全文获取类型
收费全文 | 1361篇 |
免费 | 39篇 |
国内免费 | 46篇 |
出版年
2023年 | 11篇 |
2022年 | 30篇 |
2021年 | 17篇 |
2020年 | 27篇 |
2019年 | 32篇 |
2018年 | 26篇 |
2017年 | 30篇 |
2016年 | 22篇 |
2015年 | 52篇 |
2014年 | 95篇 |
2013年 | 120篇 |
2012年 | 124篇 |
2011年 | 79篇 |
2010年 | 45篇 |
2009年 | 84篇 |
2008年 | 101篇 |
2007年 | 68篇 |
2006年 | 65篇 |
2005年 | 57篇 |
2004年 | 53篇 |
2003年 | 55篇 |
2002年 | 32篇 |
2001年 | 9篇 |
2000年 | 10篇 |
1999年 | 18篇 |
1998年 | 18篇 |
1997年 | 11篇 |
1996年 | 9篇 |
1995年 | 14篇 |
1994年 | 6篇 |
1993年 | 8篇 |
1992年 | 9篇 |
1991年 | 9篇 |
1990年 | 8篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1987年 | 11篇 |
1986年 | 2篇 |
1985年 | 10篇 |
1984年 | 16篇 |
1983年 | 8篇 |
1982年 | 11篇 |
1981年 | 3篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1976年 | 1篇 |
1973年 | 2篇 |
1972年 | 3篇 |
1971年 | 1篇 |
排序方式: 共有1446条查询结果,搜索用时 15 毫秒
81.
Evariquinone,isoemericellin, and stromemycin from a sponge derived strain of the fungus Emericella variecolor 总被引:2,自引:0,他引:2
From a strain of the fungus Emericella variecolor derived from the marine sponge Haliclona valliculata, two new natural products, evariquinone and isoemericellin, were isolated after HPLC-UV, -MS, and -NMR studies of the extract and their structures were elucidated by mass spectrometry and NMR experiments. Evariquinone showed antiproliferative activity towards KB and NCI-H460 cells at a concentration of 3.16 microg/ml. Furthermore, the fungus was found to produce the known metabolites stromemycin, shamixanthone, and 7-hydroxyemodin. Chemical degradation, NMR decoupling experiments, and spin-system simulation provided evidence for the double bonds in stromemycin to be all E-configured. ROESY experiments established the monosaccharide moiety to be glucose. 相似文献
82.
The isolation, structural elucidation, and antiprotozoal activities of habropetaline A, a novel naphthylisoquinoline alkaloid from Triphyophyllum peltatum, are described. This alkaloid had previously only been identified on line, by the LC-MS/MS-NMR-CD triad, in the crude extract of the rare and difficult-to-provide related plant species Habropetalum dawei, whose small quantities available had not permitted to isolate the compound. As predicted by quantitative structure-activity relationship (QSAR) investigations, habropetaline A exhibits strong antimalarial activity against Plasmodium falciparum, while it is inactive against other protozoal pathogens (Trypanosoma brucei rhodesience, T. cruzi, and Leishmania donovani). 相似文献
83.
中国株丙型肝炎病毒(HCV)结构区蛋白在昆虫细胞中的表达及加工 总被引:4,自引:1,他引:3
利用杆状病毒表达系统在昆虫细胞中表达了完整的中国河北株丙丙型肝炎病毒结构蛋白。免疫印迹实验结果显示,表达产物中有一系列分子量不同、可以与HCV抗体阳性病人血清反应的蛋白,表明结构蛋白被宿主细胞蛋白酶切割与加工,相应分别为20kD的核心蛋白、32kD糖基化的E1蛋白40kD的未糖化的E2蛋白和70kD糖基化的E2蛋白,另有80kD及100kD的两组前体蛋白。利用表达产物检测慢性HCV感染者血清,发现 相似文献
84.
本文对更一般的结构模型给出了参数的一种常用的仪器变量估计近似分布方差的一种算法.并且给出了未知真值x服从指数分布的例子.此算法对生物科学中统计规律的探讨有一定的应用价值. 相似文献
85.
