全文获取类型
收费全文 | 11790篇 |
免费 | 947篇 |
国内免费 | 385篇 |
专业分类
13122篇 |
出版年
2023年 | 100篇 |
2022年 | 193篇 |
2021年 | 292篇 |
2020年 | 238篇 |
2019年 | 272篇 |
2018年 | 320篇 |
2017年 | 273篇 |
2016年 | 361篇 |
2015年 | 558篇 |
2014年 | 613篇 |
2013年 | 755篇 |
2012年 | 851篇 |
2011年 | 785篇 |
2010年 | 500篇 |
2009年 | 384篇 |
2008年 | 572篇 |
2007年 | 561篇 |
2006年 | 510篇 |
2005年 | 470篇 |
2004年 | 424篇 |
2003年 | 432篇 |
2002年 | 363篇 |
2001年 | 322篇 |
2000年 | 322篇 |
1999年 | 274篇 |
1998年 | 116篇 |
1997年 | 89篇 |
1996年 | 72篇 |
1995年 | 57篇 |
1994年 | 86篇 |
1993年 | 59篇 |
1992年 | 161篇 |
1991年 | 146篇 |
1990年 | 131篇 |
1989年 | 152篇 |
1988年 | 121篇 |
1987年 | 110篇 |
1986年 | 105篇 |
1985年 | 98篇 |
1984年 | 66篇 |
1983年 | 73篇 |
1982年 | 45篇 |
1979年 | 52篇 |
1977年 | 75篇 |
1976年 | 47篇 |
1975年 | 43篇 |
1974年 | 56篇 |
1973年 | 41篇 |
1971年 | 40篇 |
1969年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Hemoglobin is encapsulated in liposomes of different lipid composition. The resulting dispersion consists primarily of multilamellar liposomes (hemosomes) of a wide particle size distribution (diameter ranging mainly between 0.1 and 1 micron). The encapsulation efficiency is significantly larger with liposomes containing negatively charged lipids as compared to liposomes made of phosphatidylcholine. The integrity of the phospholipid bilayer is maintained in the presence of hemoglobin. The reaction rate of CO binding to encapsulated hemoglobin is reduced compared to that of free hemoglobin, but it is still greater than that observed in red blood cells. Hemoglobin encapsulated in liposomes made from negatively charged phospholipids is less stable than hemoglobin entrapped in isoelectric phosphatidylcholine. The instability of hemoglobin is due to the protein interacting with the negatively charged lipid bilayer. This interaction leads in turn to hemoglobin denaturation, possibly involving the dissociation of the heme group from the heme-globin complex. The nature of the negatively charged phospholipid is important in promoting the interaction with hemoglobin, the effect being in the order phosphatidic acid greater than phosphatidylinositol congruent to phosphatidylglycerol greater than phosphatidylserine. The presence of equimolar amounts of cholesterol in the phospholipid bilayer has a stabilizing effect on hemoglobin. This effect is pronounced with saturated phospholipids, but it is also observed, though to a lesser extent, with unsaturated ones, indicating that the bilayer fluidity has a modulating effect. The presence of cholesterol possibly interferes with secondary interactions following the binding of hemoglobin to the negatively charged lipid bilayer. 相似文献
62.
Kinetics of oxygen and carbon monoxide binding to liver fluke (Dicrocoelium dendriticum) hemoglobin. An extreme case? 总被引:1,自引:0,他引:1
E E Di Iorio U T Meier J D Smit K H Winterhalter 《The Journal of biological chemistry》1985,260(4):2160-2164
The kinetics of oxygen and carbon monoxide binding to the monomeric liver fluke (Dicrocoelium dendriticum) hemoglobin have been studied. The ligand association rates are approximately 1 X 10(8) and approximately 3 X 10(8) M-1 s-1, respectively, for CO and O2 and show no pH dependence. On the contrary the ligand dissociation rates decrease by lowering the pH below 7, the pK of the transition being around 5.5. These findings, together with spectroscopic properties of the protein, are discussed in relation to the fact that, in this hemoglobin, the distal histidine is replaced by a glycine. 相似文献
63.
In the present research we have extended our work on the presence of binding sites for prolactin in the rabbit brain focusing our attention on the brain areas with high dopamine cell bodies density. Among these areas the hypothalamus showed the highest specific binding of labeled ovine prolactin (oPRL). Clearly detectible specific binding was observed also in substantia nigra, whereas in other brain regions the specific binding was very small, except for the striatum where a low but not negligible binding was found in female rabbits. The binding of 125I-oPRL showed a hormonal specificity and Scatchard analysis of the binding showed no clear difference in dissociation constant (Kd) between hypothalamus, nigra and striatum. 相似文献
64.
Structural differences in the cell binding region of human fibronectin molecules isolated from cultured normal and tumor-derived human cells 总被引:2,自引:0,他引:2
Fibronectins isolated from human plasma (pFN) and from the conditioned media of normal (N-cFN) and tumor (T-cFN) human cells were compared by cathepsin D digestion followed by immunostaining of released fragments with the monoclonal antibody 3E3, specific for the cell binding site. Two different staining patterns were obtained, one specific for pFN and N-cFN, the second common to fibronectins from the 3 different kinds of tumors studied. This indicates structural differences between N-cFN and T-cFN in the cell binding region of the fibronectin molecule. 相似文献
65.
