全文获取类型
收费全文 | 2058篇 |
免费 | 120篇 |
国内免费 | 62篇 |
出版年
2023年 | 18篇 |
2022年 | 14篇 |
2021年 | 39篇 |
2020年 | 63篇 |
2019年 | 64篇 |
2018年 | 75篇 |
2017年 | 61篇 |
2016年 | 57篇 |
2015年 | 49篇 |
2014年 | 89篇 |
2013年 | 139篇 |
2012年 | 41篇 |
2011年 | 84篇 |
2010年 | 43篇 |
2009年 | 84篇 |
2008年 | 94篇 |
2007年 | 95篇 |
2006年 | 87篇 |
2005年 | 70篇 |
2004年 | 51篇 |
2003年 | 70篇 |
2002年 | 82篇 |
2001年 | 60篇 |
2000年 | 49篇 |
1999年 | 53篇 |
1998年 | 50篇 |
1997年 | 47篇 |
1996年 | 49篇 |
1995年 | 36篇 |
1994年 | 51篇 |
1993年 | 35篇 |
1992年 | 32篇 |
1991年 | 25篇 |
1990年 | 27篇 |
1989年 | 23篇 |
1988年 | 24篇 |
1987年 | 27篇 |
1986年 | 26篇 |
1985年 | 33篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 16篇 |
1981年 | 16篇 |
1980年 | 12篇 |
1979年 | 14篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1974年 | 2篇 |
1973年 | 6篇 |
1972年 | 4篇 |
排序方式: 共有2240条查询结果,搜索用时 15 毫秒
81.
82.
In order to find novel antitumor candidate agents with high efficiency and low toxicity, 14 novel substituted 5‐anilino‐α‐glucofuranose derivatives have been designed, synthesized and evaluated for antiproliferative activities in vitro. Their structures were characterized by NMR (1H and 13C) and HR‐MS, and configuration (R/S) at C(5) was identified by two‐dimensional 1H,1H‐NOESY‐NMR spectrum. Their antiproliferative activities against human tumor cells were investigated by MTT assay. The results demonstrated that most of the synthesized compounds had antiproliferative effects comparable to the reference drugs gefitinib and lapatinib. In particular, (5R)‐5‐O‐(3‐chloro‐4‐{[5‐(4‐fluorophenyl)thiophen‐2‐yl]methyl}anilino)‐5‐deoxy‐1,2‐O‐(1‐methylethylidene)‐α‐glucofuranose ( 9da ) showed the most potent antiproliferative effects against SW480, A431 and A549 cells, with IC50 values of 8.57, 5.15 and 15.24 μm , respectively. This work suggested 5‐anilino‐α‐glucofuranose as an antitumor core structure that may open a new way to develop more potent anti‐cancer agents. 相似文献
83.
Tao Wang Huimin Luo Yaocai Bai Jianlin Li Ilias Belharouak Sheng Dai 《Liver Transplantation》2020,10(30)
Ionic liquids (ILs) are a family of nonconventional molten salts that offer many advantages, such as negligible vapor pressures, negligible flammability, wide liquidus ranges, good thermal stability, and much synthesis flexibility. The unique solvation environment of these ILs provides new reaction or flux media for controlling formation of solid‐state materials with a minimum perturbation of morphologies. A successful lithiation via ionothermal synthesis using a cost‐effective Li halide as Li source and recyclable ILs as solvents is reported here for the direct recycling of LiNi1/3Co1/3Mn1/3O2 (NCM 111) cathodes. In addition, the ionic liquids can be readily recycled and reused after ionothermal lithiation. The lithiation of spent cathodes can enable the direct recycling of spent cathode materials in lithium‐ion batteries. 相似文献
84.
85.
《Bioorganic & medicinal chemistry letters》2020,30(4):126896
A series of novel 3-benzylcoumarin-imidazolium salts were prepared and evaluated in vitro against a panel of human tumor cell lines. The results showed that the existence of 5,6-dimethyl-benzimidazole ring and substitution of the imidazolyl-3-position with a naphthylacyl group were vital for modulating cytotoxic activity. Notably, compound 38 was found to be the most potent derivative with IC50 values of 2.04–4.51 μM against five human tumor cell lines, while compound 34 were more selective to SW-480 cell lines with IC50 value 40.0-fold lower than DDP. Mechanism of action studies indicated that compound 38 can cause the G0/G1 phase cell cycle arrest and apoptosis in SMMC-7721 cell lines. 相似文献
86.
《Bioorganic & medicinal chemistry》2020,28(2):115245
Thiophene bioisosteres of potent GluN2B receptor negative allosteric modulators were prepared and evaluated pharmacologically. The five-step synthesis of 4,5,7,8-tetrahydro[7]annuleno[b]thiophen-6-one (10) was considerably improved by carboxylation of thiophene-3-carboxylic acid (8) in the first reaction step. Reductive amination and alkylation led to three homologous series of secondary and tertiary phenylalkylamines 5, 11 and 12. Metalation, reaction with 1-formylpiperidine and subsequent reduction provided hydroxymethyl derivatives 15 and 16, which had been designed as bioisosteres of phenols. 2-Bromo derivatives 18 were obtained by bromination of ketone 10 with NBS and subsequent reductive amination. High GluN2B affinity was achieved with [7]annuleno[b]thiophenes bearing a 3-phenylpropylamino or 4-phenylbutylamino moiety (e.g. 5c: Ki = 5.9 nM; 11d: Ki = 9.0 nM). Tertiary ethylamines 12 showed lower GluN2B affinity than tertiary methylamines 11 or secondary amines 5 (e.g. 5c: Ki = 5.9 nM; 11c: Ki = 6.0; 12c: Ki = 51 nM). A Br-atom or a hydroxymethyl moiety in 2-position were less tolerated by the GluN2B receptor. Very similar relationships between the structure and GluN2B affinity and structure and σ affinity, in particular σ2 affinity, were detected. A slight preference for the ifenprodil binding site of GluN2B receptors over σ1 and σ2 receptors was found for methylamines 11c (≈2-fold) and 11d (≈1.5–2-fold) as well as for bromo derivative 18c (≈3-fold). 相似文献
87.
