全文获取类型
收费全文 | 643篇 |
免费 | 21篇 |
国内免费 | 11篇 |
专业分类
化学 | 446篇 |
晶体学 | 6篇 |
力学 | 10篇 |
数学 | 90篇 |
物理学 | 123篇 |
出版年
2023年 | 2篇 |
2022年 | 15篇 |
2021年 | 16篇 |
2020年 | 18篇 |
2019年 | 7篇 |
2018年 | 12篇 |
2017年 | 13篇 |
2016年 | 14篇 |
2015年 | 21篇 |
2014年 | 19篇 |
2013年 | 42篇 |
2012年 | 34篇 |
2011年 | 47篇 |
2010年 | 36篇 |
2009年 | 24篇 |
2008年 | 43篇 |
2007年 | 51篇 |
2006年 | 36篇 |
2005年 | 34篇 |
2004年 | 30篇 |
2003年 | 21篇 |
2002年 | 19篇 |
2001年 | 10篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 7篇 |
1997年 | 7篇 |
1996年 | 2篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1971年 | 2篇 |
1890年 | 1篇 |
排序方式: 共有675条查询结果,搜索用时 31 毫秒
651.
652.
653.
654.
Jacek Namiesnik Kann Vearasilp Alina Nemirovski Hanna Leontowicz Maria Leontowicz Pawel Pasko Alma Leticia Martinez-Ayala Gustavo A. González-Aguilar Milan Suhaj Shela Gorinstein 《Applied biochemistry and biotechnology》2014,172(6):2849-2865
The aim of this study was to investigate the possibility to use the bioactive components from cape gooseberry (Physalis peruviana), blueberry (Vaccinium corymbosum), and cranberry (Vaccinium macrocarpon) extracts as a novel source against oxidation in food supplementation. The quantitative analysis of bioactive compounds (polyphenols, flavonoids, flavanols, carotenoids, and chlorophyll) was based on radical scavenging spectrophometric assays and mass spectrometry. The total phenolic content was the highest (P?<?0.05) in water extract of blueberries (46.6?±?4.2 mg GAE/g DW). The highest antioxidant activities by 2,2-diphenyl-1-picrylhydrazyl radical scavenging assay and Cupric reducing antioxidant capacity were in water extracts of blueberries, showing 108.1?±?7.2 and 131.1?±?9.6 μMTE/g DW with correlation coefficients of 0.9918 and 0.9925, and by β-carotene linoleate assay at 80.1?±?6.6 % with correlation coefficient of 0.9909, respectively. The water extracts of berries exhibited high binding properties with human serum albumin in comparison with quercetin. In conclusion, the bioactive compounds from a relatively new source of gooseberries in comparison with blueberries and cranberries have the potential as food supplementation for human health. The antioxidant and binding activities of berries depend on their bioactive compounds. 相似文献
655.
From Homoconjugated Push–Pull Chromophores to Donor–Acceptor‐Substituted Spiro Systems by Thermal Rearrangement
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Cagatay Dengiz Oliver Dumele Dr. Shin‐ichiro Kato Dr. Michal Zalibera Dr. Pawel Cias Dr. W. Bernd Schweizer Prof. Dr. Corinne Boudon Dr. Jean‐Paul Gisselbrecht Prof. Dr. Georg Gescheidt Prof. Dr. François Diederich 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(5):1279-1286
Series of homoconjugated push–pull chromophores and donor–acceptor (D–A)‐functionalized spiro compounds were synthesized, in which the electron‐donating strength of the anilino donor groups was systematically varied. The structural and optoelectronic properties of the compounds were investigated by X‐ray analysis, UV/Vis spectroscopy, electrochemistry, and computational analysis. The homoconjugated push–pull chromophores with a central bicyclo[4.2.0]octane scaffold were obtained in high yield by [2+2] cycloaddition of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) to N,N‐dialkylanilino‐ or N,N‐diarylanilino‐substituted activated alkynes. The spirocyclic compounds were formed by thermal rearrangement of the homoconjugated adducts. They also can be prepared in a one‐pot reaction starting from DDQ and anilino‐substituted alkynes. Spiro products with N,N‐diphenylanilino and N,N‐diisopropylanilino groups were isolated in high yields whereas compounds with pyrrolidino, didodecylamino, and dimethylamino substituents gave poor yields, with formation of insoluble side products. It was shown by in situ trapping experiments with TCNE that cycloreversion is possible during the thermal rearrangement, thereby liberating DDQ. In the low‐yielding transformations, DDQ oxidizes the anilino species present, presumably via an intermediate iminium ion pathway. Such a pathway is not available for the N,N‐diphenylanilino derivative and, in the case of the N,N‐diisopropylanilino derivative, would generate a strained iminium ion (A1,3 strain). The mechanism of the thermal rearrangement was investigated by EPR spectroscopy, which provides good evidence for a proposed biradical pathway starting with the homolytic cleavage of the most strained (CN)C?C(CN) bond between the fused four‐ and six‐membered rings in the homoconjugated adducts. 相似文献
656.
