全文获取类型
收费全文 | 26069篇 |
免费 | 2314篇 |
国内免费 | 1813篇 |
专业分类
30196篇 |
出版年
2024年 | 60篇 |
2023年 | 283篇 |
2022年 | 690篇 |
2021年 | 1033篇 |
2020年 | 763篇 |
2019年 | 997篇 |
2018年 | 1012篇 |
2017年 | 781篇 |
2016年 | 1095篇 |
2015年 | 1666篇 |
2014年 | 1898篇 |
2013年 | 2017篇 |
2012年 | 2350篇 |
2011年 | 2242篇 |
2010年 | 1360篇 |
2009年 | 1287篇 |
2008年 | 1470篇 |
2007年 | 1348篇 |
2006年 | 1206篇 |
2005年 | 1033篇 |
2004年 | 1028篇 |
2003年 | 871篇 |
2002年 | 765篇 |
2001年 | 492篇 |
2000年 | 402篇 |
1999年 | 345篇 |
1998年 | 271篇 |
1997年 | 207篇 |
1996年 | 183篇 |
1995年 | 148篇 |
1994年 | 131篇 |
1993年 | 79篇 |
1992年 | 102篇 |
1991年 | 96篇 |
1990年 | 99篇 |
1989年 | 84篇 |
1988年 | 53篇 |
1987年 | 56篇 |
1986年 | 38篇 |
1985年 | 35篇 |
1984年 | 42篇 |
1983年 | 18篇 |
1982年 | 15篇 |
1981年 | 9篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1973年 | 4篇 |
1967年 | 3篇 |
1966年 | 3篇 |
1965年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
Restoration of a subtropical eutrophic shallow lake in China: effects on nutrient concentrations and biological communities 总被引:1,自引:0,他引:1
Disturbances caused by rainfall are common in streams with the impact on stream inhabitants determined by the frequency, intensity, and predictability of the event. Here, we examine the response of winter-emerging Chironomidae (Diptera) to extreme flooding disturbance. In August of 2007, a severe flood impacted southeastern Minnesota, imparting stress on aquatic communities. Chironomid pupal exuviae collections were obtained biweekly from 18 southeast Minnesota streams during the following winter to assess resistance and resilience of winter-active chironomids to flooding. Streams examined were divided into moderate (2.5–10 cm), heavy (10–20 cm), or extreme (20+ cm) rainfall categories with rainfall amounts in each category representing total precipitation during the 3-day storm. Post-flood samples were compared to samples obtained from the same localities during prior winters. Our findings contradict studies of responses by Chironomidae to flooding during warmer-water conditions and show that winter-emerging Chironomidae are resistant to stress imposed by summer spates. Significantly more taxa emerged during winter after flooding as compared to historic collections, and the number of species emerging in winter was positively correlated with rainfall severity, indicating that 15 species responded opportunistically to disturbance. This indicates that winter-active Chironomidae may be resistant to increased severity of summer spates associated with climate change predictions. 相似文献
952.
953.
Lei Ding Feng Tang Wei Huang Qiu Jin Han Shen Ping Wei 《Bioorganic & medicinal chemistry letters》2013,23(20):5630-5633
A novel series of 3-pyrrolo[b]cyclohexylene-2-dihydroindolinone derivatives targeting VEGFR-2, PDGFR-β and c-Kit kinases were designed and synthesized. The molecular design was based on the SAR features of indolin-2-ones as kinase inhibitors. SAR study of the series allowed us to identify compounds possessing more potent inhibitory activities against the three kinases than sunitinb with IC50 values in the low nanomolar range in vitro. Additionally, some compounds also showed favorable antiproliferative activities against a panel of cancer cell lines (BXPC-3, T24, BGC, HEPG2 and HT29). 相似文献
954.
Li-Ping Shi Kun-Ming Jiang Jun-Jie Jiang Yi Jin Yun-Hai Tao Ke Li Xing-Hong Wang Jun Lin 《Bioorganic & medicinal chemistry letters》2013,23(21):5958-5963
A novel series of polyhalobenzonitrile quinazolin-4(3H)-one derivatives were synthesized and characterized by NMR, IR, MS, and HRMS spectra. All of the newly prepared compounds were screened for antimicrobial activities against four strains of bacteria (Gram-positive bacterial: Staphylococcus aureus and Bacillus cereus; Gram-negative bacterial: Escherichia coli and Pseudomonas aeruginosa) and one strain of fungi (Candida albicans). Among the synthesized compounds, 5-(dimethylamino)-8-(2,4,5-trichloro-isophthalonitrile) quinazolin-4(3H)-one (7k) exhibited significant activity towards Gram-positive bacterial, Gram-negative bacterial, and the fungi strains. The MIC (0.8–3.3 μg/mL) and MBC (2.6–7.8 μg/mL) for this compound were close to those of nofloxacin, chlorothalonil, and fluconazole, making it the most potent antimicrobial agents in the series. 相似文献
955.