Chunyu Fan Lingzhao Tan Chunyu Zhang Xiuhai Zhao Lushuang Gao Klaus von Gadow 《Ecology and evolution》2020,10(15):8225-8234
The relationship between biodiversity and productivity has stimulated an increasing body of research over the past decades, and this topic still occupies a central place in ecology. While most studies have focused on biomass production in quadrats or plots, few have investigated the scale‐dependent relationship from an individual plant perspective. We present an analysis of the effects of biodiversity (species diversity and functional diversity) on individual tree growth with a data set of 16,060 growth records from a 30‐ha temperate forest plot using spatially explicit individual tree‐based methods. A significant relationship between species diversity and tree growth was found at the individual tree level in our study. The magnitude and direction of biodiversity effects varies with the spatial scale. We found positive effects of species diversity on tree growth at scales exceeding 9 m. Individual tree growth rates increased when there was a greater diversity of species in the neighborhood of the focal tree, which provides evidence of a niche complementarity effect. At small scales (3–5 m), species diversity had negative effects on tree growth, suggesting that competition is more prevalent than complementarity or facilitation in these close neighborhoods. The results also revealed many confounding factors which influence tree growth, such as elevation and available sun light. We conclude that the use of individual tree‐based methods may lead to a better understanding of the biodiversity‐productivity relationship in forest communities. 相似文献
88.
Sukham Munilkumar Yumnam Rameshori W. Romen Mangang Makamguang Kamei Yengkhom Chinglemba Annam Pavan-Kumar Bijay Kali Mahapatra Arabinda Das Md. Abu Hanif Muhammad Abu Bakar Siddik 《Zeitschrift fur angewandte Ichthyologie》2021,37(2):356-358
The length-weight relationships (LWRs) of six Nemacheilid species (Schistura chindwinica, S. fasciata, S. khugae, S. minuta, S. reticulata and S. rubrimaculata) have been analyzed. Fish samples were collected on quarterly basis from March 2018 to February 2019. Sampling was performed using cast nets (mesh size 5–10 mm; about 50 sq m area covered each time and water depth was 4 ft approx.), and electrofishing (Ultrasonic Inverter Electro Fisher, 24 volts, 4 m) in the day time. The total length (TL) of individual fish was measured to 0.1 cm with a digital caliper and body weights (BW) were measured to 0.001 g with digital electronic balances. The parameters for the LWR equations were calculated, and the respective statistics such as the 95% confidence interval for parameters “a” and “b” are provided as well as the coefficient of correlation. For five species a new maximum total length has been documented. 相似文献
89.
兴都库什喜马拉雅地区高海拔树木生长对气候变化的响应
高海拔地区快速升温可能导致树木对温度响应更为敏感,而限制高海拔地区树木生长的关键气候因子以及气候变化对树木生长产生多大程度的影响尚不清楚。本研究在兴都库什喜马拉雅地区收集了73 个样点的树轮数据,包括3个优势属的树种(Abies属、Juniperus属和Picea属),样点海拔均在3000 m以上。 将时间动态规整(dynamic time warping)的方法用于建立亚区域年表,以考虑不同站点年表之间变化的同步 性。同时,定量分析了气候因子对树木生长的贡献以及树木生长与气候因子关系的时空动态。研究结果发现,73个站点年表可以聚为3类,且与其所处的生物气候区相对应,即西喜马拉雅地区,中东喜马拉雅地区和藏东南地区。在干旱的西喜马拉雅地区,树木生长与冬、春两季的降水呈正相关关系,而在湿润的藏东南地区,树木生长与冬季温度和春季降水呈正相关关系。树木生长受最低温度的影响最大,特别是冬季温度,其重要性从西到东呈现递增趋势。滑动窗口相关分析表明,在中西喜马拉雅地区,影响树木生长的冬季温度信号在减弱,然而在藏东南地区该信号随着1980年以来的快速升温而增强。本研究结果表明,若该地区升温持续,在西喜马拉雅地区可能会因变暖引起的水分亏缺而造成森林衰退,而在藏东南地区因树木生长得益于变暖而使得森林扩张。 相似文献
90.
《Journal of receptor and signal transduction research》2013,33(3):207-239
A cationic protonatable amine moiety on dopaminergic ligands forms a high affinity reinforced ionic bond with an anionic aspartic acid at position 3.32 of dopamine receptors. When present, catechol hydroxyls of the ligands form hydrogen bonds with serines at position 5.42, 5.43, and 5.46, and this network of hydrogen bonds serves to orient ligands in the binding-site crevice and increase their binding affinity. A steric clash between aromatic moieties of the ligands and aromatic amino acids of the receptor (e.g., H6.55, F6.52 or F6.51 and W6.48) is likely to be propagated in domino-like fashion along the length of TM6, which is believed to trigger activation of the receptor. Specifically, it is the change in the conformation of W6.48 from an orientation perpendicular to the plane of the lipid membrane to one that is parallel that is believed to result in activation. Molecular determinants that mediate the D4/D2-selectivity of many extremely D4-selective 1,4-DAP ligands, include a nonconserved cluster of bulky amino acids at the TM2/TM3 interface (positions 2.61, 3.28 and 3.29). 相似文献