全文获取类型
收费全文 | 22260篇 |
免费 | 2073篇 |
国内免费 | 12篇 |
专业分类
24345篇 |
出版年
2021年 | 220篇 |
2020年 | 127篇 |
2019年 | 188篇 |
2018年 | 255篇 |
2017年 | 218篇 |
2016年 | 359篇 |
2015年 | 586篇 |
2014年 | 689篇 |
2013年 | 934篇 |
2012年 | 1200篇 |
2011年 | 1292篇 |
2010年 | 857篇 |
2009年 | 765篇 |
2008年 | 1097篇 |
2007年 | 1164篇 |
2006年 | 1154篇 |
2005年 | 1159篇 |
2004年 | 1164篇 |
2003年 | 1058篇 |
2002年 | 1108篇 |
2001年 | 268篇 |
2000年 | 219篇 |
1999年 | 323篇 |
1998年 | 331篇 |
1997年 | 243篇 |
1996年 | 245篇 |
1995年 | 225篇 |
1994年 | 286篇 |
1993年 | 265篇 |
1992年 | 235篇 |
1991年 | 220篇 |
1990年 | 211篇 |
1989年 | 235篇 |
1988年 | 201篇 |
1987年 | 203篇 |
1986年 | 179篇 |
1985年 | 242篇 |
1984年 | 257篇 |
1983年 | 246篇 |
1982年 | 302篇 |
1981年 | 309篇 |
1980年 | 305篇 |
1979年 | 204篇 |
1978年 | 210篇 |
1977年 | 211篇 |
1976年 | 183篇 |
1975年 | 164篇 |
1974年 | 200篇 |
1973年 | 172篇 |
1972年 | 131篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
Donald E. Rivett Dean Hewish Alan Kirkpatrick Jerome Werkmeister 《Journal of Protein Chemistry》1999,18(3):291-295
Nine fatty acid–peptide hybrid molecules were constructed using the general formula CH3(CH2)
n
CO-Phe Asp Cys-amide and tested for their ability to inhibit cell lysis induced by the membrane-active peptide melittin. All of these molecules, where n = 4–14, inhibited the action of melittin to some extent, but the longer carbon chains were most effective. Several potential inhibitors were also constructed with conservative substitutions in the peptide portion of the molecule. All were effective to varying degrees. We concluded that in the hexapeptide inhibitor published by Blondelle et al. (1993), the role of the first three residues is only to provide hydrophobic interaction with the melittin and has no particular amino acid sequence specificity. Some of these inhibitors were found to inhibit the lytic activity of a melittin analogue which had only superficial sequence similarity to melittin and also a truncated form of melittin, indicating the generality of the action of the inhibitors.Deceased 5/4/98 相似文献
3.
ABSTRACT The phrase persistent vulnerability reflects the enduring relationship of the rural population in Ceará with a highly variable climate. Persistence underscores the historical and unyielding nature of this vulnerability. Yet contrary to once-catastrophic rates of mortality etched in a public consciousness, no one dies from severe droughts and few people flee them as in the past. Government relief and social transfers have become the institutionalized form of adaptation, giving way to the counterintuitive reality that drought stabilizes the food and income supply for poor people. We analyze how maladaptive risk reduction, which is embedded in clientilistic social relations, undermines resilience, and we examine pathways toward a more sustainable adaptive relationship. 相似文献
4.
5.
Abstract Permeabilized cells of Haemophilus influenzae incorporate wall precursors into murein material in an ampicillin-sensitive reaction. In resistant transformants that contain the low antibiotic affinity penicillin-binding proteins (PBPs) 4 and 5, the sensitivity of this incorporation reaction to ampicillin is proportionally lower, suggesting a catalytic role for these proteins in wall synthesis. We conclude that, analogous to the reaction in Escherichia coli , PBPs 4 and 5 of H. influenzae have transpeptidase activity. 相似文献
6.
Bruce R. Dorrbecker Susan H. Mercik Paul A. Kramer 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1984,336(1)
A high-performance liquid chromatographic procedure is reported for reproducibly and sensitively quantitating caffeine and its N-demethylated metabolite paraxanthine in micro-samples. A 5-μm reversed-phase radial compression column and 214-nm fixed wavelength ultraviolet detector were used to attain a sensitivity sufficient to quantitate these compounds at concentratios as low as 80 ng/ml using only 25 μl of sample. The assay is applicable to microliter samples of whole blood, serum, plasma, saliva, amniotic, cerebro-spinal and gastric fluids such as might be obtained in studies involving small animals or neonates. The utility of the assay is illustrated with caffeine and paraxanthine levels measured in several maternal and fetal fluids following constant-rate intravenous infusion of caffeine into a rabbit throughout pregnancy. 相似文献
7.
Methanobacterium thermoautotrophicum, an archaebacterium, possesses the first and last enzymes of the diaminopimelic acid pathway for lysine biosynthesis, dihydrodipicolinate synthase, and diaminopimelate decarboxylase. It does not have saccharopine dehydrogenase, the last enzyme of the aminoadipate pathway for lysine biosynthesis. The dihydrodipicolinate synthase is inhibited but not repressed by lysine. We conclude that this microbe uses the diaminopimelate pathway for synthesis of lysine.Deceased. 相似文献
8.
Studies in biodiversity-ecosystem function and conservation biology have led to the development of diversity indices that take species' functional differences into account. We identify two broad classes of indices: those that monotonically increase with species richness (MSR indices) and those that weight the contribution of each species by abundance or occurrence (weighted indices). We argue that weighted indices are easier to estimate without bias but tend to ignore information provided by rare species. Conversely, MSR indices fully incorporate information provided by rare species but are nearly always underestimated when communities are not exhaustively surveyed. This is because of the well-studied fact that additional sampling of a community may reveal previously undiscovered species. We use the rarefaction technique from species richness studies to address sample-size-induced bias when estimating functional diversity indices. Rarefaction transforms any given MSR index into a family of unbiased weighted indices, each with a different level of sensitivity to rare species. Thus rarefaction simultaneously solves the problem of bias and the problem of sensitivity to rare species. We present formulae and algorithms for conducting a functional rarefaction analysis of the two most widely cited MSR indices: functional attribute diversity (FAD) and Petchey and Gaston's functional diversity (FD). These formulae also demonstrate a relationship between three seemingly unrelated functional diversity indices: FAD, FD and Rao's quadratic entropy. Statistical theory is also provided in order to prove that all desirable statistical properties of species richness rarefaction are preserved for functional rarefaction. 相似文献
9.
10.
E. Charles Kerr E Bruce Eaton L. Thomas Netzel 《Nucleosides, nucleotides & nucleic acids》2013,32(5-6):851-866
Abstract Syntheses of two analogs of deoxyuridine with N,N-dialkylaniline chromophores are reported. 5-[3-(N-methylphenylamino)propanoyl]-2′-deoxyuridine (1) and 5-[2-(4-N,N-dimethylaminophenyl)ethyl)]-2′-deoxyuridine (2) are prepared by palladium-mediated coupling. Preparation of 2 was facilitated by in situ transient O4-trimethylsilyl protection during alkynylation which suppressed secondary cyclization of the coupling adduct. 相似文献