全文获取类型
收费全文 | 123篇 |
免费 | 2篇 |
国内免费 | 1篇 |
专业分类
化学 | 52篇 |
晶体学 | 1篇 |
数学 | 28篇 |
物理学 | 45篇 |
出版年
2017年 | 2篇 |
2016年 | 2篇 |
2015年 | 3篇 |
2014年 | 1篇 |
2013年 | 5篇 |
2012年 | 5篇 |
2011年 | 7篇 |
2010年 | 5篇 |
2009年 | 3篇 |
2008年 | 6篇 |
2007年 | 1篇 |
2006年 | 1篇 |
2005年 | 2篇 |
2003年 | 3篇 |
2002年 | 3篇 |
2001年 | 1篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1994年 | 3篇 |
1993年 | 6篇 |
1992年 | 6篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1967年 | 2篇 |
1965年 | 3篇 |
1962年 | 2篇 |
1955年 | 5篇 |
1954年 | 6篇 |
1932年 | 3篇 |
1930年 | 1篇 |
1922年 | 2篇 |
排序方式: 共有126条查询结果,搜索用时 0 毫秒
101.
Analytical methodologies with ultrasonic extraction and liquid chromatography (LC) were developed for the determination of phenolic compounds in dietary supplements containing Echinacea. The phenolic compounds determined by these methods included caftaric acid, chlorogenic acid, cynarin, echinacoside, and cichoric acid. Samples from tablets, capsules, and bags of tea blends were extracted by sonication for < or = 30 min with methanol-water (60 + 40). The extracts were centrifuged and filtered, and the filtrates were diluted and analyzed by LC using a reversed-phase column and coulometric electrochemical (EC) detection. The mobile phase was acetonitrile-ammonium formate buffer, pH 3.5 (15.3 + 84.7) containing tetrabutyl ammonium hydrogen sulfate as an ion-pairing reagent. Extraction conditions (e.g., composition of the extraction solvent and sonication time) were optimized for different types of samples. Intra- and interday analytical variations were determined, and intraday analyses were performed by 2 independent analysts using 2 different LC systems. Results were generally comparable. The LC method with EC detection showed better sensitivity and selectivity when compared with LC with ultraviolet detection, although results were similar for the 2 methods for major compounds, i.e., caftaric acid, echinacoside, and cichoric acid. The identities of these major compounds found in samples were confirmed by LC/electrospray ionization mass spectrometry. 相似文献
102.
Henrike Gehring Ramona Metzinger Christian Herwig Julia Intemann Prof. Dr. Sjoerd Harder Prof. Dr. Christian Limberg 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(5):1629-1636
After the lithiation of PYR‐H2 (PYR2?=[{NC(Me)C(H)C(Me)NC6H3(iPr)2}2(C5H3N)]2?), which is the precursor of an expanded β‐diketiminato ligand system with two binding pockets, its reaction with [NiBr2(dme)] led to a dinuclear nickel(II)–bromide complex, [(PYR)Ni(μ‐Br)NiBr] ( 1 ). The bridging bromide ligand could be selectively exchanged for a thiolate ligand to yield [(PYR)Ni(μ‐SEt)NiBr] ( 3 ). In an attempt to introduce hydride ligands, both compounds were treated with KHBEt3. This treatment afforded [(PYR)Ni(μ‐H)Ni] ( 2 ), which is a mixed valent NiI? μ‐H? NiII complex, and [(PYR‐H)Ni(μ‐SEt)Ni] ( 4 ), in which two tricoordinated NiI moieties are strongly antiferromagnetically coupled. Compound 4 is the product of an initial salt metathesis, followed by an intramolecular redox process that separates the original hydride ligand into two electrons, which reduce the metal centres, and a proton, which is trapped by one of the binding pockets, thereby converting it into an olefin ligand on one of the NiI centres. The addition of a mild acid to complex 4 leads to the elimination of H2 and the formation of a NiIINiII compound, [(PYR)Ni(μ‐SEt)NiOTf] ( 5 ), so that the original NiII(μ‐SEt)NiIIX core of compound 3 is restored. All of these compounds were fully characterized, including by X‐ray diffraction, and their molecular structures, as well as their formation processes, are discussed. 相似文献
103.
C A Shaw-Reid N L Kelleher H C Losey A M Gehring C Berg C T Walsh 《Chemistry & biology》1999,6(6):385-400
BACKGROUND: EntF is a 142 kDa four domain (condensation-adenylation-peptidyl carrier protein-thioesterase) nonribosomal peptide synthetase (NRPS) enzyme that assembles the Escherichia coli N-acyl-serine trilactone siderophore enterobactin from serine, dihydroxybenzoate (DHB) and ATP with three other enzymes (EntB, EntD and EntE). To assess how EntF forms three ester linkages and cyclotrimerizes the covalent acyl enzyme DHB-Ser-S-PCP (peptidyl carrier protein) intermediate, we mutated residues of the proposed catalytic Ser-His-Asp triad of the thioesterase (TE) domain. RESULTS: The Ser1138-->Cys mutant (kcat decreased 1000-fold compared with wild-type EntF) releases both enterobactin (75%) and linear (DHB-Ser)2 dimer (25%) as products. The His 1271-->Ala mutant (kcat decreased 10,000-fold compared with wild-type EntF) releases only enterobactin, but accumulates both DHB-Ser-O-TE and (DHB-Ser)2-O-TE acyl enzyme intermediates. Electrospray ionization and Fourier transform mass spectrometry of proteolytic digests were used to analyze the intermediates. CONCLUSIONS: These results establish that the TE domain of EntF is both a cyclotrimerizing lactone synthetase and an elongation catalyst for ester-bond formation between covalently tethered DHB-Ser moieties, a new function for chain-termination TE domains found at the carboxyl termini of multimodular NRPSs and polyketide synthases. 相似文献
104.
105.
106.
107.
108.
109.
110.
F. Honcamp H. Pringsheim A. Steingroever W. Halden A. Schäffner K. Boresch H. Ullrich H. Stremme A. Gehring C. Stapp A. Rippel E. Hiltner E. K. Feichtinger E. Möller-Arnold J. Hasenbäumer H. Niklas F. Dühring H. Neubauer H. Wiessmann Margarethev Wrangell Fürstin Andronikow A. Suchier K. K. Gedroiz H. Kuron 《Analytical and bioanalytical chemistry》1932,87(3-4):156-157