全文获取类型
收费全文 | 397126篇 |
免费 | 158128篇 |
国内免费 | 29853篇 |
出版年
2018年 | 5176篇 |
2017年 | 4809篇 |
2016年 | 6101篇 |
2015年 | 7759篇 |
2014年 | 8730篇 |
2013年 | 11317篇 |
2012年 | 12827篇 |
2011年 | 13267篇 |
2010年 | 11272篇 |
2009年 | 15376篇 |
2008年 | 12763篇 |
2007年 | 13087篇 |
2006年 | 11358篇 |
2005年 | 11069篇 |
2004年 | 10755篇 |
2003年 | 10074篇 |
2002年 | 10805篇 |
2001年 | 22169篇 |
2000年 | 20257篇 |
1999年 | 20203篇 |
1998年 | 12178篇 |
1997年 | 12130篇 |
1996年 | 11309篇 |
1995年 | 11416篇 |
1994年 | 10830篇 |
1993年 | 10278篇 |
1992年 | 18021篇 |
1991年 | 17706篇 |
1990年 | 17904篇 |
1989年 | 16842篇 |
1988年 | 15536篇 |
1987年 | 14168篇 |
1986年 | 13091篇 |
1985年 | 12413篇 |
1984年 | 9883篇 |
1983年 | 8360篇 |
1982年 | 7193篇 |
1981年 | 6435篇 |
1980年 | 6115篇 |
1979年 | 9038篇 |
1978年 | 7221篇 |
1977年 | 6707篇 |
1976年 | 6165篇 |
1975年 | 6353篇 |
1974年 | 6806篇 |
1973年 | 6682篇 |
1972年 | 6626篇 |
1971年 | 5856篇 |
1970年 | 5250篇 |
1969年 | 5166篇 |
排序方式: 共有10000条查询结果,搜索用时 359 毫秒
131.
132.
S-Protein/vitronectin is a serum glycoprotein that inhibits the lytic activity of the membrane attack complex of complement, i.e., of the complex including the proteins C5b, C6, C7, C8, and C9n. We show that intact S-protein/vitronectin or its cyanogen bromide generated fragments also inhibit the hemolysis mediated by perforin from cytotoxic T-cells at 45 and 11 microM, respectively. The glycosaminoglycan binding site of S-protein/vitronectin is responsible for the inhibition, since a synthetic peptide corresponding to a part of this highly basic domain (amino acid residues 348-360) inhibits complement- as well as perforin-mediated cytolysis. In the case of C9, the synthetic peptide binds to the acidic residues occurring in its N-terminal cysteine-rich domain (residues 101-111). Antibodies raised against this particular segment react 25-fold better with the polymerized form of C9 as compared with its monomeric form, indicating that this site becomes exposed only upon the hydrophilic-amphiphilic transition of C9. Since the cysteine-rich domain of C9 has been shown to be highly conserved in C6, C7, and C8 as well as in perforin, the inhibition of the lytic activities of these molecules by S-protein/vitronectin or by peptides corresponding to its heparin binding site may be explained by a similar mechanism. 相似文献
133.
The oxidative half-reaction of phenol hydroxylase has been studied by stopped-flow spectrophotometry. Three flavin-oxygen intermediates can be detected when the substrate is thiophenol, or m-NH2, m-OH, m-CH3, m-Cl, or p-OH phenol. Intermediate I, the flavin C(4a)-hydroperoxide, has an absorbance maximum at 380-390 nm and an extinction coefficient approximately 10,000 M-1 cm-1. Intermediate III, the flavin C(4a)-hydroxide, has an absorbance maximum at 365-375 nm and an extinction coefficient approximately 10,000 M-1 cm-1. Intermediate II has absorbance maxima of 350-390 nm and extinction coefficients of 10,000-16,000 M-1 cm-1 depending on the substrate. A Hammett plot of the logarithm of the rates of the oxygen transfer step, the conversion of intermediate I to intermediate II, gives a straight line with a slope -0.5. Fluoride ion is a product of the enzymatic reaction when 2,3,5,6-tetrafluorophenol is the substrate. These results are consistent with an electrophilic substitution mechanism for oxygen transfer. The conversions of I to II and II to III are acid-catalyzed. A kinetic isotope effect of 8 was measured for the conversion of II to III using deuterated resorcinol as substrate. The conversion of III to oxidized enzyme is base-catalyzed, suggesting that the reaction depends on the removal of the flavin N(5) proton. Product release occurs at the same time as the formation of intermediate III, or rapidly thereafter. The results are interpreted according to the ring-opened model of Entsch et al. (Entsch, B., Ballou, D. P., and Massey, V. (1976) J. Biol. Chem. 251, 2550-2563). 相似文献
134.
135.
