全文获取类型
收费全文 | 368730篇 |
免费 | 41029篇 |
国内免费 | 198篇 |
出版年
2018年 | 3330篇 |
2017年 | 3158篇 |
2016年 | 4683篇 |
2015年 | 6783篇 |
2014年 | 7659篇 |
2013年 | 10666篇 |
2012年 | 12137篇 |
2011年 | 11853篇 |
2010年 | 8132篇 |
2009年 | 7347篇 |
2008年 | 10511篇 |
2007年 | 10923篇 |
2006年 | 10060篇 |
2005年 | 9694篇 |
2004年 | 9329篇 |
2003年 | 8903篇 |
2002年 | 8942篇 |
2001年 | 19203篇 |
2000年 | 19415篇 |
1999年 | 15054篇 |
1998年 | 4774篇 |
1997年 | 4990篇 |
1996年 | 4808篇 |
1995年 | 4490篇 |
1994年 | 4379篇 |
1993年 | 4256篇 |
1992年 | 12345篇 |
1991年 | 11781篇 |
1990年 | 11359篇 |
1989年 | 10897篇 |
1988年 | 10054篇 |
1987年 | 9371篇 |
1986年 | 8617篇 |
1985年 | 8591篇 |
1984年 | 6872篇 |
1983年 | 5943篇 |
1982年 | 4419篇 |
1981年 | 3879篇 |
1980年 | 3581篇 |
1979年 | 6465篇 |
1978年 | 4883篇 |
1977年 | 4342篇 |
1976年 | 3961篇 |
1975年 | 4510篇 |
1974年 | 4758篇 |
1973年 | 4616篇 |
1972年 | 4252篇 |
1971年 | 3622篇 |
1970年 | 3264篇 |
1969年 | 3073篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Substitutions of hydrophobic amino acids reduce the amyloidogenicity of Alzheimer's disease beta A4 peptides. 总被引:6,自引:0,他引:6
C Hilbich B Kisters-Woike J Reed C L Masters K Beyreuther 《Journal of molecular biology》1992,228(2):460-473
The deposition of amyloid protein aggregates in brain is the main pathological feature of Alzheimer's disease. Their principal constituent is a peptide termed beta A4, which comprises up to 43 amino acid residues. It is highly insoluble under physiological conditions and aggregates into filaments that form very dense clusters in vivo and in vitro. Based on a beta A4 prototype sequence spanning residues 10 to 42 or 43, we have designed analogues in which hydrophobic amino acid residues in position 17 to 20 were substituted by more hydrophilic residues. Depending on the kind of newly introduced amino acids and their position within the sequence, the substitution of only two residues led to variants exhibiting a broad spectrum of different properties. Common to them was a reduced beta-sheet content after solubilization in water and in the solid state. Some of the variants showed significantly reduced amyloidogenicity: although still forming filaments, they did not aggregate into the highly condensed depositions that are typical for amyloid. In addition, they could be solubilized in 200 mM-NaCl and KCl. When mixed with beta A4 peptides bearing the natural sequence, two of the analogues could inhibit the formation of filaments in vitro. These results demonstrate that a well-preserved hydrophobic core around residues 17 to 20 of beta A4 is crucial for the formation of beta-sheet structure and the amyloid properties of beta A4. The introduction of structural alterations within this region may guide the development of reagents for the therapy of Alzheimer's disease. 相似文献
992.
993.
A. R. STRØM J. A. OLAFSEN K. H. REFSNES† H. LARSEN 《Journal of applied microbiology》1979,46(3):545-551
The time course of the conversions of chemical components in herring extracts during anaerobic growth of Proteus sp., str. NTHC 153, Aeromonas sp., str. NTHC 154, and Enterobacter sp., str. NTHC 151 (Strøm & Larsen 1979) has been studied. When the Proteus sp. or the Aeromonas sp. were inoculated into the herring extracts and incubated at 15°C under anaerobic conditions, the sugar components (i.e. mainly ribose, free and bound) were the first substrates utilized. These compounds were converted to acetate and CO2 by the use of trimethylamine oxide (TMAO) as an external hydrogen acceptor. Growth of bacteria ceased when all TMAO was reduced to trimethylamine (TMA). By adding an extra amount of TMAO to the herring extracts an increased growth of the Proteus sp. and the Aeromonas sp. ensued. The increased growth occurred concomitantly with a further conversion of TMAO to TMA and of lactate to acetate and CO2 . The Enterobacter sp., which did not utilize lactate, did not give an increased growth in herring extracts enriched with TMAO. 相似文献
994.
995.
996.
997.
998.
Seventy-eight collections of species forming the Achillea millefolium group were studied for the aglycone composition of their leaf exudates. 6-Hydroxyflavones, 6-hydroxyflavonols and their methyl ethers predominate in the exudates in various combinations. Within the polyploid complex, the diploid taxa Achillea setacea and Achillea aspleniifolia appeared to be chemically well defined. Among the polyploids, little differentiation of major accumulation trends was observed. Infraspecific variability was low in most cases, thus allowing systematic interpretations of aglycone patterns. Possible pharmacological activities are briefly discussed. 相似文献
999.
Hyperthermic treatment at 43 degrees C suppressed the growth of Ehrlich ascites tumor (EAT) cells in vitro. Incubation of EAT cells at 43 degrees C for as little as 1.5 h totally abolished the transplantability of the tumor. At the same time, the rate of cellular glucose uptake, the density of glucose transporter on the cells as well as the extent of thymidine, uridine and leucine incorporation were significantly reduced. 相似文献
1000.