对高山草甸主要植物群落结构特征及其分布格局的研究结果表明,矮嵩草草甸植物群落的丰富度最大,隶属18科,43属45种,呈多优势种植物群落;小嵩草草甸居中,隶属11科,30属35种,小嵩草(Kobresiapygmaea)为优势种;藏嵩草沼泽化草甸最小,隶属9科,21属23种,藏嵩草(K.tibetica)为优势种。其中,有9个种群为3个群落中的共有种,分别占矮嵩草草甸、小嵩草草甸和藏嵩草沼泽化草甸总种数的20.00%、25.71%和39.13%。它们在水分资源位上的生态位宽度较大。3个植物群落类型的种-面积关系呈对数曲线分布,群落的最小样方面积为0.25m2或0.5m2较适宜。种-多度分布呈对数正态分布,其分布模型的表达式如下:S(R)=S0e-(a2R 相似文献
86.
H. Nadji P.N. Diouf A. Benaboura Y. Bedard B. Riedl T. Stevanovic 《Bioresource technology》2009,100(14):3585-3592
Soda lignin, dioxane lignin and milled lignin were isolated from Alfa grass (Stipatenacissima L.). The physico-chemical characterization of three different lignins: one industrial lignin precipitated from soda spent liquor and two lignin preparations isolated under laboratory conditions from Alfa grass (also know as Esparto grass) was performed. The structures of lignins were studied by three non-destructive (FT-IR, solid state 13C NMR and UV/visible spectroscopy) and two destructive (nitrobenzene oxidation and thermogravimetric analysis) methods. Elemental analysis and the methoxyl content determination were performed in order to determine the C9 formulae for the studied lignins. The total antioxidant capacity of the studied lignins has been determined and compared to commercial antioxidants commonly used in thermoplastic industry. 相似文献
87.
Questions: How can one explicitly quantify, and separately measure, stress and disturbance gradients? How do these gradients affect functional composition in early successional plant communities and to what extent? Can we accurately predict trait composition from knowledge of these gradients? Location: Southern Quebec, Canada. Methods: Using eight environmental variables measured in 48 early successional plant communities, we estimated stress and disturbance gradients through structural equation modelling. We then measured 10 functional traits on the most abundant species of these 48 communities and calculated their community‐level mean and variance weighted by the relative abundance of each species. Finally, we related these community‐weighted means and variances to the estimated stress and disturbance gradients using general linear models or generalized additive models. Results: We obtained a well‐fitting measurement model of the stress and disturbance gradients existing in our sites. Of the 10 studied traits, only average plant reproductive height was strongly correlated with the stress (r2=0.464) and disturbance (r2=0.543) gradients. Leaf traits were not significantly related to either the stress or disturbance gradients. Conclusions: The well‐fitting measurement model of the stress and disturbance gradients, combined with the generally weak trait–environment linkages, suggests that community assembly in these early successional plant communities is driven primarily by stochastic processes linked to the history of arrival of propagules and not to trait‐based environmental filtering. 相似文献
88.
Subhra Prakash Chakrabarty Hemalatha Balaram 《Biochimica et Biophysica Acta - Proteins and Proteomics》2010,1804(9):1743-1750
Reversible zinc chelation via thiol groups of cysteines leading to modulation of activity in redox regulated proteins forms a basis for switching on–off of various biochemical processes. Silent information regulator 2 (Sir2), a NAD+ dependent deacetylase, contains a non-catalytic zinc ion coordinated by thiol groups of cysteines. Using Plasmodium falciparum Sir2 (PfSir2), we have examined the effect of zinc removal on the structure and activity of this enzyme. Our studies show that the enzyme with high affinity for zinc exhibits partial collapse of structure upon removal of the metal ion. Zinc reconstitution of apo PfSir2 led to recovery of both structure and activity highlighting the reversibility of the process. 相似文献
89.
90.
Jun Liu Jianguang Luo Hong Ye Yi Sun Zhaoxin Lu Xiaoxiong Zeng 《Carbohydrate polymers》2010,79(1):206-213
In this study, response surface methodology was employed to optimize the medium compositions for the production of exopolysaccharides (EPS) from endophytic bacterium Paenibacillus polymyxa EJS-3. Firstly, fractional factorial design was applied to evaluate the effects of different components in the medium. It was found that sucrose, yeast extract and CaCl2 influenced significantly the production of EPS. Then, steepest ascent method and central composite design were used to optimize the concentrations of the three variables. As results, the optimal medium compositions were determined as following (g/L): sucrose 188.2, yeast extract 25.8, K2HPO4 5 and CaCl2 0.34, with a corresponding yield of 35.26 g/L. In addition, both polysaccharide fractions (EPS-1 and EPS-2) from crude EPS were mainly composed of (2 → 6)-linked β-d-fructofuranosyl residues backbone with (2 → 1)-linked branches based on their structural characterization by FT-IR spectroscopy, methylation analysis and 13C NMR spectroscopy. 相似文献