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   105篇
  免费   5篇
化学   92篇
力学   3篇
数学   1篇
物理学   14篇
  2021年   2篇
  2020年   1篇
  2019年   3篇
  2017年   1篇
  2016年   3篇
  2015年   6篇
  2014年   8篇
  2013年   1篇
  2012年   7篇
  2011年   5篇
  2010年   10篇
  2009年   8篇
  2008年   11篇
  2007年   5篇
  2006年   10篇
  2005年   8篇
  2004年   8篇
  2003年   5篇
  2002年   2篇
  2001年   1篇
  2000年   1篇
  1998年   1篇
  1997年   1篇
  1996年   1篇
  1994年   1篇
排序方式: 共有110条查询结果,搜索用时 46 毫秒
81.
Pinosylvin is a natural stilbenoid found particularly in Scots pine. Stilbenoids are a group of phenolic compounds identified as protective agents against pathogens for many plants. Stilbenoids also possess health-promoting properties in humans; for instance, they are anti-inflammatory through their suppressing action on proinflammatory M1-type macrophage activation. Macrophages respond to environmental changes by polarizing towards proinflammatory M1 phenotype in infection and inflammatory diseases, or towards anti-inflammatory M2 phenotype, mediating resolution of inflammation and repair. In the present study, we investigated the effects of pinosylvin on M2-type macrophage activation, aiming to test the hypothesis that pinosylvin could polarize macrophages from M1 to M2 phenotype to support resolution of inflammation. We used lipopolysaccharide (LPS) to induce M1 phenotype and interleukin-4 (IL-4) to induce M2 phenotype in J774 murine and U937 human macrophages, and we measured expression of M1 and M2-markers. Interestingly, along with inhibiting the expression of M1-type markers, pinosylvin had an enhancing effect on the M2-type activation, shown as an increased expression of arginase-1 (Arg-1) and mannose receptor C type 1 (MRC1) in murine macrophages, and C-C motif chemokine ligands 17 and 26 (CCL17 and CCL26) in human macrophages. In IL-4-treated macrophages, pinosylvin enhanced PPAR-γ expression but had no effect on STAT6 phosphorylation. The results show, for the first time, that pinosylvin shifts macrophage polarization from the pro-inflammatory M1 phenotype towards M2 phenotype, supporting resolution of inflammation and repair.  相似文献   
82.
The wetting properties of polypropylene (PP) surfaces were modified by adjusting the dimensions of the surface nanostructure. The nanostructures were generated by injection molding with nanoporous anodized aluminum oxide (AAO) as the mold insert. Atomic layer deposition (ALD) of molybdenum nitride film was used to control the pore diameters of the AAO inserts. The original 50-nm pore diameter of AAO was adjusted by depositing films of thickness 5, 10, and 15 nm on AAO. Bis(tert-butylimido)-bis(dimethylamido)molybdenum and ammonia were used as precursors in deposition. The resulting pore diameters in the nitride-coated AAO inserts were 40, 30, and 20 nm, respectively. Injection molding of PP was conducted with the coated inserts, as well as with the non-coated insert. Besides the pore diameter, the injection mold temperature was varied with temperatures of 50, 70, and 90 degrees C tested. Water contact angles of PP casts were measured and compared with theoretical contact angles calculated from Wenzel and Cassie-Baxter theories. The highest contact angle, 140 degrees , was observed for PP molded with the AAO mold insert with 30-nm pore diameter. The Cassie-Baxter theory showed better fit than the Wenzel theory to the experimental values. With the optimal AAO mask, the nanofeatures in the molded PP pieces were 100 nm high. In explanation of this finding, it is suggested that some sticking and stretching of the nanofeatures occurs during the molding. Increase in the mold temperature increased the contact angle.  相似文献   
83.
The feasibility of desorption atmospheric pressure photoionization (DAPPI) in the direct analysis of illicit drugs was demonstrated by the analysis of confiscated drug samples of various forms such as tablets, blotter paper, and plant resin and bloom. 3,4-Methylenedioxymethamphetamine (MDMA), amphetamine, phenazepam, and buprenorphine were detected from the analyzed tablets, lysergic acid diethylamide (LSD) and bromobenzodifuranylisopropylamine (bromo-Dragonfly, ABDF) from blotter paper, and Delta(9)-tetrahydrocannabinol (THC) and cannabinol from Cannabis Sativa bloom and resin. The amphetamines, phenazepam and ABDF showed protonated molecules independent of the solvent used, whereas buprenorphine, LSD and the cannabinoids showed molecular ions with toluene and protonated molecules with acetone as the solvent.  相似文献   
84.
13C CPMAS NMR investigations of cellulose polymorphs in different pulps   总被引:2,自引:0,他引:2  
In order to obtain information about the crystallinity and polymorphs of cellulose, and the occurrence of hemicelluloses in pulp fibers, wood cellulose, bacterial cellulose, cotton linters, viscose, and celluloses in different pulps were investigated by solid state 13C CPMAS NMR spectroscopy. A mixed softwood kraft pulp and a dissolving-grade pulp were treated under strongly alkaline and acidic conditions and the effect on cellulose crystallinity was studied. The presence of different crystalline polymorphs of cellulose and the amounts of hemicelluloses are considered.  相似文献   
85.
