全文获取类型
收费全文 | 208篇 |
免费 | 8篇 |
专业分类
化学 | 183篇 |
晶体学 | 2篇 |
数学 | 14篇 |
物理学 | 17篇 |
出版年
2024年 | 1篇 |
2023年 | 2篇 |
2022年 | 6篇 |
2021年 | 7篇 |
2020年 | 6篇 |
2019年 | 6篇 |
2018年 | 8篇 |
2017年 | 4篇 |
2016年 | 4篇 |
2015年 | 6篇 |
2014年 | 5篇 |
2013年 | 10篇 |
2012年 | 14篇 |
2011年 | 17篇 |
2010年 | 6篇 |
2009年 | 7篇 |
2008年 | 18篇 |
2007年 | 12篇 |
2006年 | 11篇 |
2005年 | 4篇 |
2004年 | 8篇 |
2003年 | 5篇 |
2002年 | 4篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1996年 | 1篇 |
1993年 | 2篇 |
1985年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1977年 | 4篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1972年 | 4篇 |
1971年 | 2篇 |
1969年 | 2篇 |
1968年 | 1篇 |
1967年 | 5篇 |
1966年 | 1篇 |
1965年 | 2篇 |
排序方式: 共有216条查询结果,搜索用时 15 毫秒
71.
Kong WM Chik Z Ramachandra M Subramaniam U Aziddin RE Mohamed Z 《Molecules (Basel, Switzerland)》2011,16(9):7344-7356
The extract from Mitragyna speciosa has been widely used as an opium substitute, mainly due to its morphine-like pharmacological effects. This study investigated the effects of M. speciosa alkaloid extract (MSE) on human recombinant cytochrome P450 (CYP) enzyme activities using a modified Crespi method. As compared with the liquid chromatography-mass spectrometry method, this method has shown to be a fast and cost-effective way to perform CYP inhibition studies. The results indicated that MSE has the most potent inhibitory effect on CYP3A4 and CYP2D6, with apparent half-maximal inhibitory concentration (IC(50)) values of 0.78 μg/mL and 0.636 μg/mL, respectively. In addition, moderate inhibition was observed for CYP1A2, with an IC(50) of 39 μg/mL, and weak inhibition was detected for CYP2C19. The IC(50) of CYP2C19 could not be determined, however, because inhibition was <50%. Competitive inhibition was found for the MSE-treated CYP2D6 inhibition assay, whereas non-competitive inhibition was shown in inhibition assays using CYP3A4, CYP1A2 and CYP2C19. Quinidine (CYP2D6), ketoconazole (CYP3A4), tranylcypromine (CYP2C19) and furafylline (CYP1A2) were ACCESSused as positive controls throughout the experiments. This study shows that MSE may contribute to an herb-drug interaction if administered concomitantly with drugs that are substrates for CYP3A4, CYP2D6 and CYP1A2. 相似文献
72.
Kalenius E Beyeh NK Jänis J Rissanen K 《Chemical communications (Cambridge, England)》2011,47(9):2649-2651
Pyrogallarene shows concentration-dependent instability in dilute solutions resulting in elimination of two ketene molecules and formation of pyrogallarene lactones. This unexpected phenomenon, which is not observed with resorcinarenes, highlights the significance of the four hydroxyl groups at 2-position for the molecular characteristics of pyrogallarenes. 相似文献
73.
Kirsi S. Mikkonen Leena Pitk?nen Ville Liljestr?m Elina Mabasa Bergstr?m Ritva Serimaa Lennart Salmén Maija Tenkanen 《Cellulose (London, England)》2012,19(2):467-480
The chemical structure of rye arabinoxylan (rAX) was systematically modified, exploiting selective enzymes to mimic different
naturally occurring xylans, i.e., its degree of substitution (DS) was decreased using α-l-arabinofuranosidase, and a controlled decrease in the degree of polymerization (DP) was performed using endo-1,4-β-d-xylanase. The arabinose to xylose ratio was decreased from 0.45 to 0.27 and the weight-average molar mass was decreased from
184,000 to 49,000 g/mol. The resulting samples were used to prepare films, as such, and with 15% (wt. − %) softwood-derived
microfibrillated cellulose (MFC) to obtain novel plant-derived biocomposite materials. The enzymatic tailoring of rAX increased
the crystallinity of films, evidenced by X-ray diffraction studies, and the addition of MFC to the debranched, low DS rAX
induced the formation of ordered structures visible with polarizing optical microscopy. MFC decreased the moisture uptake
of films and increased the relative humidity of softening of the films, detected with moisture scanning dynamic mechanical
analysis. For the first time, the chemical structure of xylan was proven to significantly affect the reinforcement potential
of nano-sized cellulose, as the tensile strength of films from high DP rAXs, but not that of low DP rAXs, clearly increased
with the addition of MFC. At the same time, MFC only increased the Young’s modulus of films from rAX with high arabinose content,
regardless of DP. 相似文献
74.
Electrochemical DNA biosensors are promising tools for the fast, inexpensive and simple in vitro analysis for the determination of free radicals and antioxidants. High concentrations of antioxidants in such compounds as
phenolic acids and plant extracts, act as free radical terminators which reduce the effect of the oxidative dam-age on DNA.
