全文获取类型
收费全文 | 1561篇 |
免费 | 29篇 |
国内免费 | 5篇 |
专业分类
化学 | 966篇 |
晶体学 | 16篇 |
力学 | 24篇 |
数学 | 290篇 |
物理学 | 299篇 |
出版年
2020年 | 16篇 |
2019年 | 21篇 |
2016年 | 20篇 |
2015年 | 15篇 |
2014年 | 20篇 |
2013年 | 35篇 |
2012年 | 50篇 |
2011年 | 41篇 |
2010年 | 41篇 |
2009年 | 46篇 |
2008年 | 57篇 |
2007年 | 53篇 |
2006年 | 40篇 |
2005年 | 49篇 |
2004年 | 42篇 |
2003年 | 36篇 |
2002年 | 29篇 |
2001年 | 23篇 |
2000年 | 28篇 |
1999年 | 27篇 |
1997年 | 16篇 |
1996年 | 16篇 |
1995年 | 14篇 |
1994年 | 21篇 |
1993年 | 20篇 |
1992年 | 22篇 |
1991年 | 14篇 |
1988年 | 12篇 |
1987年 | 17篇 |
1986年 | 12篇 |
1985年 | 25篇 |
1984年 | 16篇 |
1983年 | 14篇 |
1981年 | 12篇 |
1980年 | 14篇 |
1978年 | 13篇 |
1960年 | 12篇 |
1956年 | 16篇 |
1934年 | 26篇 |
1933年 | 15篇 |
1932年 | 29篇 |
1931年 | 24篇 |
1930年 | 21篇 |
1929年 | 13篇 |
1928年 | 16篇 |
1926年 | 23篇 |
1925年 | 11篇 |
1923年 | 19篇 |
1922年 | 16篇 |
1910年 | 11篇 |
排序方式: 共有1595条查询结果,搜索用时 15 毫秒
91.
Covalent attachment of solvent-sensitive fluorescent dyes to proteins is a powerful tool for studying protein conformational changes, ligand binding, or posttranslational modifications. We report here new merocyanine dyes that make possible the quantitation of such protein activities in individual living cells. The quantum yield of the new dyes is sharply dependent on solvent polarity or viscosity, enabling them to report changes in their protein environment. This is combined with other stringent requirements needed in a live cell imaging dye, including appropriate photophysical properties (excitation >590 nm, high fluorescence quantum yield, high extinction coefficient), good photostability, minimal aggregation in water, and excellent water solubility. The dyes were derivatized with iodoacetamide and succinimidyl ester side chains for site-selective covalent attachment to proteins. A novel biosensor of Cdc42 activation made with one of the new dyes showed a 3-fold increase in fluorescence intensity in response to GTP-binding by Cdc42. The dyes reported here should be useful in the preparation of live cell biosensors for a diverse range of protein activities. 相似文献
92.
S. Kallus A. Hahn G. Golemme C. Algieri P. Timmins J.D.F. Ramsay 《Journal of Sol-Gel Science and Technology》2003,26(1-3):721-725
The synthesis of zeolite membranes and thin films using the secondary growth process is briefly described. In this process colloidal zeolite particles (sols) are prepared hydrothermally and then subsequently deposited on substrates to produce uniform layers of controlled thickness, as illustrated with silicalite and zeolite-A. The formation and growth of the zeolite sols has been investigated in situ by small angle neutron scattering (SANS). SANS measurements on silicalite sols at progressively higher concentrations have provided details of the colloid interactions which lead to zeolite gel-layer structures which are uniform and free of defects. 相似文献
93.
H. Schumann F. H. Grlitz F. E. Hahn J. Pickardt Changtao Qian Zuowei Xie 《无机化学与普通化学杂志》1992,609(3):131-138
The partial hydrolysis of [O(CH2CH2C5H4)2]Y(C5H4CH3) 1 , [O(CH2CH2C5H4)2]Y(C5H5) 3 , and [O(CH2CH2C5H4)2]Ho(C5H4CH3) 5 results in the formation of [O(CH2CH2CH2C5H4)2Y(μ-OH)2]2 2 , (C5H5)3Y(OH2) 9 and (MeC5H4)3Ho(OH2) 11 . The new compounds have been characterized by elemental analyses, IR and NMR spectra. The X-ray structural analyses shows 2 to be monoclinic, space group P21/n with a = 1146.0(3), b= 1046.6(3), c = 1514.9(3) pm, β = 94.83(2)°. The molecular structure shows bridging hydroxyl groups with a mean distance Y? O = 223.8(3) pm. 11 crystallizes in the cubic space group 14 3d with a = 1847.9(3)pm with Z = 16 molecules per unit cell. The molecules posses symmetry C3-3, the coordination is trigonal pyramidal with three methylcyclopentadienyl anions and one water molecule as ligands. The distance Ho? O is 231 pm. 相似文献
94.
Klaus Grasshoff und Harry Hahn 《Fresenius' Journal of Analytical Chemistry》1962,186(1):147-153
Zusammenfassung Es wird eine polarographische Methode zur Bestimmung von Molybdän in Stahl beschrieben. Als Trägerelektrolyt wird 0,1 m Citronensäure verwendet. Bei pH 2 treten zwei Stufen mit einem Halbstufenpotential von –0,27 und –0,65 V auf. Der Diffusionsstrom bzw. Spitzenstrom beider Stufen ist gut ausgebildet und gut reproduzierbar. Eisen sowie die üblichen in Stählen vorkommenden Elemente stören die Bestimmung nicht. Die Brauchbarkeit und die Genauigkeit der Methode wird an vier verschiedenen Legierungen mit Gehalten von 0,3–1,3% Molybdän demonstriert.V. Mitteilung: Grasshoff, K., u. H. Hahn: diese Z. 186, 132 (1962).Wir danken der Deutschen Forschungsgemeinschaft sowie dem Fonds der Chemischen Industrie für die Unterstützung dieser Arbeit. 相似文献
95.