Youguang Zheng Ming Zheng Xin Ling Yi Liu Yunsheng Xue Lin An Ning Gu Min Jin 《Bioorganic & medicinal chemistry letters》2013,23(12):3523-3530
Novel pyrazole–benzimidazole derivatives have been designed and synthesized. The entire target compounds were determined against cancer cell lines U937, K562, A549, LoVo and HT29 and were screened for Aurora A/B kinase inhibitory activity in vitro. The compounds 7a, 7b, 7i, 7k and 7l demonstrated significant cancer cell lines and Aurora A/B kinase inhibitory activities. Molecular modeling studies suggested the derivatives have bound in the active site of Aurora A kinase through the formation of four hydrogen bonds. Quantum chemical studies were carried out on these compounds to understand the structural features essential for activity. The cellular activity of 7k was also tested by immunofluorescence. 相似文献
956.
Jin Bae Weon Hyun-Jeong Ko Choong Je Ma 《Bioorganic & medicinal chemistry letters》2013,23(24):6732-6736
We isolated 2,3-dihydroxy-4-methoxyacetophenone, a neuroprotective compound from Cynenchum paniculatum in our previous study.The present study was conducted to investigate the possible neuroprotective effect of 2,3-dihydroxy-4-methoxyacetophenone that has been previously isolated from Cynenchum paniculatum on hippocampal neuronal cell line, HT22 cells and its possible cognitive-enhancing effect on scopolamine-induced amnesia in mice.Neuroprotective effect against glutamate-induced neurotoxicity in HT22 cells was evaluated by MTT assay. Also, cognitive enhancing effect against scopolamine (1 mg/kg, ip) induced learning and memory deficit was measured by Morris water maze test. Oral administered of 2,3-dihydroxy-4-methoxyacetophenone (1, 10, 20, 40 and 50 mg/kg) to amnesic mice induced by scopolamine. In Morris water maze test, 2,3-dihydroxy-4-methoxyacetophenone (50 mg/kg) improved the impairment of spatial memory induced by scopolamine. 2,3-Dihydroxy-4-methoxyacetophenone protect HT22 cells on glutamate induced cell-death in a dose-dependent manner (EC50 value: 10.94 μM). Furthermore, 2,3-dihydroxy-4-methoxyacetophenone was found to inhibit [Ca2+] accumulation in HT22 cells and had antioxidantive activity. The results showed that 2,3-dihydroxy-4-methoxyacetophenone exert neuroprotective and cognitive-enhancing activities through its antioxidant activity. We suggest that 2,3-dihydroxy-4-methoxyacetophenone improves cognitive function and may be helpful for the treatment of Alzheimer’s disease. 相似文献
957.
Tian-Xing Shi Shu Wang Ke-Wu Zeng Peng-Fei Tu Yong Jiang 《Bioorganic & medicinal chemistry letters》2013,23(21):5904-5908
Five new phenolic glycosides, tenuisides A–E (1?5), and a new megastigmane glycoside, tenuiside F (6), along with seventeen known compounds (7–23) were isolated from the aerial parts of Polygala tenuifolia Willd. Their structures were established by detailed analysis of NMR and HRESIMS spectroscopic data, and the absolute configurations of compounds 5 and 6 were determined by CD spectra and in-NMR-tube Mosher’s method. The inhibitory effects of these compounds were evaluated on NO production in LPS-activated BV-2 microglia cells. Compound 17 showed the strongest activity, with an IC50 value of 7.4 μM, while compounds 1, 8, 14, and 18 showed the moderate activities, with IC50 values of 16.2–38.5 μM. And their primary structure–activity relationships (SARs) of NO inhibitory effects were also briefly discussed. 相似文献
958.
Xianwen Cao Jing Jiang Shoude Zhang Lili Zhu Juan Zou Yanyan Diao Weilie Xiao Lei Shan Handong Sun Weidong Zhang Jin Huang Honglin Li 《Bioorganic & medicinal chemistry letters》2013,23(11):3329-3333
Eleven compounds were identified as estrogen receptor modulators from an in-house natural product database (NPD) by structure-based virtual screening for ERα and ERβ. Among them, 3 compounds were confirmed as ER agonists and 8 compounds were confirmed as ER antagonists by yeast two-hybrid (Y2H) assay, with EC50 values ranging from several micromolar to 100 micromolar. In this study, a novel series of cycloartane triterpenoids isolated from Schisandra glaucescens Diels was found to have ER antagonistic effect, the most potent antagonist of which exhibited activity with EC50 value of 2.55 and 4.68 μM for ERα and ERβ, respectively. Moreover, the types of modulation and subtype selectivity were also investigated through molecular docking simulation. 相似文献
959.
Seong Rim Byeon Hyunjin Vincent Kim Mijin Jeon Young Gil Ahn Maeng Sup Kim Jae Yang Kong Hye Yun Kim Young Soo Kim Dong Jin Kim 《Bioorganic & medicinal chemistry letters》2013,23(11):3467-3469
Alzheimer’s disease drug discovery regarding exploration into the molecules and processes has focused on the intrinsic causes of the brain disorder correlated with the accumulation of amyloid-β. An anti-amyloidogenic bis-styrylbenzene derivative, KMS80013, showed excellent oral bioavailability (F = 46.2%), facilitated brain penetration (26%, iv) in mouse and target specific in vivo efficacy in acute AD mouse model attenuating the cognitive deficiency in Y-maze test. Acute toxicity (LD50 >2000 mg/kg) and hERG channel inhibition (14% at 10 μM) results indicated safety of KMS80013. 相似文献
960.