首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   379篇
  免费   19篇
  国内免费   29篇
化学   297篇
力学   5篇
综合类   6篇
数学   49篇
物理学   70篇
  2023年   9篇
  2022年   8篇
  2021年   28篇
  2020年   11篇
  2019年   13篇
  2018年   12篇
  2017年   11篇
  2016年   19篇
  2015年   19篇
  2014年   14篇
  2013年   25篇
  2012年   18篇
  2011年   18篇
  2010年   14篇
  2009年   25篇
  2008年   19篇
  2007年   22篇
  2006年   22篇
  2005年   19篇
  2004年   10篇
  2003年   11篇
  2002年   14篇
  2001年   12篇
  2000年   6篇
  1999年   1篇
  1998年   3篇
  1997年   2篇
  1996年   1篇
  1995年   3篇
  1994年   1篇
  1991年   3篇
  1988年   1篇
  1987年   3篇
  1985年   3篇
  1984年   2篇
  1983年   3篇
  1982年   4篇
  1981年   7篇
  1980年   3篇
  1979年   1篇
  1978年   3篇
  1977年   3篇
  1976年   1篇
排序方式: 共有427条查询结果,搜索用时 22 毫秒
1.
A simple and rapid method was established and validated for the simultaneous quantification of 10 saponins, namely ginsenosides-Rb1, Rb2, Rb3, Rc, Rd, Rg1, Rg2, Re, Rf and Notoginsenside R1, in Chinese Shizhu Panax by ultra performance liquid chromatography coupled with an electrospray mass spectrometry (UPLC-ESI-MS). In addition, the contents of the analytes in different parts of Chinese Shizhu Panax were also analysed. The results showed that the concentration of saponins had a reference to the different parts of Chinese Shizhu Panax. The established method could be used as a new analytical approach for assessment of the quantity of Chinese Shizhu Panax.  相似文献   
2.
A new assay based on ultrafiltration, liquid chromatography and mass spectrometry was developed for the rapid screening and identification of the ligands for α‐glucosidase from the extract of Panax japonicus. Six saponins were identified as α‐glucosidase inhibitors. Subsequently, the specific binding ligands, namely, notoginsenoside R1, ginsenoside Rb1, chikusetsusaponin V, chikusetsusaponin IV, chikusetsusaponin IVa, and ginsenoside Rd (the purities were 94.18, 95.43, 96.09, 93.26, 94.50, 93.86%, respectively) were separated by counter‐current chromatography using two‐phase solvent systems composed of tert‐butyl methyl ether, acetonitrile, 0.1% aqueous formic acid (3.8:1.0:4.4, v/v/v) and the solvent system composed of methylene chloride, isopropanol, methanol, 0.1% aqueous formic acid (5.8:1.0:6.0:2.2, v/v/v). The results demonstrate that ultrafiltration, liquid chromatography and mass spectrometry combined with high‐speed counter‐current chromatography might provide not only a powerful tool for screening and isolating α‐glucosidase inhibitors in complex samples but also a useful platform for discovering bioactive compounds for the prevention and treatment of diabetes mellitus.  相似文献   
3.
人参、西洋参和三七为人参属近缘物种,形态和化学成分相似,均具有连作障碍的特性。人参属中药连作后,土壤环境逐渐变得不适宜其生长,植株长势变弱易患病,导致生长发育受到抑制,出现严重的根腐病、须根脱落,甚至植株死亡现象。而造成人参属连作障碍的主要原因之一是化感物质的自毒作用,人参残体降解物作为人参化感物质主要来源之一,会导致种子发芽率下降,种苗死亡率升高、根腐病等病害加重,最终造成连作障碍。在人参属中药连作过程中,通过化感物质与土壤交流,也会直接或间接影响土壤理化性质的变化。对人参属中药腐解化感作用的研究进展以及腐解对土壤成分的影响进行综述,以期为人参属中药腐解化感作用及土壤利用的相关研究提供借鉴。  相似文献   
4.
采用电感耦合等离子体质谱法测定了山东、吉林、美国和加拿大4个产地西洋参中50种矿物元素的含量,研究了不同产地西洋参矿物元素的差别和转换系数,构建了西洋参的矿物元素指纹图谱。以各产地矿物元素含量的平均值构建了山东、吉林、美国和加拿大产西洋参的矿物元素标准指纹图谱。采用SPSS 20.0计算了各西洋参矿物元素指纹图谱与其矿物元素标准指纹图谱的相似度,确定了山东、吉林、美国和加拿大产西洋参矿物元素指纹图谱的相似度阈值分别为0.93、0.91、0.98和0.93。通过比较未知产地西洋参矿物元素指纹图谱与矿物元素标准指纹图谱的相似度,进行西洋参的产地判别。采用20批未知产地西洋参样品验证模型的准确性,正确率为85%。此外,研究表明,不同生长年限和不同部位西洋参样品对所建立的西洋参产地鉴别方法无影响。  相似文献   
5.