96.
W. Lorenz J. Kusche H. Hahn E. Werle 《Fresenius' Journal of Analytical Chemistry》1968,243(1):259-263
Summary The activities of diamine oxidases (DAO) from submaxillary glands, parotid glands and kidneys, from human serum during the pregnancy and from pig serum and of DAO from pea seedlings and the activities of urease and histidine ammonia-lyase have been determined by measuring NH4
+ released from the corresponding substrates. Ammonia has been determined with the aid of glutamate dehydrogenase reaction in the coupled optic test. The specifity and sensitivity of the method, by which still 3×10–9 Mol of NH4
+ could be measured, is excellent, the performance simple and rapid. The sensitivity could be enhanced by fluorometric determination of NAD+ in strong alkali solution. A method for the preparation of tissues for the spectrophotometric test of DAO is described. The importance of DAO determination in different diseases is briefly discussed.
Supported by a grant from Deutsche Forschungsgemeinschaft. 相似文献
Zusammenfassung Die Aktivität von Diaminoxydasen aus Speicheldrüsen und Nieren, aus menschlichem Serum während der Schwangerschaft und Schweineserum und aus Erbsenkeimlingen sowie die Aktivität der Urease und Histidase wurden anhand der NH3-Abspaltung aus ihren entsprechenden Substraten bestimmt. Das entstehende Ammoniak wurde dabei mit Hilfe der Glutamatdehydrogenasereaktion im gekoppelten optischen Test gemessen. Spezifität und Empfindlichkeit der Methode, mit der noch 3 · 10–9 Mol NH4 + bestimmt werden können, sind ausgezeichnet, die Durchführung einfach und rasch. Die Empfindlichkeit konnte außerdem noch durch die fluorimetrische Bestimmung von NAD+ gesteigert werden. Für die Bestimmung der Diaminoxydasen im Gewebe wird eine Methode der Aufarbeitung des Gewebes angegeben, die praktisch keinen Verlust an Gesamtaktivität gegenüber dem Ausgangshomogenat aufweist. Die Bedeutung der Diaminoxydasebestimmung für die Klinik wird kurz diskutiert.
Supported by a grant from Deutsche Forschungsgemeinschaft. 相似文献
97.
98.
Kinetic and polymer analytical results obtained with the systems Me2Si(Ind)2ZrMe2/[B(C6F4Si(i-Pr)3)4]− [(C6H5)3C]+ (2) and Me2Si(Ind)2ZrMe2/[B(C6F5)4]− [(C6H5)3C]+ (3) in olefin polymerizations are compared (Me: methyl; Ind: indenyl). Both systems show that the polymerization rate increases with increasing metallocene concentration in a surplus due to the formation of binuclear species. The expected influence of increasing cation-anion distance on the stereo-errors of the poly(propylene)s when changing from system 3 into system 2 could not be detected. 相似文献
99.
Modifying the Steric Properties in the Second Coordination Sphere of Designed Peptides Leads to Enhancement of Nitrite Reductase Activity 下载免费PDF全文
Dr. Karl J. Koebke Dr. Fangting Yu Elvin Salerno Casey Van Stappen Dr. Alison G. Tebo Prof. James E. Penner‐Hahn Prof. Vincent L. Pecoraro 《Angewandte Chemie (International ed. in English)》2018,57(15):3954-3957
Protein design is a useful strategy to interrogate the protein structure‐function relationship. We demonstrate using a highly modular 3‐stranded coiled coil (TRI‐peptide system) that a functional type 2 copper center exhibiting copper nitrite reductase (NiR) activity exhibits the highest homogeneous catalytic efficiency under aqueous conditions for the reduction of nitrite to NO and H2O. Modification of the amino acids in the second coordination sphere of the copper center increases the nitrite reductase activity up to 75‐fold compared to previously reported systems. We find also that steric bulk can be used to enforce a three‐coordinate CuI in a site, which tends toward two‐coordination with decreased steric bulk. This study demonstrates the importance of the second coordination sphere environment both for controlling metal‐center ligation and enhancing the catalytic efficiency of metalloenzymes and their analogues. 相似文献
100.
2-Iminothiazole, an isoform of 2-aminothiazole, is a scaffold of synthetic and medicinal significance. We have reported an efficient method by which alkyl group of 2-alkyliminothiazoles is changed into other alkyl groups by isothiocyanates. In this article, a detailed mechanistic aspect, and the scopes and limitations were disclosed. All the reactions were carried out in toluene at 105 °C without any additive. The reaction is a reversible process and the equilibrium is determined by the reactivity of both reactants, in which the more electron-withdrawing alkyl or aryl groups at the 2-imino group or isothiocyanate showed higher reactivities. With this simple method, we effectively altered the alkyl group attached on the imine nitrogen. A synthetic problem in 2-iminothiazole chemistry, synthesis of amino acid-derived 2-iminothiazole was solved in a very simple manner. Using suitably designed 2-iminothiazole substrate, the electrophilic reactivity of various isothiocyanates could be empirically compared by this exchange reaction. Moreover, successful exchange reaction using isocyanates instead of isothiocyanates broadened the utility of this reaction. 相似文献