Hilton CL Crowfoot JM Rempala P King BT 《Journal of the American Chemical Society》2008,130(40):13392-13399
The title compound, 1-Hex, was synthesized by the Zr-mediated biphenylation of 4,4'-dihexyloctabromobiphenyl using (Li(THF)4)2 x Zr(biphe)3, where biphe is the 2,2'-biphenyldiyl ligand, in 5% isolated yield. Two independent X-ray diffraction analyses revealed that arene 1-Hex possesses a highly strained and hindered aryl-aryl single bond. This bond causes the phenanthro[9,10-b]triphenylene (PTP) moieties to twist (anthracene subunit dihedral, 69 degrees); the interlocked, helical, homochiral PTP moieties give rise to effective D2 symmetry. The calculated adiabatic homolytic bond dissociation energy of this strained bond is only 67 kcal/mol, but nonetheless the bond exhibits a surprisingly normal length (1.49 A); the reason is elongation only slowly releases strain. Variable temperature NMR revealed two dynamic processes: hexyl rotation (12.0 +/- 0.4 kcal/mol) and inversion of chirality (15.2 +/- 0.6 kcal/mol). DFT calculations provide rate-determining transitions states, whose energies agree with measured values, and provide insight to the mechanism of these processes. Rotation about the central bond is not involved in either observed process. Calculations demonstrate that rotation does not involve a simple torsion of the equilibrium structure, but rather a complex movement with a barrier of 49 kcal/mol from a slipped-parallel, C(2h) intermediate. 相似文献
657.
Henryk Kozlowski Anna Janicka-Klos Pawel Stanczak Daniela Valensin Gianni Valensin Kinga Kulon 《Coordination chemistry reviews》2008,252(10-11):1069
The prion proteins may play a critical role in copper homeostasis and the antioxidant activity in the brain. This review presents the state of art in the studies on Cu2+ prion systems. The proteins discussed are from different species from mammals to fishes. All proteins are His-rich and the research discussed clearly indicates the basic role of imidazole side chains and the adjacent amide nitrogen atoms in metal ion binding. Prions represent the family of proteins with new mode of Cu2+ binding which includes the amide nitrogen coordination. The multi-imidazole coordination is also likely and it can play a critical role in the antioxidant activity of the copper–prion complexes. The combination of the imidazole and amide nitrogen atoms to Cu2+ ions could also be relevant in histidine-rich peptide antibiotics including demegen. The impact of peptide sequence and His positions on copper binding ability is also discussed. 相似文献
658.
Urban PL Goodall DM Carvalho AZ Bergström ET Van Schepdael A Bruce NC 《Journal of chromatography. A》2008,1206(1):52-63
Active pixel sensor UV area imaging and capacitively coupled contactless conductivity detection have been applied in an electrophoretically mediated microanalysis (EMMA) assay for substrate specificity of tyramine oxidase (Arthrobacter sp.). Use of the UV area imaging detector to monitor four windows in a capillary with three loops provided intrinsic self-referencing for all species and identified tyramine and 2-phenethylamine as the only reactive components in a multi-compound mixture. Continuous engagement EMMA experiments showed significant benefits by comparison with plug-plug EMMA, improving sensitivity by extending enzyme-substrate interaction times and allowing measurement of time-dependent reaction in the substrate zones passing the four windows. 相似文献
659.
Tyrosinase/laccase bienzyme biosensor for amperometric determination of phenolic compounds 总被引:1,自引:0,他引:1
Jolanta Kochana Pawel Nowak Anna Jarosz-Wilkoazka Marta Biero 《Microchemical Journal》2008,89(2):171-174
Tyrosinase/laccase bienzyme biosensor for amperometric determination of phenolic compounds was constructed. Enzymes were immobilized in titania gel matrix. The obtained biosensor was successfully used for determination of 2,6-dimethoxyphenol, 4-tertbutylcatechol, 4-methylcatechol, 3-chlorophenol and catechol. The highest sensitivity and the widest linear range were noticed for catechol, 234 mA L mol− 1 and 2.0 × 10− 7–3.2 × 10− 5 mol/L, respectively. Detection limit for catechol, at signal-to-noise ratio of 3 was 1.3 × 10− 7 mol/L. 相似文献
660.
Using tRNA molecule as an example, we evaluate the applicability of the Poisson-Boltzmann model to highly charged systems such as nucleic acids. Particularly, we describe the effect of explicit crystallographic divalent ions and water molecules, ionic strength of the solvent, and the linear approximation to the Poisson-Boltzmann equation on the electrostatic potential and electrostatic free energy. We calculate and compare typical similarity indices and measures, such as Hodgkin index and root mean square deviation. Finally, we introduce a modification to the nonlinear Poisson-Boltzmann equation, which accounts in a simple way for the finite size of mobile ions, by applying a cutoff in the concentration formula for ionic distribution at regions of high electrostatic potentials. We test the influence of this ionic concentration cutoff on the electrostatic properties of tRNA. 相似文献