Many insects use chemicals synthesized in exocrine glands and stored in reservoirs to protect themselves. Two chemically defended insects were used as models for the development of a new rapid analytical method based on desorption atmospheric pressure photoionization-mass spectrometry (DAPPI-MS). The distribution of defensive chemicals on the insect body surface was studied. Since these chemicals are predominantly nonpolar, DAPPI was a suitable analytical method. Repeatability of DAPPI-MS signals and effects related to non-planarity and roughness of samples were investigated using acrylic sheets uniformly covered with an analyte. After that, analytical figures of merit of the technique were determined. The spatial distribution of (E)-1-nitropentadec-1-ene, a toxic nitro compound synthesized by soldiers of the termite Prorhinotermes simplex, was investigated. Then, the spatial distribution of the unsaturated aldehydes (E)-hex-2-enal, (E)-4-oxohex-2-enal, (E)-oct-2-enal, (E,E)-deca-2,4-dienal and (E)-dec-2-enal was monitored in the stink bug Graphosoma lineatum. Chemicals present on the body surface were scanned along the median line of the insect from the head to the abdomen and vice versa, employing either the MS or MS2 mode. In this fast and simple way, the opening of the frontal gland on the frons of termite soldiers and the position of the frontal gland reservoir, extending deep into the abdominal cavity, were localized. In the stink bug, the opening of the metathoracic scent glands (ostiole) on the ventral side of the thorax as well as the gland reservoir in the median position under the ventral surface of the anterior abdomen were detected and localized. The developed method has future prospects in routine laboratory use in life sciences.  相似文献   
86.
87.
Steroids have important roles in the progress of pregnancy, and their study in maternal urine is a non-invasive method to monitor the steroid metabolome and its possible abnormalities. However, the current screening techniques of choice, namely immunoassays and gas and liquid chromatography–mass spectrometry, do not offer means for the rapid and non-targeted multi-analyte studies of large sample sets. In this study, we explore the feasibility of two ambient mass spectrometry methods in steroid fingerprinting. Urine samples from pregnant women were screened by desorption electrospray ionization (DESI) and desorption atmospheric pressure photoionization (DAPPI) Orbitrap high resolution mass spectrometry (HRMS). The urine samples were processed by solid phase extraction for the DESI measurements and by enzymatic hydrolysis and liquid–liquid-extraction for DAPPI. Consequently, steroid glucuronides and sulfates were detected by negative ion mode DESI–HRMS, and free steroids by positive ion mode DAPPI–HRMS. In DESI, signals of eleven steroid metabolite ions were found to increase as the pregnancy proceeded, and in DAPPI ten steroid ions showed at least an order of magnitude increase during pregnancy. In DESI, the increase was seen for ions corresponding to C18 and C21 steroid glucuronides, while DAPPI detected increased excretion of C19 and C21 steroids. Thus both techniques show promise for the steroid marker screening in pregnancy.  相似文献   
88.
It is well documented since the early days of the development of atmospheric pressure ionization methods, which operate in the gas phase, that cluster ions are ubiquitous. This holds true for atmospheric pressure chemical ionization, as well as for more recent techniques, such as atmospheric pressure photoionization, direct analysis in real time, and many more. In fact, it is well established that cluster ions are the primary carriers of the net charge generated. Nevertheless, cluster ion chemistry has only been sporadically included in the numerous proposed ionization mechanisms leading to charged target analytes, which are often protonated molecules. This paper series, consisting of two parts, attempts to highlight the role of cluster ion chemistry with regard to the generation of analyte ions. In addition, the impact of the changing reaction matrix and the non-thermal collisions of ions en route from the atmospheric pressure ion source to the high vacuum analyzer region are discussed. This work addresses such issues as extent of protonation versus deuteration, the extent of analyte fragmentation, as well as highly variable ionization efficiencies, among others. In Part 1, the nature of the reagent ion generation is examined, as well as the extent of thermodynamic versus kinetic control of the resulting ion population entering the analyzer region.
Figure
?  相似文献   
89.
Pseudomonas syringae pathovars possess multiple levansucrases with still unclear specific roles for bacteria. We have cloned and expressed three levansucrase genes, lsc1, lsc2 and lsc3, from P. syringae DC3000 in Escherichia coli. Levansucrases synthesize a high molecular weight fructan polymer, levan, from sucrose and in the case of some levansucrases, fructooligosaccharides (FOS) with potential prebiotic effects are also produced. The ability of purified Lsc3 protein of DC3000 to synthesize FOS was tested using prolonged incubation time and varied concentrations of sugar substrates. Thin‐layer chromatography (TLC) analysis of reaction products disclosed formation of FOS from both sucrose and raffinose, revealing a new catalytic property for P. syringae levansucrases. In order to analyze Lsc3‐produced FOS in underivatized form, we optimized a novel method recently introduced in carbohydrate research, based on fully automated chip‐based nanoelectrospray ionization (nanoESI) high‐capacity ion trap mass spectrometry (HCT‐MS). Uding chip‐based nanoESI MS in negative ion mode, FOS, with degrees of polymerization up to five, were detected in reaction mixtures of Lsc3 with sucrose and raffinose. For confirmation, further structural analysis by tandem mass spectrometry (MS/MS) employing collision‐induced dissociation at low energies was performed. To validate the method, commercial inulin‐derived FOS preparations Orafti®P95 and Orafti®Synergy1, which are currently used as prebiotics, were used as controls. By chip‐based nanoESI HCT‐MS, similar FOS distribution was observed in these reference mixtures. Thereby, the obtained data allowed us to postulate that FOS produced by the Lsc3 protein of P. syringae DC3000 may be prebiotic as well. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   
90.
[首页] « 上一页 [2] [3] [4] [5] [6] [7] [8] 9 [10] [11] 下一页 » 末  页»
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