Concentration factor and biological half-life of 54Mn were determined in three species representing an ecologically and economically important food chain. Green algae (Chlorella spp.), Daphnia magna and yellow perch (Perca flavescens) were exposed to 54Mn in water and assayed for 54Mn uptake. Steady state concentration factors computed from the laboratory data for algae, Daphnia and perch were 4230, 17 000 and 11, respectively. Respective biological half-lives were 1.6, 1.2 and 8.3 days. 相似文献
136.
A statistical method for correlating tRNA sequence with amino acid specificity. 总被引:3,自引:1,他引:2
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A statistical method for finding the nucleotide positions in tRNA sequences that correlate with amino acid specificity has been developed. The procedure involves finding the subset of nucleotide positions and groups of positions where the marginal density of one amino acid tRNA class does not overlap that of any other amino acid class. The procedure is an application of a statistical method known as the Expectation Maximization algorithm. 相似文献
137.
Coordinate regulation of ribosomal protein mRNA level in regenerating rat liver. Study with the corresponding mouse cloned cDNAs. 总被引:17,自引:6,他引:11
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Cloned mouse ribosomal protein (rp) cDNAs exhibit extensive homology with the corresponding rat sequences. The size of the rp-mRNAs and complexity of the rp-genes are very similar in the two species. Using the mouse rp-recombinant DNAs we find that the relative abundance of rat L7, L13, L18, L30, L32/33 and S16 mRNAs increases after partial hepatectomy. Their maximal level is about twice that of normal rat liver, and is achieved 12-18 h after the operation, while the relative abundance of albumin mRNA decreases to half the normal values 12 h after partial hepatectomy. This concomitant increase in the relative content of these rp-mRNAs indicates coordinate regulation of their level in the rat. The dissimilar behavior of L10 and L19 rp-mRNA suggests additional control mechanisms of rp-mRNA levels in the regenerating rat liver. 相似文献
138.
139.
The stoichiometry of the Ca2+ pump in human erythrocyte vesicles: modulation by Ca2+, Mg2+ and calmodulin 总被引:1,自引:0,他引:1
Active Ca2+ uptake and the associated (Ca2+ + Mg2+)-ATPase activity were studied under the same conditions in an inside-out vesicle preparation of human red blood cells made essentially by the procedure of Quist and Roufogalis (Journal of Supramolecular Structure 6, 375-381, 1977). Some preparations were treated with 1 mM EDTA at 30 degrees to further deplete them of endogenous levels of calmodulin. As the Ca2+ taken up by the EDTA-treated inside-out vesicles, as well as the non-EDTA treated vesicles, was maintained after addition of 4.1 mM EGTA, the vesicles were shown to be impermeable to the passive leak of Ca2+ over the time course of the experiments. In the absence of added calmodulin, both active Ca2+ uptake and (Ca2+ + Mg2+)-ATPase were sensitive to free Ca2+ over a four log unit concentration range (0.7 microM to 300 microM Ca2+) at 6.4 mM MgCl2. Below 24 microM Ca2+ the stoichiometry of calcium transported per phosphate liberated was close to 2:1, both in EDTA and non-EDTA treated vesicles. Above 50 microM Ca2+ the stoichiometry approached 1:1. When MgCl2 was reduced from 6.4 mM to 1.0 mM, the stoichiometry remained close to 2:1 over the whole range of Ca2+ concentrations examined. In contrast to the results at 6.4 mM MgCl2, the Ca2+ pump was maximally activated at about 2 microM free Ca2+ and significantly inhibited above this concentration at 1 mM MgCl2. Calmodulin (0.5-2.0 microgram/ml) had little effect on the stoichiometry in any of the conditions examined. The possible significance of a variable stoichiometry of the Ca2+ pump in the red blood cell is discussed. 相似文献
140.
The difficulty in predicting the consequences of interactions between different cytokine networks has increased with the expansion
of the T helper (Th) cell universe and the discovery of numerous B lymphocyte-derived cytokines. Consequently, it is now difficult
to conceptualize a straightforward view of the contribution of these disturbances to the pathogenesis of primary Sj?gren's
syndrome (SS). Th1 cells, which produce interferon-γ and IL-2, and Th17 cells, which make IL-17 and TNF-α, have been cast
in the leading roles of the play. However, the complex role of T-cell subsets in SS is accentuated by the reciprocal effects
of Th17 cells and regulatory T cells found in salivary glands of SS patients. Furthermore, B lymphocyte polarization into
type-1 B effector (Be1) and Be2 cells and B-cell modulating factors of the TNF family, most notably the B-cell-activating
factor (BAFF), and their prominent role in SS are additional complicating factors. Whereas Th17 cells orchestrate autoreactive
germinal centers, local BAFF would repress the generation of Th17 cells. Such new insights into interconnected cytokines in
primary SS may lead to new treatments for these patients. 相似文献