A mos oncogene-containing retrovirus, myeloproliferative sarcoma virus, transforms rat thyroid epithelial cells and irreversibly blocks their differentiation pattern. 总被引:8,自引:1,他引:7 下载免费PDF全文
A Fusco G Portella P P Di Fiore M T Berlingieri R Di Lauro A B Schneider G Vecchio 《Journal of virology》1985,56(1):284-292
66.
M Di Giambattista G Ide Y Engelborghs C Cocito 《The Journal of biological chemistry》1984,259(10):6334-6339
The two virginiamycin components VM and VS interact synergistically with bacterial ribosomes in vitro and in vivo. Ribosome affinity for virginiamycin S increases about 10-fold upon incubation with virginiamycin M. This effect has been previously traced by spectrofluorimetric measurement based on the enhancement of virginiamycin S fluorescence upon its binding to the 50 S ribosomal subunit. In the present work the action of two virginiamycin S fluorescence quenchers, acrylamide and iodide, has been explored to gather information about the accessibility of ribosome-bound virginiamycin S and the variation of the accessibility level in the presence of virginiamycin M. Both acrylamide (non-ionized quencher) and iodide (ionized quencher) proved powerful quenchers of free virginiamycin S solutions. Since a comparable effect was obtained on 3- hydroxypicolinamide , the latter was indicated as the part of the molecule involved in the fluorescence effect. Fluorescence quenching by either agent was of the dynamic, i.e. collisional, type. Such an inference was based on the fact that these quenchers merely modified the emission spectrum (not the absorption spectrum), the bimolecular rate constant for the quenching process decreased linearly with the viscosity of the medium (static-type quenching is viscosity-independent), and that linear Stern-Volmer plots were obtained. The quenching ability of both agents underwent a sharp decrease in the presence of ribosomes; however, the Stern-Volmer equation was followed only in the case of acrylamide, whereas Lehrer 's relationship had to be applied in the case of iodide. When ribosomes were incubated with virginiamycin M, the fluorescence quenching ability of acrylamide and iodide was significantly reduced. Conclusions are as follows: a) the 3- hydroxypicolinyl residue of virginiamycin S is buried within an open well on the ribosome surface and is likely to be involved in the interaction with the binding site; b) the accessibility to the well is partly controlled by electrostatic forces; c) interaction of ribosomes with virginiamycin M entails a conformational change whereby the access to the well is reduced. These findings provide a molecular explanation for the previously observed increase of the association constant of virginiamycin S to ribosomes incubated with virginiamycin M which was found to be due to the decrease of the dissociation rate constant (the association rate constant remains practically the same). 相似文献
67.
L. Manzocchi C. Tonelli G. Gavazzi N. Di Fonzo C. Soave 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1986,72(6):778-781
Summary This paper reports that the opaque-6 (o6) mutation of maize, which causes seedling lethality and interferes in the endosperm with the synthesis of zeins and b-32 protein, is a proline requiring mutant functionally allelic to proline-1 (pro-1). Furthermore, immunological studies on the b-32 content of ten independently originated o6 and pro-1 alleles demonstrated that four alleles contain an apparently normal b-32 protein while the others are either devoid of it or contain trace amounts of cross-reacting proteins of lower molecular weight. 相似文献
68.
C Di Ilio A Arduini G Del Boccio G La Rovere G Federici 《Clinical physiology and biochemistry》1986,4(2):120-124
The erythrocytes of multiple sclerosis patients with elevated superoxide dismutase levels were tested for the activities of glutathione redox cycle enzymes. No differences were observed between multiple sclerosis and normal control erythrocytes when the activities were referred to either hemoglobin concentration or lactate dehydrogenase content. Our results indicate that no adaptative changes occur in the activities of glutathione redox cycle enzymes in erythrocytes of multiple sclerosis subjects as a consequence of an elevated superoxide dismutase level. 相似文献
69.
J Di Salvo D Gifford J R Vandenheede W Merlevede 《Biochemical and biophysical research communications》1983,111(3):912-918
A spontaneously active (Mr greater than 350,000) and an ATPMg-dependent phosphatase (Mr congruent to 140,000) were identified in bovine aortic smooth muscle. The spontaneously active phosphatase was effective in dephosphorylating both phosphorylase a (240nmol32P/min/mg) and phosphorylated myosin light chains (1000nmol32P/min/mg). In contrast, the ATPMg-dependent phosphatase was only effective in dephosphorylating phosphorylase a (400nmol32P/min/mg). Phosphorylase phosphatase activity of the ATPMg-dependent enzyme was suppressed by the well-characterized modulator protein (inhibitor-2), whereas the activity of the spontaneously active enzyme was unaffected. The aortic spontaneously active phosphatase did not convert to an ATPMg-dependent form when it was stored at 4 degrees or incubated at 30 degrees C in either the presence or absence of modulator protein. These findings suggest that spontaneous and ATPMg-dependent phosphatase activities described in these studies are probably ascribable to different enzymes. Since both phosphorylase and myosin light chains are phosphorylated when smooth muscle contracts these phosphatases may participate in coordinating arterial contractility and metabolism. 相似文献
70.
In various applications one faces the problem of estimating a signal from discontinuous observations. For example, in biomedical applications the signal may be the ‘state’ of a given organ and one observes through an external counter the amount of radioactivity sequestered by the organ after injection of a radioactive tracer. Here the problem is studied in the context of nonlinear filtering when the signal can be modelled as either a random variable or a diffusion process, and the observations have a continuous and a purely discontinuous component; both components may be affected by the signal. When the signal is a random variable an explicitly computable solution is obtained; for the diffusion case the solution is given as a sequence of approximating filters that can be computed recursively. 相似文献