Wild ginseng has better pharmacological effects than cultivated ginseng. However, its industrialization is limited by the inability to grow wild ginseng on a large scale. Herein, we demonstrate how to optimize ginseng production through cultivation, and how to enhance the concentrations of specific ginsenosides through fermentation. In the study, we also evaluated the ability of fermented cultured wild ginseng root extract (HLJG0701-β) to inhibit acetylcholinesterase (AChE), as well as its neuroprotective effects and antioxidant activity. In in vitro tests, HLJG0701-β inhibited AChE activity and exerted neuroprotective and antioxidant effects (showing increased catalyst activity but decreased reactive oxygen species concentration). In in vivo tests, after HLJG0701-β was orally administered at doses of 0, 125, 250, and 500 mg/kg in an animal model of memory impairment, behavioral evaluation (Morris water maze test and Y-maze task test) was performed. The levels of AChE, acetylcholine (ACh), blood catalase (CAT), and malondialdehyde (MDA) in brain tissues were measured. The results showed that HLJG0701-β produced the best results at a dose of 250 mg/kg or more. The neuroprotective mechanism of HLJG0701-β was determined to involve the inhibition of AChE activity and a decrease in oxidative stress. In summary, both in vitro and in vivo tests confirmed that HJG0701-β administration can lead to memory improvement.  相似文献   
6.
将PEG/PAA凝胶浸入铁/柠檬酸(Fe3+/CA)溶液中,自然光照一定时间可实现凝胶-溶胶的相互转化,而且转化行为受溶液的组成及其p H影响。只有当溶液中[Fe3+]0∶[CA]0≤1∶2,且溶液的p H在1~6之间,该转化才会发生,通过改变溶液中[Fe3+]0∶[CA]0的值或溶液的p H可以有效地控制其转化速率。此外,结果还表明,当凝胶转化为溶胶后,向其中加入高价金属阳离子,例如Fe3+、Al3+、Ce4+、Cu2+等,溶胶又会再次转化为凝胶。由于这种新颖的特性,该凝胶有望应用于药物释放、细胞培养等领域。  相似文献   
7.
Exploration of new natural compounds is of vital significance for drug discovery and development. The conventional approaches by systematic phytochemical isolation are low-efficiency and consume masses of organic solvent. This study presents an integrated strategy that combines offline comprehensive two-dimensional liquid chromatography, hybrid linear ion-trap/Orbitrap mass spectrometry, and NMR analysis (2D LC/LTQ-Orbitrap-MS/NMR), aimed to establish a green protocol for the efficient discovery of new natural molecules. A comprehensive chemical analysis of the total ginsenosides of stems and leaves of Panax ginseng (SLP), a cardiovascular disease medicine, was performed following this strategy. An offline 2D LC system was constructed with an orthogonality of 0.79 and a practical peak capacity of 11,000. The much greener UHPLC separation and LTQ-Orbitrap-MS detection by data-dependent high-energy C-trap dissociation (HCD)/dynamic exclusion were employed for separation and characterization of ginsenosides from thirteen fractionated SLP samples. Consequently, a total of 646 ginsenosides were characterized, and 427 have not been isolated from the genus of Panax L. The ginsenosides identified from SLP exhibited distinct sapogenin diversity and molecular isomerism. NMR analysis was finally employed to verify and offer complementary structural information to MS-oriented characterization. The established 2D LC/LTQ-Orbitrap-MS/NMR approach outperforms the conventional approaches in respect of significantly improved efficiency, much less use of drug materials and organic solvent. The integrated strategy enables a deep investigation on the therapeutic basis of an herbal medicine, and facilitates new compounds discovery in an efficient and environmentally friendly manner as well.  相似文献   
8.
《Analytical letters》2012,45(8):1341-1354
The radix of Panax notoginseng is an important herbal resource for clinics, commercial markets, and the health industry worldwide. The quality of P. notoginseng roots is known to be associated with its cultivation age. This study used 1H-nuclear-magnetic-resonance-spectrometry-based metabolic profiling to characterize P. notoginseng roots. Twenty metabolites—including four ginsenosides—contributed to the composition of P. notoginseng according to age. A partial least-squares regression model using a combined data set from two solvent systems was the best predictor of cultivation age. Finally, receiver-operating-characteristic analysis was used to screen potential markers of P. notoginseng root age. These results may be applied to the development of medicinal and nutraceutical products using P. notoginseng roots.  相似文献   
9.
Design and development of a domino cyclative approach for the synthesis of new polycyclic γ-butenolides from β-aryl-Z-enoate propargylic alcohols, through the interception of an intermediate of the Z-enoate-assisted Meyer–Schuster rearrangement, has been reported. A systematic NMR analysis of various derivatives of this class revealed and supported the potential atropisomerism associated with them. These molecules represent first examples of butenolide ring-based atropisomeric compounds in organic chemistry. The synthetic process involves a synchronous construction of both rings with concurrent creation of the potential stereogenic rotational axis.  相似文献   
10.
Phytochemical investigation of the rhizomes of Panax japonicus C. A. Meyer (Araliaceae) resulted in the isolation of two new dammarane‐type triterpenoid saponins, yesanchinoside R1 ( 1 ) and yesanchinoside R2 ( 2 ), together with one new natural product, 6′′′‐O‐acetylginsenoside Re ( 3 ). In addition, 25 known compounds, including 23 triterpenoid saponins, 4 – 26 , β‐sitosterol 3‐Oβ‐D ‐glucopyranoside ( 27 ), and ecdysterone ( 28 ), were also identified. The known saponins 12, 15 , and 18 – 22 were reported for the first time from the title plant. Their structures were elucidated on the basis of detailed spectroscopic analyses, including 1D‐ and 2D‐NMR techniques, as well as acidic hydrolysis.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号