全文获取类型
收费全文 | 19492篇 |
免费 | 1382篇 |
国内免费 | 5篇 |
出版年
2023年 | 179篇 |
2022年 | 184篇 |
2021年 | 640篇 |
2020年 | 464篇 |
2019年 | 626篇 |
2018年 | 690篇 |
2017年 | 652篇 |
2016年 | 842篇 |
2015年 | 1145篇 |
2014年 | 1184篇 |
2013年 | 1408篇 |
2012年 | 1592篇 |
2011年 | 1511篇 |
2010年 | 955篇 |
2009年 | 810篇 |
2008年 | 1016篇 |
2007年 | 958篇 |
2006年 | 858篇 |
2005年 | 748篇 |
2004年 | 677篇 |
2003年 | 615篇 |
2002年 | 543篇 |
2001年 | 315篇 |
2000年 | 273篇 |
1999年 | 243篇 |
1998年 | 117篇 |
1997年 | 126篇 |
1996年 | 92篇 |
1995年 | 76篇 |
1994年 | 70篇 |
1993年 | 70篇 |
1992年 | 124篇 |
1991年 | 95篇 |
1990年 | 73篇 |
1989年 | 71篇 |
1988年 | 75篇 |
1987年 | 75篇 |
1986年 | 56篇 |
1985年 | 56篇 |
1984年 | 67篇 |
1983年 | 41篇 |
1982年 | 36篇 |
1981年 | 45篇 |
1980年 | 31篇 |
1979年 | 43篇 |
1977年 | 28篇 |
1976年 | 34篇 |
1975年 | 38篇 |
1974年 | 28篇 |
1973年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
21.
22.
Harry Le Vine Pedro Cuatrecasas 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,672(3):248-261
A cytosolic, macromolecular factor required for the cholera toxin-dependent activation of pigeon erythrocyte adenylate cyclase and cholera toxin-dependent ADP-ribosylation of a membrane-bound 43 000 dalton polypeptide has been purified 1100-fold from horse erythrocyte cytosol using organic solvent precipitation and heat treatment. This factor, 13 000 daltons, does not absorb to anionic or cationic exchange resins, is sensitive to trypsin or 10% trichloroacetic acid and is not extractable by diethyl ether. Activation of adenylate cyclase by cholera toxin requires the simultaneous presence of ATP (including possible trace GTP), NAD+, dithiothreitol, cholera toxin, membranes and the cytosolic macromolecular factor. Reversal of cholera toxin activation of adenylate cyclase, and of the toxin-dependent ADP-ribosylation, requires the presence of the cytosolic factor. The ability of the purified cytosolic factor to influence the hormonal sensitivity of liver membrane adenylate cyclase may provide clues to its physiological functions. 相似文献
23.
The synthesis and characterization of rare-earth (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) thiocyanate adducts with tripiperidinophosphine oxide (tpppO) with general formula (RE)(SCN)3(tpppO)3 are reported. Conductance measurements in acetonitrile indicate the non-electrolytic nature of the complexes. Infrared absorption spectra evidence that the SCN− ion coordinates through the nitrogen atom (isothiocyanate form) and that tpppO coordinates through the phosphoryl oxygen. X-ray powder patterns suggest the existence of three different crystal forms: (1) La; (2) an isomorphous series including Ce, Nd and Pr; and (3) another isomorphous series, including Sm, Gd, Eu, Ho, Er, Tb, Lu and Y. The visible spectra of the Nd adduct and the calculated parameters β = 0.98, b1/2 = 0.072 and δ = 1.06 indicate that the metal-ligand bonds are essentially electrostactic. The emission spectra of the Eu compound showed 5D0 → 7FJ bands (J = 0, 1, 2), suggesting a C3v symmetry for the coordination polyhedron. The lifetime of the 5D0 state is 1.28 ms. The emission spectra of the Tb complex presented 5D4 → 7FJ bands (J = 4, 5, 6) and the Dy complex showed the 4F9/2 → 6H13/2 band. The structure of the Pr complex showed that the coordination polyhedron is a trigonal antiprism, with the isothiocyanate anions in one base and three tpppO ligands in the other. Thermal analyses (TG-DTG) were carried out for the Ce, Nd and Gd adducts. Mass losses start between 250 and 334 °C. The final residues at 1300 °C are the corresponding phosphates. 相似文献
24.
25.
There are an increasing number of studies that are now focussing on the influence of climate change on species’ distributions. However, access to predictive climatic datasets for future scenarios is difficult due to their specific formats and/or the need to be geographically downscaled. The TYN dataset is freely available to users and provides a synthetic format with several climatic models and IPCC future climate scenarios. Moreover, the CRU historical dataset (1901–2000) is also available which allows users to create baseline models for current climatic variables. E‐clic is a free, user‐friendly software package that offers three different ways to convert these two datasets into a spatially explicit raster format which is compatible with the most common geographic information systems and usable on different platforms. 相似文献
26.
G. Matthew Hebdon Harry Le Vine Naji E. Sahyoun Claus J. Schmitges Pedro Cuatrecasas 《Life sciences》1980,26(17):1385-1396
3T3C2 mouse fibroblasts rendered permeable to (α?32P)NAD+ show cholera toxin-dependent labeling of a 45,000 m.w. protein and of a doublet of polypeptides around 52,000 m.w. These same bands are ADP-ribosylated in broken cells. Membranes prepared from pigeon erythrocytes pretreated with choleragen show a decrease in subsequent cholera toxin-specific ADP-ribosylation of a 43,000 m.w. polypeptide. Both whole cell and broken cell adenylate cyclase activation and toxin-specific ADP-ribosylation are reversed specifically by low pH and high concentrations of toxin and nicotinamide in all systems. Thus ADP-ribosylation appears to be relevant to the molecular action of choleragen in whole cells as well as in broken cells. 相似文献
27.
28.
29.
David W. Thomas Amabile K. Matida Célio L. Silva Thuioshi Ioneda 《Chemistry and physics of lipids》1979,23(3):267-282
The purification procedure of 6,6′-diesters of trehalose from Corynebacterium diphtheriae was modified and the isolated substance was analysed by mass spectrometry as its permethylated derivative. The fatty acid moiety released from the glycolipid after alkaline hydrolysis was studied by mass spectral analysis of the O-methylated and O-acetylated methyl ester derivatives. By argentation thin-layer chromatography, three species of O-acetylated methyl esters were recognized, corresponding to saturated, mono-unsaturated and di-unsaturated α-branched-β-hydroxylated fatty acids. The double bond was located by ozonolysis of the O-acetylated methyl ester derivatives, by gas chromatography of the reaction product and mass spectrometry of the effluent from the gas chromatograph. The main components of each species of α-branched-β-hydroxylated fatty acids found in the gly colipid fraction of C. diphtheriae were 2-tetradecyl-3-hydroxyoctadecanoic acid (C32H64O3, corynomycolic acid), 2-tetradecyl-3-hydroxy-11-octadecenoic acid (C32H62O3, corynomycolenic acid), 2-tetradec-7′-enyl-3-hydroxy octadecanoic acid (C32H62O3) and 2-tetradec-7′-enyl-3-hydroxy-11-octadecenoic acid (C32H60O3, corynomycoldienic acid). The glycolipid fraction from C. diphtheriae is obviously a complex mixture of 6,6′-diesters of trehalose. 相似文献
30.