The electrochemical behavior of three representative phenolic acids, caffeic acid, gallic acid and trolox were studied by
cyclic voltammetry. Moreover, the determination of the above antioxidants under the optimized conditions (scan rate, deposition
potential and time) using differential pulse voltammetry was also investigated. In vitro studies focused on their antioxidative effect were performed by adsorptive transfer stripping voltammetry and dsDNA biosensor.
Using Fenton’s system, with FeSO4 and H2O2 was chosen as a strong oxidative system. This biosensor was applied as a screening antioxidant test in order to estimate
the antioxidant capacity of aqueous herb extracts.
相似文献
75.
Christian Klemps Dipl.‐Chem. Elina Payet Lionel Magna Dr. Lucien Saussine Dr. Xavier F. Le Goff Dr. Pascal Le Floch Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(33):8259-8268
Chromium(III) complexes bearing R′N(CH2PR2)2 (PCNCP) ligands have been prepared. Upon activation with methylaluminoxane, these complexes proved to be effective in the selective tri‐ and tetramerization of ethylene. The formation of either 1‐hexene or 1‐octene was found to be highly dependent on the steric bulk of the substituents R on the phosphine moieties. This observation was rationalized by using density functional theory calculations on selected steps of the metallacyclic mechanism of the ethylene oligomerization reaction. 相似文献
76.
In order to utilise wood and wood fibres in advanced materials, a better understanding of the mechanical material characteristics
and the interactions among the components is necessary. For this purpose, FTIR was explored together with mechanical loading
as a means of studying the molecular responses to the loading of spruce wood and cellulose paper material. A linear shift
of absorption bands was detected as the loading was applied. In relation to the applied stress these shifts were higher under
moist conditions than under dry ones but they were similar with regard to the strains applied. There were no shifts detected
in bands related to lignin or the hemicelluloses. The results are interpreted as reflecting a parallel arrangement of the
load bearing component, the cellulose ordered structure, and the moisture accessible regions in the cellulose microfibril
structure. This therefore represents an equal strain loaded system. 相似文献
77.
Melecchi MI Péres VF Dariva C Zini CA Abad FC Martinez MM Caramão EB 《Ultrasonics sonochemistry》2006,13(3):242-250
The influence of several experimental parameters on the ultrasonic extraction of Hibiscus tiliaceus L. flowers were investigated: extraction time, solvent polarity, sample amount, solvent volume and sample particle size. It was concluded that the most influential variables were extraction time and solvent polarity. The optimized procedure employed 5 g of ground flowers, 150 mL of methanol and 140 min of extraction. The extracts were fractionated using preparative silica columns and the resulting fractions were analyzed by GC/MS. Some saturated hydrocarbons, fatty acids, fatty acid methyl esters, phytosterols, and vitamin E were identified in the plant extracts. 相似文献
78.
Kaisa Helttunen Riia Annala Aku Suhonen Juho Iloniemi Elina Kalenius Gemma Aragay Pablo Ballester Heikki M. Tuononen Maija Nissinen 《化学:亚洲杂志》2019,14(5):647-654
The anion‐binding properties of three closely related oligoamide foldamers were studied using NMR spectroscopy, isothermal titration calorimetry and mass spectrometry, as well as DFT calculations. The 1H NMR spectra of the foldamers in [D6]acetone solution revealed partial preorganization by intramolecular hydrogen bonding, which creates a suitable cavity for anion binding. The limited size of the cavity, however, enabled efficient binding by the inner amide protons only for the chloride anion resulting in the formation of a thermodynamically stable 1:1 complex. All 1:1 chloride complexes displayed a significant favourable contribution of the entropy term. Most likely, this is due to the release of ordered solvent molecules solvating the free foldamer and the anion to the bulk solution upon complex formation. The introduction of electron‐withdrawing substituents in foldamers 2 and 3 had only a slight effect on the thermodynamic constants for chloride binding compared to the parent receptor. Remarkably, the binding of chloride to foldamer 3 not only produced the expected 1:1 complex but also open aggregates with 1:2 (host:anion) stoichiometry. 相似文献
79.
Shatsauskas Anton L. Saibulina Elina R. Gatilov Yurii V. Kostyuchenko Anastasia S. Fisyuk Alexander S. 《Chemistry of Heterocyclic Compounds》2019,55(11):1080-1086
Chemistry of Heterocyclic Compounds - Reactivity of 2-methyl-6-phenylbenzo[c][1,7]naphthyridine in alkylation, oxidation, and electrophilic substitution reactions has been studied. The reaction of... 相似文献
80.
V. M. Kazakova O. G. Sokol G. G. Dvoryantseva I. S. Musatova A. S. Elina 《Chemistry of Heterocyclic Compounds》1980,16(3):284-287
The anion radicals formed in the electrochemical reduction of 2,3-dimethyl-quinoxoline and its N,N′-dioxide, the 6-methoxy
and 6-chloro derivatives of the N,N′-dioxide, and the preparation quinoxidine in DMF were studied. Polarographic analysis
showed that the first step in the reduction is a one-electron step and is reversible for all of the compounds except quinoxidine.
However, the EPR spectra of the primary anion radicals were recorded only for 2,3-dimethylquinoxaline and its 1,4-dioxide.
For the remaining compounds we were able to obtain anion radicals of only the reduction products, the structures of which
in a number of cases were established from the EPR spectra.
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No 3, pp. 376–380, March, 1980. 相似文献