remains a pressing need for the development of new antiviral agents that can be used not only as first line therapeutic candidates, but also in the antiretroviral-experienced patient population. 相似文献
176.
Antonio Lopez Martin Jose Maria Algueta Miguel Lucia Acosta Jaime Ramírez‐Angulo Ramón Gonzalez Carvajal 《ETRI Journal》2011,33(3):393-400
A systematic approach for the design of two‐stage class AB CMOS unity‐gain buffers is proposed. It is based on the inclusion of a class AB operation to class A Miller amplifier topologies in unity‐gain negative feedback by a simple technique that does not modify quiescent currents, supply requirements, noise performance, or static power. Three design examples are fabricated in a 0.5 µm CMOS process. Measurement results show slew rate improvement factors of approximately 100 for the class AB buffers versus their class A counterparts for the same quiescent power consumption (< 200 µW). 相似文献
177.
Ren Y Acuña UM Jiménez F García R Mejía M Chai H Gallucci JC Farnsworth NR Soejarto DD Carcache de Blanco EJ Kinghorn AD 《Tetrahedron》2012,68(12):2671-2678
Six new (1-6) and eight known germacranolide-type sesquiterpene lactones, along with several known phenylpropanol coumarates and methylated flavonoids, were isolated from the leaves of Piptocoma rufescens, collected in the Dominican Republic. The new compounds were identified by analysis of their spectroscopic data, with the molecular structure of 3 being established by single-crystal X-ray diffraction. The absolute configurations of the sesquiterpene lactones isolated were determined from their CD and NOESY NMR spectra, together with the analysis of Mosher ester reactions. Bioassay screening results showed the majority of the sesquiterpene lactones isolated (1-13) to be highly cytotoxic toward the HT-29 human colon cancer cell line, with the most potent compound being 15-deoxygoyazensolide (10, IC(50), 0.26 μM). In addition, several of the sesquiterpene lactones exhibited NF-κB (p65) inhibitory activity. 相似文献
178.
Rubén Marín-Barrios Francisco M. Guerra Ana Leticia García-Cabeza F. Javier Moreno-Dorado Guillermo M. Massanet 《Tetrahedron》2012,68(4):1105-1108
The multivariate optimization method known as simplex is applied to the Kharasch–Sosnovsky allylic oxidation of double bonds. By applying this method, the amounts of three variables (copper source, oxidant, and additive) are optimized at the same time. Under the conditions thus obtained the reaction takes place in a considerable shorter time, being the alkene the limiting reagent. These conditions are applied to some monoterpenes and sesquiterpenes leading regioselectively to the corresponding benzoate esters, opening a route to the employment of this reaction in the synthesis of more complex molecules. 相似文献
179.
Víctor Yañez Rodríguez Miguel Ángel del Águila María José Iglesias Fernando López Ortiz 《Tetrahedron》2012,68(36):7355-7362
Directed ortho lithiation of diphenylphosphinic acid and subsequent electrophilic trapping provides mono ortho-functionalized derivatives including enantiopure γ-aminophosphinic acids in moderate yields. Copper catalyzed coupling of the ortho anion leads to biphenyl-2,2′-diylbis(phenylphosphinic acid), a phosphorus analogue of biphenyl-2,2′-dicarboxylic acid. Preliminary studies of the metal-binding abilities of this O,O-chelating ligand towards a series of metal cations are included. 相似文献
180.